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Optical emission variability in the intermediate polars FO Aquarii, V1223 Sagittarii and AO Piscium
Martell, Phillip John, Ph.D.
The Ohio State University, 1990
Copyright ©1990 by Martell, Phillip John. All rights reserved.
UMI 300 N. Zeeb Rd. Ann Arbor, MI 48106
OPTICAL EMISSION LINE VARIABILITY IN THE INTERMEDIATE POLARS
FO AQUARII, V1223 SAGITTARII AND AO PISCIUM
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree Doctor of Philosophy in the Graduate
School of the Ohio State University
By
Phillip John Martell, B.S., M.S.
*****
The Ohio State University
1990
Dissertation Committee: Approved by
Dr. Ronald H. Kaitchuck
Dr. Gerald H. Newsom Advisor Dr. Arne Slettebak Department of Astronomy Copyright by Phillip John Martell 1990 ACKNOWLEDGMENTS
I am indebted to my advisor, Dr. Ronald H. Kaitchuck, for his insightful guidance of the work leading to this dissertation. His patience, encouragement, and perpetual good humor were deeply appreciated. For invaluable assistance at the 1.8 m telescope during my summer in Flagstaff, I thank Dr. R. Mark Wagner and Ray Bertram.
Thanks are due to the Department of Astronomy for providing funds in support of the stay in Flagstaff. Drs. Arne Slettebak and Gerald H.
Newsom, members of my dissertation committee, are acknowledged for their useful comments and suggestions. I am profoundly grateful to my parents for their love and guidance, and for their support of this endeavor. VITA
December 22, 1950... Born - Menominee, Michigan
197 3...... B.S., University of Wisconsin-Madison, Madison, Wisconsin
197 4 ...... Teaching Certificate, University of Wisconsin- Madison, Madison, Wisconsin
1974 - 80...... Teacher of physics, physical science and mathematics, Community High School, West Chicago, Illinois
1980 - 82...... Graduate Teaching Assistant, Department of Physics, Iowa State University, Ames, Iowa
1982...... M.S., Iowa State University, Ames, Iowa
1982 - 1985...... Visiting Instructor, Physics Department, University of Wisconsin-River Falls, River Falls, Wisconsin
1985 - 1990...... Graduate Teaching Associate (at intervals), Department of Astronomy, The Ohio State University, Columbus, Ohio
1986 - 1988...... Graduate Research Associate (at intervals), Department of Astronomy, The Ohio State University, Columbus, Ohio
PUBLICATIONS
"Identification of a Red Optical Counterpart of Cygnus X-3", R.M. Wagner, T.J. Kreidl, P.J. Martell, and J. Beaver, in CCDs in Astronomy, ed. G.H. Jacoby (Astronomical Society of the Pacific: San Francisco), p. 361. (1990)
- iii - FIELDS OF STUDY
Major Field: Astronomy
Studies in Infrared Colors of Late-Type Stars. Professor Robert F. Wing.
Studies in Cataclysmic Variables. Professor Ronald H. Kaitchuck. TABLE OF CONTENTS
ACKNOWLEDGMENTS...... ii
VITA...... iii
LIST OF TABLES...... vii
LIST OF FIGURES...... viii
CHAPTER PAGE
I. INTRODUCTION...... 1
Cataclysmic Variables...... 1 Magnetic CVs...... 5 The Optical Emission from Intermediate Polars...... 8 This Thesis...... 12
II. OBSERVATIONS , DATA REDUCTION AND ANALYSIS...... 15
Observations...... 15 Instrumentation...... 15 Observing Program...... 16 Data Reduction...... 19 Methods of Analysis...... 23 Phase Binning...... 23 Magnitudes...... 23 Emission Line Profile Measurements...... 23 Gray-Scale Images...... 24
III. FO AQUARII...... 26
Introduction...... 26 Observations...... 29 Spectra...... 33 Continuum...... 38 Hydrogen Lines...... 38 He II A4686...... 48 Period Search...... 48 Analysis...... 57 The "Rotational Disturbance" and the System Inclination...... 57
v The Nature of the 21 Minute Modulation...... 62 The Orbital Continuum Modulation...... 70 Discussion...... 70
IV. V1223 Sagittarii...... 79
Introduction...... 79 Observations...... 82 Spectra...... 85 Continuum...... 93 Hydrogen Lines...... 93 He II A4686...... 96 Period Search...... 100 Analysis...... 105 The Structure of the Hj8 Line...... 105 The Orbit Phasing of Radial Velocity and Continuum...... 109 The Spin Modulation...... 110 Discussion...... 110 The Orbit Modulation...... 110 Evidence for a Wind...... 114 The Spin Modulation...... 114
V. AO Piscium...... 116
Introduction...... 116 Observations...... 121 Spectra...... 123 Continuum...... 129 Hydrogen Lines...... 129 He II A4686...... 133 Analysis...... 137 The Continuum Phasing on the Orbit Cycle...... 137 The H/3 Flux Modulation and the Absorption Feature...... 144 The Spin Modulation...... 145 Discussion...... 146 The Orbit Modulation and System Phasing...... 146 The Spin Modulation...... 151
VI. SUMMARY...... 154
Spin Modulations...... 154 Orbit Modulations...... 159 Reprocessing and Winds...... 162 A Final Word...... 163
LIST OF REFERENCES...... 165
- vi - LIST OF TABLES
TABLE PAGE
1. Journal of Observations...... 17
2. False alarm probabilities for detected frequencies, for FO Aqr...... 49
3. False alarm probabilities for detected frequencies, for V1223 Sgr...... 101
4. Summary of spin modulations...... 155
- vii - LIST OF FIGURES
FIGURE PAGE
1. Schematic diagram of an intermediate polar...... 9
2. FO Aqr. Composite spectrum formed by averaging all spectra...... 30
3. FO Aqr. Gray-scale image on the 4.8 hour phasing...... 31
4. FO Aqr. Gray-scale image on the 21 minute phasing...... 32
5. FO Aqr. Continuum light curve on the 4.8 hour phasing.... 34
6. FO Aqr. Continuum light curve onthe 21 minute phasing.... 35
7. FO Aqr. flux on the 4.8 hour phasing...... 36
8. FO Aqr. Hj3 radial velocity on the 4.8 hour phasing...... 37
9. FO Aqr. H/J flux on the 21 minute phasing...... 39
10. FO Aqr. Hj8 radial velocity on the 21 minute phasing 40
11. FO Aqr. Hj8 FWHM on the 21 minute phasing...... 41
12. FO Aqr. He II A4686 flux on the 4.8 hour phasing...... 42
13. FO Aqr. He II A4686 radial velocity on the 4.8 hour phasing...... 43
14. FO Aqr. He II A4686 flux on the 21 minute phasing...... 44
15. FO Aqr. He II A4686 radial velocity on the 21 minute phasing...... 45
16. FO Aqr. He II A4686 FWHM on the 21 minute phasing...... 46
17. FO Aqr. Power spectrum of the continuum magnitude...... 50
18. FO Aqr. Power spectra of (a) Hft flux, and (b) Hjd radial velocity...... 51
- viii - 19. FO Aqr. H/9 radial velocity on the 19.5 minute phasing.... 52
20. FO Aqr. Power spectra of He II A4686 (a) flux, and (b) radial velocity...... 53
21. FO Aqr. He II A4686 radial velocity on the 19.5 minute phasing...... 54
22. FO Aqr. He II A4686 radial velocity on the 4.8 hour phasing, with a sinusoid fit subtracted...... 56
23 (a). FO Aqr. He II A4686 radial velocity at flux maximum of the 21 minute cycle, on the 4.8 hour phasing...... 58
23 (b). FO Aqr. He II A4686 radial velocity at flux minimum of the 21 minute cycle, on the 4.8 hour.phasing...... 59
24. FO Aqr. Gray-scale image of He II A4686 at maximum and minimum of the 21 minute period, on the 4.8 hour phasing...... 60
25. FO Aqr. He II A4686 profiles at maximum (vector) and minimum (histogram) FWHM of the 21 minute cycle...... 63
26. FO Aqr. Flux modulation of the He II A4686 profile on the 21 minute phasing "sliced" at -600 km s"^ (left) and at +600 km s (right)...... 64
27. FO Aqr. He II A4686 residual wind component flux on the 21 minute phasing...... 66
28. FO Aqr. Gray-scale image of the He II A4686 residual wind component, on the 21 minute phasing...... 67
29. FO Aqr. Schematic view of the wind reprocessing model.... 72
30. V1223 Sgr. Magnitude vs time for 6 September 1988, illustrating the monotonic decrease in brightness during the night...... 84
31. V1223 Sgr. Composite spectrum formed by averaging all spectra taken in July 1988...... 86
32. V1223 Sgr. Gray-scale image of the July 1988 spectra, on the 3.4 hour phasing...... 88
33. V1223 Sgr. Gray-scale image of the July 1988 spectra, in the neighorhood of H/9, on the 3.4 hour phasing...... 89
- ix - 34. V1223 Sgr. Gray-scale image of the July 1988 spectra, in the neighborhood of He II A4686, on the 3.4 hour phasing...... 90
35. V1223 Sgr. Gray-scale image of 6 September 1988 spectra, in the neighborhood of tj/3, on the 12.4 minute phasing...... 91
36. V1223 Sgr. Continuum light curves for July and September 1988, on the 3.4 hour phasing...... 92
37. V1223 Sgr. Continuum light curve for September 1988 on the 13.2 minute phasing...... 94
38. V1223 Sgr. radial velocity for July 1988 on the 3.4 hour phasing...... 95
39. V1223 Sgr. E/3 radial velocity for 6 September 1988 on the 12.4 minute phasing...... 97
40. V1223 Sgr. He II A4686 radial velocity for 5 and 6 September 1988 on the 12.4 minute phasing...... 98
41. V1223 Sgr. He II A4686 radial velocity for 5 and 6 September 1988 on the 14.2 minute phasing...... 99
42. V1223 Sgr. Power spectrum of the continuum magnitude for 6 September 1988...... 102
43. V1223 Sgr. Power spectrum of H/? radial velocity for 6 September 1988...... 103
44. V1223 Sgr. Power spectrum of He II A4686 radial velocity for 5 and 6 September 1988...... 104
45. V1223 Sgr. Gray-scale image of the H/3 residual wind component on the 3.4 hour phasing...... 106
46. V1223 Sgr. The H/J residual wind profile...... 107
47. V1223 Sgr. Radial velocity for Gaussian fits to the red side of H/J, on the 3.4 hour phasing...... 108
48. V1223 Sgr. (a) Orbit phasing if the emission lines originate in the secondary star, (b) Orbit phasing if the emission lines originate in the disk. In either case, continuum maximum occurs at phase 0.88...... 112
- x - 49. AO Psc. Composite spectrum formed by averaging all spectra...... 124
50. AO Psc. Gray-scale image of all spectra on the 3.6 hour phasing...... 126
51. AO Psc. Gray-scale image of all spectra, in the neighborhood of H/9, on the 3.6 hour phasing...... 127
52. AO Psc. Composite spectrum formed by averaging spectra in the 3.6 hour phase interval 0.56-0.72...... 128
53. AO Psc. Gray-scale image in the neighborhood of He II A4686, on the 3.6 hour phasing...... 130
54. AO Psc. Gray-scale image on the 14.3 minute phasing...... 131
55. AO Psc. (a) (top) Gray scale image, in the neighborhood of H/3, on the 13.4 minute phasing. (b) (bottom) Gray scale image, in the neighborhood of He II A4686, on,the 13.4 minute phasing...... 132
56. AO Psc. Hj9 flux on the 3.6 hour phasing...... 134
57. AO Psc. H/3 radial velocity, measured in the wings of the line, on the 3.6 hour phasing...... 135
58. AO Psc. Hj9 radial velocity on the 13.4 minute phasing 136
59. AO Psc. He II A4686 flux on the 3.6 hour phasing . 138
60. AO Psc. He II A4686 radial velocity on the 3.6 hour phasing...... 139
61. AO Psc. He II A4686 flux on the 14.3 minute phasing...... 140
62. AO Psc. He II A4686 radial velocity on the 13.4 minute phasing...... 141
63. AO Psc. He II A4686 flux on the 13.4 minute phasing. 142
64. AO Psc. He II A4686 FWHM on the 13.4 minute phasing. 143
65. AO Psc. H/3 at zero velocity (vector) and maximum blueshift (histogram), on the 13.4 minute phasing...... 147
66. AO Psc. H£ at zero velocity (vector) and maximum redshift (histogram), on the 13.4 minute phasing...... 148
- xi - 67. AO Psc. Orbital phasing based on the assumption that the wings of H)8 originate in the disk...... 150
68. FO Aqr. Orbital phasing based on the assumption that the continuum light originates at the inner face of the secondary star...... 161
- xii - CHAPTER I
INTRODUCTION
CATACLYSMIC VARIABLES
Intermediate polars (IPs) constitute a subclass of the cataclysmic variables (CVs). Observationally, CVs typically exhibit broad emission lines (—10 3 km s -1 or more) of H, He I, and sometimes He II.
The Balmer decrement is commonly found to be very flat. The continua are typically very blue, and variations in both continuum and lines are observed which can be periodic or non-periodic. Continua often show random variations at low amplitude (Am - 0.1) called flickering.
Emission lines typically show periodic Doppler shifts and changes in integrated flux and FWHM on timescales of minutes to hours. Some members of the class (the novae, recurrent novae and dwarf novae) display outburts on semi-periodic timescales ranging from days to years, with amplitudes ranging up to 10 or more magnitudes (Robinson,
1976). Such outburst activity gave this class of objects its name.
Cataclysmic variables are believed to be semi-detached, interacting binary stars (Wade and Ward, 1985). Systems usually consist of a white dwarf primary and a late type (K or M) dwarf secondary star. The secondary star fills its Roche lobe, spilling matter through the LI point at the local thermal speed (-10 km s'-*-).
1 2
The matter, in the form of a stream, is diverted from the line of
centers of the stars by the coriolis acceleration. It then becomes highly supersonic. Thus, because the timescale for free-fall is much
less than the timescale for thermal expansion of the stream, the
accretion stream remains very narrow (Lubow and Shu 1975). The stream
orbits the primary, colliding with itself. At the collision point,
the gas is shock heated. The primary is now surrounded by a ring of
gas, rotating differentially, setting up shear. Viscosity dissipates
the shear energy, resulting in heating and subsequent radiation by the
gas. The loss of energy also causes the gas to sink farther into the
gravitational potential well of the primary, but, in order to conserve
angular momentum, some of the gas in the initial ring must move to
larger radii. Thus, a disk is formed (Pringle 1981).
The light from a CV should have several sources: primary star,
secondary star, accretion disk, accretion stream, hot spot and
boundary layer, the latter to be defined below. The luminosity of the
disk is expected to be proportional to the rate of mass accretion (M)
onto the white dwarf (mass ^ and radius R^) (Pringle 1981):
Ldisk ~ GVV 2V (D For a 1 Mg white dwarf, M — 1016 g s-^ and - 6 x 108 cm, yielding
Ldisk ~ 1033 erg s"1, so that the disk luminosity should greatly
exceed the light from the secondary and primary stars. At least in
its inner parts, where it is expected to be optically thick in the
continuum, the disk probably radiates locally like a blackbody. The
resulting continuum spectrum integrated over the whole disk resembles 3
a blackbody curve with a flattened center (Frank, King and Raine
1985).
The boundary layer is the region which separates the surface of the white dwarf from the assumed Keplerian flow at the inner edge of the disk. Within this region, the angular velocity of the gas must decrease to the angular rotational velocity of the white dwarf. The thickness of this region is believed to be much less than the radius of the white dwarf (Frank, King and Raine 1985). The reader will note that equation (1) expresses only one-half of the total available accretion energy. The remaining half must be dissipated in the boundary layer. At low accretion rates, the boundary layer has low optical depth. This results in the emission of x-rays characteristic of high temperatures (-108 K), i.e., hard x-rays (Pringle and Savonije
1979; Patterson and Raymond 1985a), as the shock region is viewed directly. At high accretion rates, the boundary layer is expected to be optically thick, resulting in thermalization, and the emission of photons in the ultraviolet and soft x-ray regions (T - 105 K) (Pringle
1977; Patterson and Raymond 1985b).
The hotspot region marks the location of a shock at the intersection between the accretion disk and the accretion stream from the secondary star, and is typically a source of emission line and continuum radiation. The models of Flannery (1975) predict temperatures in this region of order 20,000 K. The hotspot region is widely believed to be the source of the s-wave distortion commonly 4
observed in CV emission lines (Wade and Ward 1985; Honeycutt,
Kaitchuck and Schlegel 1987).
In addition to the hotspot region, line emission evidently originates in the outer parts of the accretion disk, a region believed to be optically thin in the optical continuum as a result of the low temperatures (~6000 K) expected there (Williams 1980). The great breadth of the lines is usually attributed to the Doppler shift produced by the Keplerian motion in the disk. This is suggested by the constancy of the ratio AX/X from line-to-line (Wade and Ward
1985). In addition, the lines are often double-peaked. The latter phenomenon is best understood in terms of the "dipole" pattern of loci of constant radial velocity on the surface of a disk (see, for example, Horne and Marsh 1986). The absence of double-peaked emission lines, however, does not imply the absence of a disk. Systems viewed at low inclination, for example, are unlikely to show double-peaked lines (Robinson 1976). It is the case, however, that some systems at high inclination also show single-peaked profiles. It is possible in these cases that the emission near line center has been filled in by a wind, or that Stark broadening, when convolved with a disk profile, produces a single emission peak (Lin et al. 1988).
In the above considerations, it is assumed that line emission results from viscous dissipation in the outer regions of the disk.
Another source of line emission could arise from irradiation of parts of the disk, or of a disk corona (e.g., Jameson et al. 1980), by the 5
hot boundary layer.
MAGNETIC CVs
The picture of disk accretion outlined above is evidently altered if the white dwarf possesses an appreciable magnetic field, in which case the field intervenes to control the accretion at some distance from the white dwarf. There are (at least) two possible cases, represented by the two currently recognized classes of magnetic CVs:
The polars (or AM Her stars) and the intermediate polars (or DQ Her stars). The difference between these two classes lies in the degree of synchronization between the white dwarf spin period and the orbit period. In the former case, the spin is phase-locked with the orbit;
in the latter, the spin and orbit are not synchronized (Watson 1986).
For a typical intermediate polar, the orbit period exceeds the spin period by a factor of 10-20 or more.
The region within which the accreting gas is controlled by the magnetic field (the "magnetosphere") is defined by the Alfven radius,
R& . For radial inflow, Ra is defined as the radius at which the ram pressure of the gas equals the magnetic pressure (Frank, King and
Raine 1985):
pv2 - B2/8tt. (2)
In the case of the polars, Ra evidently is of the same order as the separation between the stars; gas is entrained at the LI point, and accretion proceeds along "tubes" which terminate at the magnetic pole 6
of the white dwarf (Wade and Ward 1985). For the intermediate polars,
Ra is thought to be less than the outer radius of the accretion disk, so that the disk is disrupted. (However, for an alternative view, in which no disk is thought possible, see Hameury, King and Lasota,
1986). The fields associated with the polars have been detected by observations of linear and circular polarization (thus the name
"polar"), evidently arising from cyclotron emission near the magnetic poles, with strengths determined to be of order 107 Gauss (see, for example, Schmidt 1985). However, there are only three cases of such detection among the intermediate polars: DQ Her (Swedlund, Kemp, and
Wolstencroft 1974; Kemp, Swedlund, and Wolstencroft 1974), AE Aqr
(Szkody, Michalsky, and Stokes 1982; Cropper 1986) and BG CMi
(Penning, Schmidt and Liebert 1986; West, Berriman and Schmidt 1987).
Only the last of these three is unquestioned. The absence of
detectable polarization in most of the intermediate polars is
generally taken as support for the notion that the magnetic fields
associated with these objects are weak enough to allow the existence
of truncated accretion disks. It should be noted, however, that the
question of the magnetic field strength in the intermediate polars is
a matter of dispute (see, for example, Norton and Watson 1989a).
In the case of either the polars or the intermediate polars, the
channeling of material onto the magnetic poles of the white dwarf
results in a strong shock, high temperature (T ~ 10® K) and the
efficient production of x-rays. The x-ray emission originating in 7
the hot post-shock region is dominated by optically thin bremsstrahlung (Frank, King and Raine 1985), and yields a hard
component for energies above -2 keV. In addition, a portion of the
accretion energy liberated at the shock is transported to the white
dwarf surface, resulting in heating of the surface (T ~ 106) and the production of blackbody emission of soft x-rays (Watson 1986).
As discussed above, the typical intermediate polar includes a white
dwarf with spin period short when compared to the system orbit period.
A modulation of the x-ray emission at the spin period might therefore
be expected, and is indeed observed. In fact, one usually assumes
that a detected high-frequency x-ray modulation is due to the rotation
of the white dwarf. There are two main spin-phase dependent effects
which could produce the observed modulations: (1) occultation of the
x-ray source by the body of the white dwarf; and, (2) photoelectric
absorption. The former effect is obviously independent of energy;
because the cross-section for photoelectric absorption goes as ,
the latter effect depends on the energy at which the modulation is
observed. That is, the modulation depth increases with decreasing
energy. X-ray observations of intermediate polars confirm the
modulation depth/energy relation predicted by photoelectric absorption
(Norton and Watson 1989b) . And the latter authors have shown that
models incorporating both occultation and photoelectric absorption are
required to explain the observations. 8
THE OPTICAL EMISSION FROM INTERMEDIATE POLARS
The current population of intermediate polars numbers 13. Figure 1 depicts schematically a typical IP. In the optical, IPs typically exhibit strong coherent modulations on - time scales of minutes to hours. The long-period modulations are usually associated with the system orbital motion, and the explanations invoked for these modulations are essentially identical to those invoked for non magnetic CVs: Continuum modulations are due to the changing aspect of the hotspot and/or inner surface of a secondary which is heated by radiation from the vicinity of the white dwarf. In this case, that radiation field is time-dependent. Emission lines will likely originate in gas at locales distant from the system center of mass
(e.g., disk, hotspot, secondary star), thus will be modulated in radial velocity on the orbital period. Lines will also be modulated in flux if, for example, there is significant contribution to the line flux from a hotspot optically thick in the lines, or from the inner face of the secondary star.
In addition, intermediate polars should produce optical modulations not observed in normal CVs. It is likely that optical radiation, both in the continuum and in line emission, will originate in the magnetosphere of the white dwarf. This radiation will then be pulsed at the white dwarf rotation period (i.e., the x-ray period). In this case, radial velocity variations could be caused by two effects: (1) motion of the gas about the white dwarf spin axis; or, (2) a spin- I 9 \ CD \ I (a)
(b)
Figure 1 Schematic view of an intermediate polar. (a) View in the plane of the disk. The flow of matter from the disk to the magnetic poles of the white is indicated by arrows. The rotation axis of the white dwarf is assumed to be perpendicular to the plane of the accretion disk. (b) View from above the plane of the disk. The secondary star lies on the left, the disk on the right. The shaded area at the edge of the disk is the hotspot. The dashed circle encloses the region within which accretion is dominated by the magnetic field of the white dwarf. 10 phase dependent aspect effect in which streaming of gas in the accretion column is viewed at different angles in the course of the spin period. A form of the latter model is favored by Mason, Rosen and Hellier (1988). But it would seem likely that both effects operate. Flux variations could arise if there is rotational phase- dependent occultation of the radiating gas by the body of the white dwarf or by other parts of the accretion column, or if the radiating gas is optically thick.
Spin-pulsed radiation could also be produced by reprocessing of the hard x-rays in axisymmetric parts of the accretion disk (see, for example, Penning 1985). In the simple model presented by the latter author, a narrow, rotating, x-ray beam falls upon a limited range of disk radii, essentially creating a moving spot on the face of the disk. Radial velocity modulation arises as the angle between the line of sight to the disk and the velocity vector of the Keplerian flow in the spot varies over the spin period. Flux modulation in this model would arise as a consequence of the fact that the disk thickness increases with disk radius (Shakura and Sunyaev 1973), so that the back side of the disk will present a larger projected surface area to the viewer. This model produces no modulation at all, of course, in the case where the system inclination is zero.
Another source of modulation not observed in ordinary CVs is that due to reprocessing of the x-ray beam at fixed targets in the rotating system frame. In this case, a modulation appears with frequency w-fl, 11
where w is the white dwarf spin frequency, and 0 is the system orbit frequency. This reprocessing frequency, sometimes labeled the beat frequency, will be referred to in this work as the first lower sideband, as it is simply one of a series of difference and sum frequencies (sidebands) which can appear in these systems (Warner
1986). The phenomenon of modulation at the first lower sideband was
first noticed in the continuum light of the intermediate polar AO Psc by Patterson and Price (1981).
Two candidates for sites of non-axisymmetrie reprocessing in the
intermediate polars are discussed most frequently: the inner surface
of the secondary star and the hotspot region. The hotspot region is usually assumed to be elevated above the general level of the disk
edge due to shock heating (Lin 1975), and thus can form a significant
target for the x-ray beam. There would seem to be an additional
potential site for reprocessing which is not much discussed in the
literature of intermediate polars, namely the part of the accretion
stream which flows over the disk (Lubow and Shu 1976; Livio, Soker
and Dgani 1986; Lubow 1989).
It is sometimes argued that the three intermediate polars (AE Aqr,
DQ Her and V533 Her) having the shortest spin periods (of order 1 minute) should be assigned to a separate class - the DQ Her stars.
There are two reasons cited for this opinion: (1) The spin periods of these objects are significantly smaller than the next smallest spin period (that of GK Per at 351 s); and, (2) only one of the three (AE 12
Aqr) has been detected in x-rays. However, as the model for the intermediate polars outlined above is generally accepted as descriptive of the DQ Her stars as well, this author regards the latter as members of the intermediate polar class. The properties of intermediate polars have been reviewed by, among others, Cordova and
Mason (1983), Warner (1983, 1985), Lamb (1985), Mason (1985), Watson
(1986) and Osborne (1988).
THIS THESIS
Apart from EX Hya and DQ Her, few of the intermediate polars have been subjected to thorough spectroscopic examination. In particular, very little extensive time-resolved and flux-calibrated spectroscopy has been performed. In the case of the intermediate polars, good time resolution is needed in order to resolve all of the known periodicities in a system. The matter of using flux-calibrated spectra is not trivial. Most studies have relied upon equivalent widths instead of fluxes, and equivalent widths can be difficult to interpret as they result from the level of both line and continuum flux. Because the systems are relatively dim (B-magnitude typically
-13.5), individual spectra will be very noisy, and so must be obtained in large numbers for coaddition in phase. Such a process implies, of course, observation of many cycles.
With a good set of spectra in hand, it should be possible to study in detail the behavior of the emission lines and the continuum as a 13
function of phase for each of the relevant periods, and hence draw
conclusions about the locale of emission at each period. The particular form, phasing and amplitude of plots of radial velocity,
line flux and FWHM will yield hints as to the locales of emission
responsible for each periodicity. It is important to try to establish
the relative phasing between line parameters whenever possible, as
phase shifts imply different origins for different lines. In
particular, the phase shift between spin modulated line flux and
radial velocity would serve as a constraint on the models discussed
above.
One must also pay careful attention to the specific shapes of line
profiles. It is certain to be the case in the intermediate polars that
line profiles are composites of contributions from several different
sources. A modulation at a particular period, for example, will
likely be a modulation of a line component, rather than of the whole
line. Isolation of these components will yield more information about
the source of the periodicity than can be obtained from a study of the
behavior of the whole line.
This study concerns itself with three intermediate polars: FO
Aquarii, V1223 Sagittarii, and AO Piscium. The choice of these three
was the result of several constraints: availability of telescope time,
accessiblity of the objects from a northern hemisphere observing site,
and the desire to obtain comprehensive coverage of all relevant
periods were among the most important. The next chapter deals with 14 the methods of observation, data reduction and analysis used to study these objects. Chapters 3, 4 and 5 each constitute a self-contained study of, respectively, FO Aqr, V1223 Sgr and AO Psc. The final chapter is a summation of the results of this thesis.
/ CHAPTER II
OBSERVATIONS, DATA REDUCTION AND ANALYSIS
OBSERVATIONS
Instrumentation
All data used in this thesis are based on observations made with the
1.8m telescope of The Ohio State and Ohio Wesleyan Universities. The telescope is located on Anderson Mesa, near Flagstaff, Arizona, at an altitude of ~7000 feet. It is operated jointly with the Lowell
Observatory. The data in this study consist entirely of spectra obtained with the Ohio State Intensified Image Dissector Scanner (IDS)
Spectrograph attached to the 1.8 m telescope. A description of this instrument follows; additional information may be found in Byard et al. (1981).
The IDS spectrograph is of Cassegrain design with 10 cm optics.
Light from the object enters the spectrograph through one of two apertures ("East" and "West"), is collimated, and falls upon a grating. A variety of filters, located on a wheel behind the apertures, may be used. The beam emerging from the diffraction grating is imaged by a Schmidt-Cassegrain camera onto the photocathode of a set of three stacked image tubes. The tubes are surrounded by a
15 16 methanol refrigeration unit. A voltage of 24 kV across the image
tubes has a multiplying effect, resulting in the formation of an
enhanced image of the two spectra (one from each aperture) on the
phosphor of the last tube in the chain. The light from this phosphor
falls upon a photocathode at the entrance to the image dissector. The
electrons emerging from the photocathode are scanned across a small
entrance aperture by a magnetic field due to coils which surround the
image dissector. Electrons which pass through the aperture are
multiplied by a dynode chain, and the signal read by IDS counting
circuits. Each spectrum spans 2048 pixels (or channels).
The IDS effectively uses the finite decay time of the image tube
phosphor as a temporary storage device. The scan time across the
photocathode is set to be less than the typical decay time, so that
there is a strong likelihood that each photon "hit" is counted. But
this method of operation also means that photons are likely to be
counted more than once, so that the IDS is not a photon-counting
detector. As a consequence, Poisson error analysis cannot, in
general, be applied to IDS output.
Observing Program
Table 1 gives a journal of the observations used in this study. The
total number of spectra obtained was 1806. Observations of FO Aqr
were made by R.H. Kaitchuck; those of V1223 Sgr and AO Psc were made
by P.J. Martell. All spectra were obtained using the IDS in a beam-
switching mode. In this mode, integrations on the object are 17
Table 1
Journal of Observations
Obj ect Date (UT) Number of Spectra Integration Time per spectrum (seconds)
FO Aqr 4 Sep 1986 74 2 0 0 5 Sep 1986 64 2 0 0 6 Sep 1986 1 0 0 2 0 0 7 Sep 1986 18 2 0 0
V1223 Sgr 4 Jul 1988 14 1 2 0 4 Jul 1988 54 60 5 Jul 1988 70 60 13 Jul 1988 104 60 24 Jul 1988 26 60 1 Sep 1988 2 0 60 5 Sep 1988 80 60 6 Sep 1988 80 60 7 Sep 1988 81 60
AO Psc 19 Aug 1988 93 60 23 Aug 1988 106 60 1 Sep 1988 2 0 60 2 Sep 1988 148 60 3 Sep 1988 . 137 60 4 Sep 1988 145 60 5 Sep 1988 86 60 6 Sep 1988 152 60 7 Sep 1988 134 60 18 alternated between west and east apertures. The empty aperture is thus simultaneously taking a sky background spectrum. The usual practice has been to move the object to the other aperture after each integration, and integrate again. This was, in fact the mode in which data were taken for FO Aqr. However, it was found that this method, when applied to the shorter integration times required for good resolution of the shorter periodicities in V1223 Sgr and AO Psc, resulted in far too much "dead time"; i.e., time spent between integrations. An alternative method, in which an object resides in a single aperture for several integrations, before switching of apertures, was therefore employed for the latter two obj ects. A computer program which carried out this alternative mode of operation was designed and implemented by the author of this study.
In all cases, an 1800 line mm“^ grating was employed, centered at approximately 4650 A. This results in a bandpass of -800 A. Also, for all observations, 7.3" diameter circular apertures were used. At the beginning (or sometimes at the end) of a night, it is necessary to take several spectra ("flat field" spectra) of a quartz-halogen continuum source. The purpose here is to remove the pixel-to-pixel variations found in the IDS. In order that the flat field spectrum not contribute significantly to the signal-to-noise in the final spectrum, long integrations are necessary. For the F0 Aqr data, flat- field integration times of one hour were used. For most of the remainder of the data, flat fields were obtained over a span of two hours. 19
At intervals, it is necessary to take spectra of He and Fe-Ne lamps for purposes of wavelength calibration. A good wavelength calibration is required here as part of the object of this study is an examination of the radial velocity modulations found in these systems. The wavelength calibration is typically performed once every 30-40 minutes. The Fe-Ne lamp was usually observed through a yellow filter, and the He lamp through the ND1. Integration times were typically 200 seconds. However, toward the end of the 1988 run, the He lamp began to fade, finally burning out. During the interval of fading, the He lamp was often observed without a filter. The absence of the He lamp in the latter stages of the 1988 rtin resulted in a reduction of the number of lines available for wavelength calibration in the bandpass from 16 to 1 0 .
Finally, in order to establish a flux calibration for the observations, it is necessary to observe flux standard stars in the course of an evening. For most of the nights shown here, two flux standards were observed, usually near the beginning and end of the evening, although on occasion three standards were observed. The standards were selected from among the list of Stone (Stone 1974;
1977) and the Oke white dwarfs.
DATA REDUCTION
The data reduction for this thesis was carried-out within the IRS 20
(Interactive Reduction System) environment, first on the Astronomy
Department PDP 11/73 (FO Aqr) , later on the VAX system. A raw IDS data file consists of values of counts for each channel of a spectrum from each aperture. The spectra from a single night are first divided into "east" and "west" files according to the aperture of origin of the spectrum in question. At this time, the data are also converted into IRS format. For the remainder of the reduction, through flux calibration, east and west files are kept separate as the IDS east and west apertures are characterized by different response curves.
A wavelength calibration is then determined for each aperture from the spectra of the Fe-Ne and He lamps taken in the course of the night. The usual practice is to combine Fe-Ne and He spectra which bracket a subset of object spectra. The calibration for the bracketed spectra is then calculated using the IRS command WAVFIT, which permits mapping of wavelength onto IDS pixels with a polynomial of order as high as seven. In this study, polynomials of order five or six were usually used. This process results in the accumulation of several calibration files (or dispersion curves) spread throughout a night.
The next step is to subtract a suitable sky spectrum from the object spectrum. For the FO Aqr data, this spectrum originates in either the following integration (when the object is in the west aperture), or in the preceeding integration (when the object is in the east aperture). That is, after an integration on the object in the west aperture, it is moved to the east, so that the following 21 integration finds the west aperture vacant, i.e., taking a west sky spectrum. The object spectrum is then divided by the integration time, which results in the storage of a counts/second value in each channel.
Each object spectrum is now divided by a flat field spectrum. The purpose here is, as described above, to remove channel-to-channel variations. The flat field spectrum used in this procedure is the sum of the continuum lamp spectra taken on the night of the observations.
The wavelength dispersion solution is applied next, using the IRS command APPLDC.
At this point, an extinction correction is applied to each object spectrum using the IRS command EXTINC. Airmass information is stored
in each IDS file when the observation is made. The extinction curve used for the correction is a mean extinction determined at Lowell
Observatory by Tug, White and Lockwood (1977). There is reason to believe, however, that, in some cases, the mean extinction curve was not a good representation of the actual extinction. This matter is
dicussed further in succeeding chapters.
In the next step, the original polynomial dispersion curve is
replaced by a linear solution, using the IRS REBIN command. It was
the usual practice here to choose a solution having 2048 channels with
a channel width of 0.4 A. In some cases (FO Aqr in particular) the
spectra were then lightly smoothed, using the IRS SMOOTH command. 22
Finally, the spectra are flux calibrated. In the case of the FO
Aqr spectra, the IRS FLUX command was used to determine a response function for each aperture by comparing actual counts/sec/channel recorded for standard stars with catalogued fluxes for those stars.
Up to this point, the standard star spectra have been reduced in a manner identical to that applied to the program objects. The last step is to apply the response map to the program objects with IRS
APFLUX command, resulting in data having units erg s 1 cm 9 A t 1 . The spectra are now ready for the application of various analysis procedures.
Much of the reduction procedure described above has been condensed into reduction programs (IRS "Macros") provided by R.H. Kaitchuck. In the case of V1223 Sgr and AO Psc, the spectra were treated slightly differently. Here the object typically resided in one aperture for five one-minute integrations, before beam switching. The sky spectrum to be subtracted from each of these five object spectra was obtained by averaging the next (for west files) or preceding (for east files) five integrations in the same (now vacant) aperture. In addition, the spectra of V1223 Sgr and AO Psc were flux calibrated by applying the
QFLUX command after having established a quantum efficiency map of each of the apertures with the QEFF command. Numerous applications of
FLUX and QEFF to data sets showed that there is no significant difference between the two methods of flux calibration. New reduction programs were written by the author of this work in order to effect
these changes in the reduction routine. 23
METHODS OF ANALYSIS
Phase Binning
The spectra are typically coadded into phase bins in order to study the phase dependent behavior of the systems treated in this study.
Short integration times are used specifically in order to time-resolve the high frequency modulations in these systems. The coadded spectra have greatly enhanced signal-to-noise, but the opportunity to detect non-periodic events is lost. Indeed, coaddition of spectra taken over many nights is based on the assumption that a system is reasonably stable.
Magnitudes
All magnitudes calculated in this study are approximate blue magnitudes calculated from the following formula:
m = -2.51og(f) - 13.53, (3) where f is the integrated flux over the whole spectrum. In most cases, however, the major emission lines have been omitted from the magnitude calculation, so that these are effectively continuum magnitudes. The formula is due to R.H. Kaitchuck. Errors for magnitudes derived from binned spectra are errors in the mean, and are typically ±0.03.
Emission Line Profile Measurement
In course of this study, several methods have been used to extract 24
information from the emission lines. As a quick method of obtaining line fluxes, the author of this work has written a program which obtains line fluxes by integrating over the line and subtracting a continuum fit. No assumptions about the line profile shape are made.
An alternative to this is a rather more sophisticated method which
fits a Gaussian function to the line profile. The program which performs this calculation is due to R.H. Kaitchuck. The results of
Gauss-fitting a line are location of line centroid, integrated flux
and FWHM. The errors in each of these quantities is derived from a x minimization routine which calculates the error per data point from O the input data. The x search and error calculation programs are
largely due to Chandler (1965). In a few cases, a method of finding a
line center due to Schneider and Young (1980), and further elaborated by Shafter (1983), and by Horne, Wade and Szkody (1986), has been
employed. This method defines two Gaussians a specified distance
either side of line center. The Gaussian-weighted flux is measured in
each, and when these weighted fluxes are equal, the line center is
found. This method is especially useful in determining radial velocities based on the motion of line wings, and in this work is
referred to as the "RV" method.
Gray-Scale Images
The gray-scale imaging technique is largely due to the work of R.K.
Honeycutt, R.H. Kaitchuck and E.M. Schlegel (see, for example, 25
Honeycutt, Kaitchuck and Schlegel, 1987), and is very useful in determining the behavior of emission lines as a function of the phase of any system periodicity. The method is very similar to creation of a
"trailed spectrum", except that all the data at hand are utilized to display phase-binned spectra in phase order. Each spectrum in an image has had the continnum subtracted out. Each image displays 16 levels of gray, with the maximum flux level set at black. CHAPTER III
FO AQUARII
INTRODUCTION
Discovered as an x-ray source (H2215-086) by Marshall et al. (1979),
FO Aqr was identified as a cataclysmic variable by Patterson and
Steiner (1983), who found photometric modulations at 20.9 minutes and
4.03 hr. The latter period was identified with the orbit modulation.
A controversy subsequently arose over the value of the orbital period.
Shafter and Targan (1982) found an orbital modulation in Balmer line radial velocity, but could not distinguish between orbital periods of
4.025 hr and 4.85 hr, although they favored the latter. The difficulty arises because both of these candidate periods are commensurate with 24 hr. Shafter and Macry (1987), using both photometric and spectroscopic data, arrived at an orbit period of
4.032 hr. Investigators subsequently assumed this to be the correct orbital period until Osborne and Mukai (1989) found the true period to be 4.85 hr, a conclusion supported by Patterson (1988).
The 21 minute photometric modulation and numerous upper and lower sidebands have been detected in a variety of circumstances (Shafter and Targan 1982; Patterson and Steiner 1983; Sherrington, Jameson and
Bailey 1984; Berriman et al. 1986; Shafter and Macry 1987; Steiman-
26 27
Cameron, Imamura and Steiman-Cameron 1989; Osborne and Mukai 1989;
Chiapetti et al. 1989). Patterson and Steiner (1983) favored a model in which the 2 1 minute signal originates in axisymmetric reprocessing of the x-ray beam. They also found the first lower sideband in their data, and associated it with the reprocessing of x-rays at a site which is fixed in the rotating frame. Pakull (referenced in Patterson and Steiner 1983) found an additional photometric modulation with period near the first upper sideband, as did Semeniuk and Kaluzny
(1988) and Chiapetti et al. (1989). The latter authors have in addition found numerous upper and lower sidebands in their photometry.
Taking advantage of a long baseline of observation, Steiman-Cameron et al. (1989) and Osborne and Mukai (1989) have recently arrived at improved ephemerides for the 2 1 minute modulation.
The expected connection between the pulsed optical emission and the x-ray emission was made by Cook et al. (1984). Using the EXOSAT
ME detector in the range 2-10 keV, they found an x-ray modulation having a period close to 21 minutes. Subsequent EXOSAT observations by Chiapetti et al. (1988, 1989) have confirmed the presence of the 21 minute modulation and revealed the first two upper sidebands in the x-rays. We note that the association of the x-ray modulation with the
2 1 minute period has led most investigators to the conclusion that 2 1 minutes is, indeed, the white dwarf rotation period.
Optical spectra of FO Aqr (e.g., Shafter and Targan 1982; Patterson and Steiner 1983) exhibit the strong Balmer emission lines common to 28 cataclysmic variable stars, in addition to high excitation lines
(e.g., He II A4686) characteristic of magnetic systems. The emission lines are not especially broad (-1000 km s"^ FWZI, Shafter and Targan
1982), nor is there a discernible double-peaked structure. No conspicuous emission s-wave is found in the Balmer lines, although the
He II A4686 profile is clearly contaminated by an s-wave, and both H/3 and He I A4471 appear to exhibit absorption s-waves (Hellier, Mason and Cropper 1990). The latter authors propose a model in which the absorption s-waves originate in material projected out of the system plane in the vicinity of the hot spot. Hellier, Mason and Cropper
(1989) assert that their data show the signature of a classical disk
"rotational disturbance", indicative of a disk eclipse. If this is the case, then FO Aqr is a high inclination system.
The emission lines exhibit modulations in radial velocity, V/R ratio and equivalent width on the 2 1 minute period and at the system sidebands as well (Penning 1985; Chiapetti et al. 1989; Hellier, Mason and Cropper 1990). A variety of explanations for the line behavior have been suggested. Penning (1985) proposed a disk reprocessing model in which the 2 1 minute velocity variations are due to illumination of axisymmetric parts of the disk, resulting in emission from gas moving at the local Keplerian velocity. Based on time- resolved spectroscopy, Chiapetti et al. argued that the line emission modulated at 21 minutes originates in the accretion stream. Hellier,
Mason and Cropper (1990) have asserted that a correlation between the phasing of the 21 minute continuum and He II A4686 V/R (in the sense 29
that maximum V/R occurs near continuum maximum) provides support for the notion that the spin-modulated emission originates in an
"accretion curtain" located near the magnetic poles of the white dwarf.
OBSERVATIONS
The observations of FO Aqr were obtained on 4-7 September 1986 (UT).
A total of 256 spectra were obtained with coverage in excess of one orbit cycle on two of the four nights. The integration times were 200 s for all observations. The sky quality was excellent and spectrophotometric standard stars were observed in order to flux calibrate the observations.
For most of the analysis, the spectra were coadded into 25 phase bins on the 4.85 hr orbital modulation, using the ephemeris of Osborne and Mukai (1989), and 10 bins on the 21 minute modulation, using the ephemeris of Sherrington, Jameson, and Bailey (1984). In the former case, a phase shift of 0.040 has been added in order to force minimum continuum light to correspond to phase zero. In the latter case, a phase shift of 0.600 has been added, in order to bring maximum light to phase zero. These are the phasing conventions adopted in all previous studies, and they are used here in order to avoid confusion.
The decision to use an older spin period instead of one of the more recently determined values (e.g., Steiman-Cameron, Imamura and
Steiman-Cameron 1989; Osborne and Mukai 1989) results in an error, at 30
CNI I
O
rto CP CM i i 4200 4400 4600 4800 5000 Wavelength (A) Figure 2. FO Aqr. Composite spectrum formed by averaging all spectra. J HORIZ RESOLUTION = 1 VERT RESOLUTION = 8 « SPECTRA = 2 5 H WRAPS = 12 FIRST PIXEL BLACK LEVEL = 700 WHITE LEUEL = 450 ROLL OVER PIXELS? = N ONE MAX F O A Q R / A .S—HOUR ORBITAL PERIOD - 4 3 0 0 -4 -4 0 0 -4500 4600 A 700 4800 4900 5000 I I ' ir ..t e t Figure 3. FO Aqr. Gray-scale image on the 4.8 hour phasing. HORIZ RESOLUTION = 1 VERT RESOLUTION * 16 H SPECTRA = 1 0 H WRAPS = 5 FIRST PIXEL = 1 BLACK LEVEL = 750 WHITE LEVEL = 450 ROLL OVER PIXELS? = N ONE MAX FO AQR. 21—MINUTE PERIOD Figure 4. FO Aqr. Gray-scale image on the 21 minute phasing. 10fO 33 worst, of 0.001 cycle over the span of our observations. For each of the beat periods, an ephemeris has been defined which takes the epoch to be that of Sherrington, Jameson and Bailey (1984) for the 21 minute modulation. In the remainder of this work the frequency of the 21 minute modulation will be designated as w, although it will be argued that the white dwarf spin period is actually 19.5 minutes. The orbital frequency will be designated as 0 . SPECTRA Figure 2 shows the grand average of all the spectra. In a manner typical of a magnetic binary, He II A4686 is comparable in strength to the H/3 and Hy lines. Weaker lines of He I A4388, 4471, 4713, 4922, 5016, He II A4542, and the CIII/NIII blend at A4640 are also present. Figures 3 and 4 show gray-scale images of the spectral line behavior on the 4.85 hr and the 21 minute periods, respectively (see Honeycutt, Kaitchuck and Schlegel 1987 for a description of this imaging technique). The appearance of He II A4686 in Figure 3 is similar to the s-wave seen in many other cataclysmic binaries (see Honeycutt, Kaitchuck and Schlegel 1987 for examples). It is worthwhile noting that the s-wave almost completely disappears near orbit phase 0 . 0 over an interval spanning approximately 0.2 in phase. There appears to be a second, but shallower, weakening of the s-wave near phase 0.7, after which the s-wave fails to regain its former strength. The H/3 line appears to have a complex structure which is difficult to characterize. At times it appears double peaked, suggesting the Approx. B Magnitude 13.8 13.6 Figure 5. F0 Aqr. Continuum light curve on the 4.8the houron phasing. curve Continuum light F0 Aqr. 5. Figure 05 1.5 1 0.5 0 ■ ■ ■ 8—or Phase —Hour .8 4 ■ ■ ■ ■ ■ ■ ■■ 34 Approx. B Magnitude 13.7 13.6 13.5 Figure 6. FO Aqr. Continuum light curve on the 21the FO Continuumlightminute curveAqr. on 6. Figure 05 1.5 1 0.5 0 phasing. 1—iue Phase —Minute21 35 36 I £ o CO a) in K) T o x Li_ a cn 0 -+-j c 0 0.5 1.5 4.8—Hour Phase Figure 7. FO Aqr. flux on the 4.8 hour phasing. Radial Velocity (km s 1) o o o o o Figure 8. FO Aqr. H6 theradial4.8 on phasing. FO H6 hour velocity Aqr. Figure8. 0 0.5 8Hour Phase r u o .8-H 4 37 1.5 38 presence of a disk. The appearance of the He I lines at A4922 and 5016 in Figure 3 is less simply characterized. The lines appear double peaked through part of the orbit cycle, but this could be the result of the interplay of two (or more) components having different .pn33radial velocity phasings. CONTINUUM Because the spectra are flux calibrated it is possible to produce continuum light curves by summing the flux across the bandpass while excluding those channels that contain strong emission lines. Figure 5 depicts the resulting continuum light curve phased on the orbital period while Figure 6 shows the light curve of the 21 minute cycle. The orbital continuum light curve appears to exhibit some asymmetry about phase 0 .0 , with the rise to maximum light slightly steeper than the decline to minimum. The declining branch of the curve also appears to be distinctly noisier than the ascending branch. The 21 minute light curve is found to be nearly sinusoidal. HYDROGEN LINES Due to the response of the IDS system the signal-to-noise ratio at H7 is significantly lower than at H/9. For this reason only the behavior of H/9 will be discussed. However, to within the uncertainties of the data, the results for H7 are completely consistent with H/9. Figures 7 and 8 depict the H/8 integrated line 39 O Li_ CM 0 0.5 1 1.5 21—Minute Phase Figure 9. F0 Aqr. Ep flux on the 21 minute phasing. Radial Velocity (km s o m O Figure 10. FO Aqr. Hp radial velocity on the 21theradial on velocity Hp minute FO Aqr. 10.Figure 0 phasing. 21—Minute Phase 0.5 1 40 1.5 FWHM (km s ) 1000 1100 Figure 11. F0 Aqr. FWHM on the 21the onphasing. minute FWHM F0 Aqr. 11.Figure 0 2 1 —M i n u t e P h a s e 0.5 1 41 1.5 Integrated Flux (10 13 erg s 1 cm 2) CN flux and radial velocity as a function of the orbital period as determined by a single Gaussian fit to the line profiles. A cursory inspection of the flux plot gives the impression that there is a phase shift between the H/9 flux and the continuum flux (Figure 5). However, the two modulations match nearly perfectly, except in the phase interval -0.65-0.90. The decline in the H/9 flux seems to have preceded the decline in continuum flux. The radial velocity curve is highly distorted, most likely due to the presence of multiple line components, as suggested by the gray-scale image. Note that this radial velocity curve becomes most "disturbed" in the interval of H/9 minimum. Figure 9 shows the flux modulation of the H/9 line on the 21 minute period. This modulation is strong, and similar in shape to the 21 minute continuum light curve. Figure 10 shows the H/9 radial velocity curve on the 21 minute cycle. Whether this curve represents a radial velocity modulation is unclear. Furthermore, a period search of the radial velocity measures of the individual spectra failed to detect any periodicty at 21 minutes. However, as Figure 11 shows, the H/9 line width clearly changes and is in phase with the flux variations. It should be borne in mind that these plots, and those in the next section, are based on a fit of a single function to the entire line profile. This will mask the behavior of individual components that might make up the profile. This issue will be addressed later. 48 HE II A4686 Inasmuch as the He II A4542 line is very weak, the results presented here for He II are limited to A4686. Figure 12 depicts the flux behavior on the orbital cycle. The line is clearly modulated, but in a more complex manner than Hy9. Figure 13 shows the radial velocitycurve, which has an amplitude of 151 ±7 km s~^. There is a deviation from sine-like behavior in the phase interval of -0.9 to 0.2. A similar feature appears in plots of the V/R ratio and was used by Hellier, Mason and Cropper (1989) as evidence for the "rotational disturbance" characteristic of the eclipse of an accretion disk. It will be shown below that this is not the case. Figures 14, 15 and 16 show the line flux, radial velocity and FWHM as a function of the 21 minute period. The flux variation leads that of Hj8 by - 0.1 in phase. There is a low amplitude radial velocity modulation which is essentially anti-phased with that of the flux. The most unusual behavior, however, is that of the line width seen in Figure 16. Between phases 0.0 and 0.3 the line width grows from 1000 to 1150 km s"^-. After phase 0.3 the width abruptly drops to -900 km -1 1 s by phase 0.5 and then recovers to -1000 km s by phase 0.6. An explanation for this behavior will be presented later. PERIOD SEARCH The continuum magnitudes, line fluxes and radial velocities determined from the individual spectra were subjected to period searches using 49 Table 2 False Alarm Probabilities for Detected Frequencies for FO Aqr Source w-20 w-Q to co+fl 0 (24.4 min) (22.5 min) (20.9 min) (19.5 min) (4.8 hr) Continuum 0 . 0 0 0 0 . 0 0 0 H/9 Flux 0 .0 0 0 a 0 . 0 0 0 Radial Vel. 0 . 1 2 0 0 . 0 0 0 He II A4686 Flux 0 .0 0 2 b 0 .0 0 0 a 0 . 0 0 0 Radial Vel 0.130b 0 .0 0 1 a 0.446b 0 . 0 0 0 a A sinusoid at 0 was subtracted prior to the calculation of the false alarm probability. Sinusoids at u> and 0 were subtracted prior to the calculation of of the false alarm probability. Power CM O Figure 17. FO Aqr. Power spectrum of the continuum magnitude.continuum the spectrum of Power FO Aqr. 17. Figure 0 40 0 600 500 400 300 F r e q u e n c(r y a d / d a y ) 50 o in 400 600 400 600 Frequency (rad/day) Frequency (rad/day) Figure 18. F0 Aqr. Power spectrm of (a) HjS flux, and (b) Hp radial velocity. Radial Velocity (km s o in O iue 9 F q. ailvlct nte 19.5the minute on radial velocity Aqr. F0 19.Figure 0 phasing. 0.5 19.5—Min Phase 52 Power (1986). In most cases the data have been pre-whitened by subtraction, in succession, of one or more sinusoids at known lower frequencies, prior to calculating the power spectrum. This was done in order to avoid confusion between alias peaks and real additional peaks in the periodograms. Table 2 summarizes the results and gives the approximate false alarm probability for each period. Only those detections represented by strong peaks and low false alarm probabilities are mentioned here. The power spectrum for the continuum magnitudes is displayed in Figure 17 and shows prominent peaks at the 21 minute period and its 1 day aliases. Although not present in the periodogram, a well-defined but low-amplitude (0.05 mag) modulation in magnitude is found when the data are folded on the 22.5 minute period. This modulation rather resembles a sawtooth, which may explain its absence from the periodogram. Figure 18 depicts the power spectrum of the H/9 flux and radial velocity. The flux reveals a period at 21 minutes, while the radial velocity shows a peak corresponding to 19.5 minutes (w+O). Figure 19 shows a plot of the radial velocity phased on the 19.5 minute period. While the amplitude is low, the variation is remarkably well defined. The power spectra for the line flux and radial velocity of He II A4686 are shown in Figure 20. The 19.5 minute period (w+0) is also detected in this line, admittedly at rather high false alarm Velocity Difference (km s ) I 200 -100 0 100 Figure 22. FO Aqr. He II >4686 radial velocity on the4.8on IIhour radial velocity He >4686 FO Aqr. Figure22. 0 phasing with a sinusoidsubtracted.fit a with phasing 0.5 4.8—Hour Phase 1 56 1.5 57 probability. Nonetheless, a plot of radial velocity phased on this period (Figure 21) appears to show a modulation which is remarkably similar to the 19.5 minute H/3 radial velocity modulation. The power spectrum of He II A4686 also reveals radial velocity detections at 21 minutes and 24.4 minutes (w-20). The He II line flux shows significant peaks at 21 minutes and 22.5 minutes (w-0) . We note that while detection of these beat frequencies in the lines have been reported elsewhere (Chiapetti et al. 1989; Hellier, Mason and Cropper 1990) under a variety of circumstances, this is the first work in which they have been reported to be unambiguously present in line fluxes. ANALYSIS The "Rotational Disturbance” and the System Inclination Hellier, Mason and Cropper (1989) have claimed the detection of an eclipse of an accretion disk in FO Aqr. This claim is based on an analysis of the He II A4686 line in which the profile was split in half about its rest wavelength. A plot of the ratio of the equivalent widths of the two halves (V/R) as a function of orbital phase was made. Between orbital phase -0.9 and -0.2 the ratio varies in a way expected from the eclipse of a rotating disk; i.e., the ratio first becomes small when the blueshifted side of the disk is occulted and then large when the redshifted side is occulted. However, their conclusion is incorrect. A "rotational disturbance" should Figure 23 (a). FO Aqr. He II ^4686 radial velocity at fluxradial II atvelocity He ^4686 FO Aqr. Figure (a).23 Radial Velocity (km s 1) O o CM o 0 maximum of the 21 minute cycle,21the4.8 on minute of maximum hour phasing. hour .-or Phase 4.8-Hour 1 58 Figure 23 (b). FO Aqr. He 11^4686 radial radial velocityflux at 11^4686 He Aqr. FO Figure (b).23 Radial Velocity (km s 1) O CM o o 0 minimum of the 21 minute cycle,onthe 21 oftheminute 4.8 minimum hour phasing. hour .-or Phase 4.8-Hour 1 t(b) 59 HORIZ RESOLUTION « 5 VERT RESOLUTION - 1 6 I* SPECTRA - 1 0 M WRAPS ■ FIRST PIXEL ■ 434 BLACK LEVEL - 7 0 0 WHITE LEVEL - 3 0 0 ROLL OVER P IX E L S ? - N ONE MAX FO AQR AT 21-M IN HE XX MAXIMUM US ORBITAL PHASE •4 <6 <6.0 <4720 4740 4620 <4<£><40 I a B I ■ .00 .20 . 4 0 . 6 0 . 8 0 .00 .20 . 4 0 HORIZ RESOLUTION = 3 VERT RESOLUTION = 1 6 # SPECTRA = 1 0 tt WRAPS = FIRST PIXEL = 434 BLACK LEVEL « 7 0 0 WHITE LEVEL - 5 0 0 ROLL OVER P IX E L S ? = N ONE MAX FO AQR AT HE II 21-M IN MINIMUM US ORBITAL PHASE 4620 <4 <£> -40 <4 — .20 — . 4 0 — . 6 0 H - . 8 0 F-.00 ~ ,2 B - — . 4 0 Figure 24. FO Aqr. Gray—scale image of He II ^4686 at maximum (top) and minimum (bottom) flux CT> of the 21 minute period, on the 4.8 hour phasing. o 61 really be studied in a radial velocity curve. In Figure 13 the signature of such an event at first appears to be apparent near phase 0.9. But there are immediately two problems. First, the event is phase-shifted 180° with respect to the expected location (i.e. at inferior conjunction of the white dwarf rather than superior conjunction). It might be argued that a phase shift is expected if the line is dominated by an s-wave. But if that were the case, the disk eclipse would be very difficult to see because the disk components would be relatively weak. Secondly, the nature of the "disturbance" itself is not consistent with that of a rotating disk. Figure 22 shows the result of subtracting a sinusoid fit to the data, using a least-squares minimization routine, in which the "disturbed" interval between phases 0.92 and 0.16 has been excluded from the fit. The classical rotational disturbance should show symmetric excursions to either side of rest velocity. Instead, the "disturbance" is almost entirely a blueward excursion. Finally, Figure 23 depicts two radial velocity curves constructed from two subsets of the data: one near flux maximum of the 2 1 minute He II A4686 cycle, and one near minimum. The former curve has been fitted with a sinusoid using a least squares minimization routine, with the two "low" points near phase 0.0 excluded from the fit. It is clear that the "rotational disturbance" is present only at flux maximum. Clearly, the existence of an accretion disk cannot depend on the phase of the 21 minute cycle. Figure 24 shows an enlarged gray scale image for these two data subsets. In particular, note the 62 change in the phase interval 0.0 to 0.2. At maximum light there is a blueshifted component (— 2 0 0 km s"^) that is absent at minimum light. It is this component that leads to the strange behavior of the radial velocity curve and the V/R ratios. It may be present at all orbital phases, but is evidently difficult to detect except at phase 0 . 0 to 0.2, where the s-wave component of the He II line appears to weaken considerably. This feature, which most likely originates in a wind, will be discussed further below. Thus, the most compelling argument for a high orbital inclination has been removed. It is of interest to compare the emission line profiles of FO Aqr with those of the eclipsing intermediate polar EX Hya. The hydrogen emission lines in EX Hya are clearly double peaked, as expected from an accretion disk viewed at high inclination. It is far from obvious that the hydrogen lines in FO Aqr are double peaked in this manner. The emission line widths are also much different: -2300 km s-^ (FWHM) in EX Hya and only -1000 km s"^ (FWHM) in FO Aqr. The latter width is much more compatible with a moderate inclination (30-60°). The Nature of the 21 Minute Modulation A fundamental question is the nature of the 21 minute emission line modulation. The manner in which the line is modulated can potentially reveal something about the emission line source and the nature of intermediate polars. What portion of the line profile changes overthe 4650 4700 4750 Wavelength (it) Figure 25. FO Aqr. He II A 4686 profiles at maximum (vector) and minimum (histogram) FWHM of the 21 minute cycle. — 1 —, t — i— r i— i— i— i— |— i— i— i— i— | r--i—, i |— r i— i— i— |— i— i---- 1— i— t — CM ol 1 1 \ E E O ■ ■ ■ O T— 1 ■ . 1 1 - (0 (0 CP cn L. ■ ■ L. “ a) ■ ■ a> in lO - ■ ■ — m in — _ T T O ■ ■ * O r— - i— * v—' ■ ■—' ■ X “ *’ X ZJ ■ " ■ ■ - ■ ■ ■ Ll_ Ll. ■ ■ ■ ■ " ■ ■ - 1 « i- i t 1 i i * i 1 i i • i - * 1 •--1 --1- 1---L 1 1-1 I I 1 1 - 0 0.5 1 1.5 0 0.5 1 1.5 21—Minute Phase 21—Minute Phase Figure 26. FO Aqr. Flux modulation of the He II 4686 profile on the 21 minute phasing "sliced" at -600 km/s (left), and at +600 km/s (right). -p-O' 65 course of the 21 minute cycle? Is it the entire line that changes, or is it some component of the line? It is difficult to characterize the change in the profile between the two images in Figure 24 other than to say that the left half of the profile seems to be affected more. An obvious diagnostic procedure is to difference the two images. However, there is a potential pitfall in this technique if there are any radial velocity variations on the 21 minute cycle. In that case there would be a systematic "misregistration" of the profiles during the subtraction process. This would make the results difficult to interpret. A different technique is needed to extract the modulated portion of the line profile. A comparison of the He II A4686 profiles at flux maximum and minimum of the 2 1 minute cycle shows that only the blue half of the profile changes significantly. This is also seen when profiles at the extremes of line width, phase 0.3 and 0.5 of the 21 minute cycle in Figure 16, are compared. Figure 25 shows that essentially all the change occurs in the blue wing of the profile. This behavior strongly suggests that the He II A4686 profile, as seen on the 21 minute cycle, consists of at least two components. The first is a strong, constant component made up of the orbit-averaged contribution from the s-wave and any accretion disk. The second component is in the blue wing of the first component and is modulated on the 21 minute cycle. In order to test this hypothesis, a program was written which measures the integrated flux contained in "slices" Integrated Flux (10 13 erg s 1 cm 2) o m Figure 27. FO Aqr. He 11^4686 residual wind componentflux on residual wind 11^4686 He FO Aqr. Figure27. 0 the21phasing. minute 21—Minute Phase 0.5 1 66 1.5 HORIZ RESOLUTION * 5 VERT RESOLUTION a 16 H SPECTRA * 1 0 ** WRAPS - 5 FIRST PIXEL - 434 BLACK LEVEL = 6 2 5 WHITE LEVEL » 5 0 0 ROLL OVER P IX E L S ? = N ONE MAX FO AQR; HEX I 46>S6> SAUSSIAN SUBTRACTED Figure 28. FO Aqr. Gray-scale image of the He II ^4686 residual wind component, on the 21 minute phasing. ON 68 one channel wide at selected radial velocities located symmetrically with respect to line center. In this case, the channel width is 0.8 A. The result for slices of the He II A4686 profiles at ±600 km s"^ is shown in Figure 26. The flux at -600 km s-^ shows a clear modulation, while that at +600 km s-^ does not. Furthermore, the fact that the minimum of the modulation at -600 km s"^ approximately coincides with the level of the scatter at +600 km s-^ shows that flux is added and subtracted on the blue side of the line, as has already been suggested. The standard deviation in the fluxes in Figure 26 is - ±0.35 x 10 IS erg s -1-9 cm . This uncertainty was determined by applying the slice program to several intervals in the continuum immediately blueward of He II A4686. Because of the extreme line width at 21 minute phase 0.3 seen in Figure 16, it would appear that the modulated component is predominantly seen in the extreme blue wing at this phase. Furthermore, this is near its minimum strength. So, this phase presents the best opportunity to deconvolve the two components. The red-most -60% of the profile at phase 0.3 was fit with a Gaussian function. This profile was then subtracted from the phase-binned spectra through the 21 minute cycle. This process appears to have rather cleanly removed the "static" component. Figure 27 shows the line flux of the residual profile as a function of 2 1 minute phase. The flux curve has an amplitude and phasing that are the same as that of the unconvolved profile in Figure 14. This is consistent with the as stamp t ion that the residual blue component is almost entirely 69 responsible for the modulation of the He II A4686 line. The gray-scale image of the residual component is shown in Figure 28. It can be seen that the residual component is itself comprised of two components of unequal strength and with a phase shift of -0 .1 . The stronger component rests at about -200 km s"^ and reaches maximum strength at about phase 0.0. The second component is located at about -500 km s"^ and reaches flux maximum at about phase 0.1. It's worth noting that the interplay between these two components can also explain the line width behavior seen in Figure 16. The peak at phase 0.3 in that figure is the result of the growing importance of the component at -500 km s -1 as the one at -200 km s 1 fades. Shortly thereafter both components fade, resulting in the line width minimum at phase 0.5. Note that Figure 16 is based on the convolved profiles and therefore serves as evidence for the reality of the double component structure of the residual profile. The full range of behavior of the He II A4686 line flux, width and radial velocity on the 2 1 minute cycle can be explained by the behavior of these two components in the blue wing. The H£ profile also shows a blue asymmetry, but all attempts to deconvolve the profile by the technique discussed above failed. Apparently, the blueward component is not as cleanly separated from the main component as is the case for He II A4686. In fact, the behavior on the 2 1 minute cycle of any components in the blue wing of H/J must be significantly different from those in He II A4686. This is 70 seen by comparing the FWHM plots of each line in Figures 11 and 16. The Orbital Continuum Modulation While the observations presented here do not supply a reliable radial velocity curve for the orbital motion of the white dwarf, it is still possible to infer the phase of stellar conjunction. If the side of the secondary star which faces the white dwarf is significantly heated by x-rays, the continuum light will be modulated on the orbital period with a minimum at superior conjunction of the white dwarf. This means that orbital phase 0 . 0 is approximately superior conjunction of the white dwarf. The asymmetry in the light curve may be interpreted as due to the presence of an additional source of continuum light which is more readily viewed in the interval between phases 0.0 and 0.5. The most likely candidate for this source is the hot spot, the x-ray heated face of which will be most directly observed in this interval. DISCUSSION It appears that almost all the modulation of the line flux, width and radial velocity on the 21 minute cycle of He II A4686 (Figures 14 - 16) can be accounted for by the interplay of the two residual components in the blue wing. Because of this, it is very important to understand the origin of these emission components. The lack of any obvious radial velocity variations on the 2 1 minute cycle makes it unlikely that they originate in gas that is entrained in the magnetic 71 field of the white dwarf, or in reprocessing in axisymmetric parts of the disk, as would almost certainly be the case if the system lies at high inclination. This, combined with their strong blueshifts, strongly suggest a wind origin. Furthermore, the fact that there are two components with different phasing and velocities suggests that there are two sites in the binary system for this wind. The phase shift between these two components is consistent with the expected angular separation of the secondary star and disk hot spot, as seen from the white dwarf. The implied picture is that of x-ray induced winds off the secondary star and the hot spot region. The He II A4686 emission is due to x-ray reprocessing at these wind sites, and is modulated by the sweeping of an x-ray "beam" from the rotating white dwarf. This "beam" is most likely not a highly collimated pattern, but rather one that has a broad distribution in azimuth around the white dwarf. Because both the x-rays and the emission lines are modulated on the 21 minute period, the observed x-rays must also be a product of reprocessing, and the true white dwarf spin period is 19.5 minutes. The 19.5 minute radial velocity variations uncovered in this study must be due to a portion of the gas entrained in the magnetic field and rotating with the white dwarf. The fact that neither H/3 nor He II A4686 exhibits a flux modulation at 19.5 minute implies that the emitting gas is optically thin in the lines and furthermore that the gas is not significantly occulted in the course of the 19.5 minute Figure 29. Schematic view of the wind reprocessing model. The presence of winds near the inner face of the secondary and near the hotspot is indicated by the dashed curves. In the model presented here, the winds are induced by irradiation of the secondary and hotspot by x-rays orginating near the magnetic pole of the white dwarf, which is located at the center of the accretion disk (right). 73 cycle. Support for the wind-reprocessing picture comes from the fact that both the orbital flux modulation of He II A4686 and the 21 minute modulation have very nearly the same amplitude. This strongly implies a common source for both modulations. This can be explained by reprocessing off a fairly optically thick wind, which will show a strong modulation with viewing angle. That is, it will appear bright when the observer views the escaping reprocessed radiation fairly directly (orbital phase ~0.5) and dim when the reprocessed radiation must escape through the full thickness of the wind sheet (orbital phase -0.0). The amplitude of the 21 minute cycle will be about the same as the orbital modulation because it is due to the presence and absence of "reflected" radiation from these same surfaces. However, the amplitude of the 2 1 minute modulation should be strongly dependent on the orbital phase, with the greatest amplitude near phase 0.5 and the smallest around 0.0. This can be studied by differencing the line flux at maximum and minimum light through the orbital period. The results show that the amplitude varies by a factor of - 2 over the orbit cycle, and is indeed largest near orbital phase 0.5. A schematic view of this wind reprocessing model may be found in Figure 29. In considering this reprocessing model, it is important to try to establish the primary reprocessing sites for other components of the light. The modulation of the continuum light appears to be in phase 74 with the modulation of the -200 Ion s"^ component of the He II A4686 line. This implies that the 21 minute continuum pulse is largely due to reprocessing at the secondary star. The phasing of the 21 minute x-ray modulation in the context of this two component reprocessing model is unclear. Cook, Watson and McHardy (1984) present an x-ray ephemeris which is compatible, within their uncertainty, with coincidence of the x-ray and continuum maxima. Chiapetti et al. (1989) draw the same conclusion. While these results, when combined with those presented here, imply an x-ray modulation originating in the secondary star wind, it appears, in considering the uncertainties quoted by these authors, and especially in examining Figure 7 in Chiapetti et al., that locating the 21 minute x-ray maximum somewhat later in phase than the continuum maximum cannot be ruled out. In this case, the x-ray pulse would originate at the hot spot region. For this reprocessing model to work it is necessary for the x-ray emission from the white dwarf to be largely confined to the orbital plane of a system which is not at unusually high inclination. Otherwise the dominant x-ray period would be 19.5 minutes rather than 21 minutes. As stated earlier, there is in fact no compelling evidence for a high inclination in this system. There is a suggestion of an occultation of the s-wave emitting region near orbital phase 0 . 0 (see Figure 12). However, the occulting body need not be the secondary star. The wind off the impact site or the secondary star, 75 being optically thick, can provide this occultation and do so at a fairly low inclination because the wind extends out of the orbital plane. Confinement of the x-rays from the white dwarf to the system plane implies large x-ray optical depth out of the plane. This could occur if the white dwarf magnetic axis lies at large colatitude, creating a barrier of magnetically entrained gas along a direction out of the plane. There is a well-known relationship in intermediate polars between the modulation amplitude and the x-ray energy (i.e., wavelength) (e.g., Norton and Watson 1989b), in the sense that modulation amplitude increases as x-ray energy decreases. The usual explanation for this behavior is that the modulation is due to variable photoelectric absorption by accreting gas carried in the white dwarf's magnetic field. The optical depth to the x-ray source near the white dwarf's magnetic pole appears to change as this material is carried in and out of the line of sight during the rotation cycle. FO Aqr also displays this amplitude/energy dependence in its x-ray modulation (Chiappetti et al. 1989; Norton and Watson 1989b). There are two ways in which the model presented here can produce the observed amplitude/energy dependence. Either (a) the wind sites act as fairly faithful mirrors, allowing us to view the amplitude/energy relationship produced by photoelectric absorption at the white dwarf, or (b) the reprocessing at the wind sites can produce a reflected spectrum which mimics the dependence. 76 The requirement in the -first case is that the spectrum of the x- rays leaving the wind sites does not have a radically different slope from the incident spectrum or the amplitude/energy relationship will be lost. This can be accomplished when a hard x-ray flux incident on a stellar atmosphere induces a wind (Basko, Sunyaev and Titarchuck 1974; Titarchuck 1987). This wind is strongly heated by the x-rays, resulting in a high degree of ionization. In the absence of this wind, the secondary gas reflects an incident bremsstrahlung spectrum with considerable loss of flux at low energies, a result of photoelectric absorption, which is most effective at low energies. However, the high degree of ionization in the hot wind reduces the effect of photoelectric absorption, and enhances the importance of Thomson scattering as an opacity source. As a result, the reprocessed spectrum at low energies is elevated. If the optical depth to scattering becomes large (r~l), the reflected spectrum retains much the same shape as the incident spectrum from low to high energies. So the wind sites act as crude mirrors, reflecting the behavior of the variable x-ray source at the white dwarf. In the second case, the reprocessing of x-rays alone provides an energy/amplitude relationship. One assumes that the source at the white dwarf is variable in flux but constant in spectral shape, and require that the cooling time scale in the wind be short compared to the 21 minute reprocessing period of the white dwarf. The work of Cominsky, London and Klein (1987) shows that this time scale for reprocessing in the atmosphere of the secondary star is on the order 77 of 10 9 seconds. The density in the wind is less than the atmosphere and probably cools even faster. At 21 minute x-ray maximum, the reflected low-energy spectrum will be elevated, as Thomson scattering dominates, while at x-ray minimum the presence of significant photoelectric absorption will depress the flux of the reflected spectrum at low energies in the sense that the lowest energies will be most deeply reduced. The result is the same as that for the standard picture. The only difference is that the site of the photoelectric absorption has been moved away from the white dwarf. Of course, it is quite possible that a combination of these two mechanisms is present. This reprocessing model provides a natural explanation for the absence of detected optical polarization in FO Aqr: The optical pulse is dominated by scattered light; any intrinsic polarization carried by light incident on the reprocessing site will be lost by virtue of the scattering process (Barrett and Chanmugam 1984). It is not entirely clear how to reconcile these conclusions with the observation of the a b s o r p t i o n s-wave in FO Aqr reported by Hellier, Mason, and Cropper (1990). There is only a suggestion of this feature in the He I A4471 line in the data presented here. This is somewhat surprising because a comparison of their Figure 1 with Figure 2 of this work shows that the two data sets are very comparable in quality. However, there is a hint that physical conditions in the system were different for the two data sets. Figure 1 of Hellier, Mason and Cropper clearly shows an 0 II A4416 feature. This line is 78 completely absent in this data set. Unfortunately, we cannot determine if other line strengths were also affected, because these authors have published only equivalent widths for the lines. It may not be a coincidence that the 7 velocity of the absorption s-wave matches that of the strongest wind emission component reported here. It Is possible that, because of a small change in the wind geometry, a cooler portion of the wind sheet is sometimes seen projected across the face of the disk, producing blue-shifted atomic absorption lines. CHAPTER IV V1223 SAGITTARII INTRODUCTION Like many another of its kind, V1223 Sgr was initially discovered as an x-ray source (4U 1849-31; Forman et al. 1978). A number of x-ray position determinations of increasing precision followed (Reid et al. 1980; McHardy et al. 1981). Steiner et al. (1981) finally associated the previously catalogued irregular variable V1223 Sgr (Kukarkin et al. 1973) with the x-ray source. Steiner et al. (1981) were the first to make photometric observations (in the Johnson UBV system) of V1223 Sgr. Power spectrum analysis of 4 nights of monitoring the star in the B-band revealed the presence of a modulation having period 13.2 minutes. No other periodicities were seen. They concluded that the modulation at 13.2 minutes represented the first lower sideband of the system, implying a shorter period for the white dwarf spin. However, in the absence of an orbital period, the value of the spin period could not be predicted. A search of Einstein and Ariel 5 archival date revealed no periodicites in the x-ray flux, although it must be admitted that the the available data sets were sparse. The issue of the x-ray period of this system was settled by Osborne et al. (1984), who found 80 a period of 12.4 minutes in EXOSAT ME and LE data. In accord with the conventional interpretation, 12.4 minutes is regarded as the white dwarf rotation period. More extensive photometric observations of this system have been conducted by numerous investigators (King and Williams 1983; Warner and Cropper, 1984; Watts et al. 1985; van Paradijs et al. 1985; van Amerongen, Augusteijn and van Paradijs 1987; Jablonski and Steiner 1987). A primary result of this work has been ever increasing precision in the determination of the ephemerides for the modulations in this system. Warner and Cropper (1984) were the first to detect a photometric orbit modulation. Applying Fourier analysis to their data, they arrived at an orbit period of 3.37 hours. In a subset of their data, they found, in addition, a modulation with period 14.2 minutes. In light of their conjecture that the white dwarf rotation period is 13.2 minutes, these authors identified 14.2 minutes as the period of the first lower sideband of this system. In view of the subsequently discovered x-ray period, this period is actually that of the second lower sideband; van Paradijs et al. (1985) and Jablonski and Steiner (1987) also report intermittent detection of the 14.2 minute modulation. So far as is known by the author of this work, the spin (12.4 minute) modulation has not been detected photometrically. Very little spectroscopy of V1223 Sgr has appeared in the literature. Watts et al. (1985) obtained 67 high resolution spectra in the wavelength range A3950-4900 over approximately one-half of the 81 orbit cycle. The spectra, when binned on the orbit phasing, exhibit highly variable emission line profiles over that part of the orbit cycle which is accessible. The Balmer lines and He II A4686 all show signs of variation in FWHM and equivalent width as a function of orbit phase. At the 13.2 minute period, Watts et al. find some modulation in Hy and H/? equivalent width. An unusual feature in their spectra is a broad absorption dip just blueward of H5, giving the impression of a P Cygni profile. The implied outflow velocity is ~850 km s"^. Penning (1985), in a survey of a subset of the intermediate polars, found significant orbital radial velocity modulation of the Balmer lines (Hy, H/?) and in He II A4686, in a set of spectra covering approximately 0.8 orbit cycle. It was found that the He II A4686 modulation lags that of the Balmer lines by 0.2 in the orbit phase, implying, in the context of the usual model for CVs, that the Balmer lines are dominated by disk emission, while the He II A4686 line probably originates in the hot spot. Penning also claims detection, using Fourier methods, of radial velocity modulation in the principal emission lines, at the white dwarf spin period of 12.4 minutes. V1223 Sgr appears to have undergone numerous and significant non periodic brightness changes on a variety of time scales. Beleserene (1981), and Garnavich and Szkody (1988), in searches of the Harvard plate collection, found several instances in which the magnitude dropped by 2-3 mag, and an extended (-12 year) period of irregular variations at a brightness level somewhat below normal, van Amerongen 82 and van Paradijs (1989) report a flaring event, in which the continuum flux and line fluxes reached levels 2-3 times normal, and remained elevated for an estimated period of 6-12 hours. Warner and Cropper (1984) found linear trends in some of their photometry, which have time scales of hours, in which the brightness typically varies by -0.15 mag. Watts et al. (1985) noticed much the same sort of phenomenon when the system brightened by -0.12 mag in the R band over a duration of one orbit cycle. Warner and Cropper (1984) and Watts et al. (1985) also found flaring activity, in which the star brightens typically by 20%, on time scales of tens of seconds. In addition, Warner and Cropper report the presence of significant high frequency flickering, a phenomenon not uncommon among CVs. Several investigators (e.g., Warner and Cropper 1984; Watts et al. 1985; van Amerongen, Augusteijn and van Paradijs 1987) have found the amplitude of both the 13.2 minute and orbit modulations to be quite variable. Indeed, in some data sets the orbit photometric modulation becomes indiscernable. OBSERVATIONS The data set for V1223 Sgr consists of 529 spectra obtained on 4, 5, 13 and 24 July 1988, and on 1, 5, 6 and 7 September 1988 (all times UT). For 14 of the spectra, the integration time was 120 seconds; the remaining spectra represent 60 second integrations. Much care has been exercised in an attempt to choose only the best spectra from this set in the analysis below. Spectra obtained on 24 July and on 1 83 September have been discarded, due to very poor transparency on those nights. In addition, selected spectra from several other nights have been removed, usually because of instrument failure, or other problems that resulted in anomalously low integrated fluxes. The final working set consists of 441 spectra. Despite these measures, these data are rendered noisier than the FO Aqr spectra by two effects. In the first place, there are numerous photometric instabilities intrinsic to this system which have been noted by other observers (see the Introduction). And secondly, V1223 Sgr lies at very low declination (~ -31°) with respect to the observing site at Flagstaff, requiring observations at very large airmass (typically 2.5 - 4.5). At these airmasses, image enlargement and motion create problems: it becomes increasingly difficult to hold the entire stellar image in the IDS aperture during integration. In addition, it is possible that the mean extinction determined for Flagstaff (Tug, White and Lockwood 1977) is inappropriate on many occasions at large airmass. Figure 30, which is a plot of magnitude vs time for the observation of 6 September, illustrates the airmass problem. The span in air mass is ~2.6-3.8. While the general linear trend in the data suggests a possible connection with a similar feature noted by Warner and Cropper (1984), or even a flare event (van Amerongen and van Paradijs 1989), it appears as though a poor match between real and mean extinction provides the most likely explanation. Attempts at relieving this problem by using the data (or flux Approx. B-Magnitude iue3. 12 g. Magnitude V1223 timeSgr.forvs Figure30. 0 duringthe night. illustratingthe monotonicdecreasein brightness 0.5 lpe Tm (Hours) TimeElapsed ■ ■ ■ 6 September 1988, 1.5 84 85 standards) to derive an extinction curve (using IRS TRUEXT) produced ambiguous results. In any case, this was a problem on only two nights. For purposes of much of the following discussion, the data set has been divided into a July set and a September set. This is due to the fact that there is a systematic difference of -0 . 2 mag in the flux levels of the two sets. We do not believe this difference arises from a flux calibration error; rather, it likely represents an intrinsic change in system brightness between the two observing runs. In phasing the spectra on the orbit (3.4 hour) cycle (Q) and on the 13.2 minute period (w-Q), the photometric ephemerides of Jablonski and Steiner (1987) have been used. In both of these ephemerides, maximum light occurs at phase 0.0. At the spin (12.4 minute) period (w), the period used in folding the data is one due to Osborne et al. (1985), and the epoch occurs on 5 September 1988. It might seem reasonable to use as the spin epoch that given for the x-ray modulation by Osborne et al. (1985), in order to determine the phasing of the x-rays relative to the optical light. In practice, this proved unworkable as there is considerable uncertainty in the published x-ray epoch. SPECTRA A composite spectrum of the July data is shown in Figure 31. The spectrum from September is similar in character. The Balmer lines (Hy, H/J) and He II A4686 are the most prominent features. In this object He II A4686 is particularly narrow (FWHM ~ 550 km s“^). Other 86 '/||| i 4200 4400 4600 4800 5000 Wavelength (A) Figure 31. V1223 Sgr. Composite spectrum formed by averaging all spectra taken in July 1988. 87 lines present include He I A4388, 4471, and 4922; and the CIII/NIII blend at A4640. It is unclear whether He I A5016 and He II A4542 are present. It should be noted that while the emission lines are not obviously double-peaked in the composite spectrum, the Balmer lines appear to be intermittently double-peaked through the orbit cycle (this is especially pronounced in H7 ). On the other hand, the He II A4686 line never shows an unambiguously double-peaked structure. Figure 32 depicts a gray-scale image of the July spectra binned on the orbital (3.37 hour) phasing. The portion of the last image centered on Hy9 is shown in Figure 33, which reveals a remarkably asymmetric line profile: The line is strongly modulated in radial velocity on the red side, while no obvious modulation appears on the blue side. The September spectra exhibit much the same asymmetry, although somewhat less distinctly. The He II A4686 profile (Figure 34), which is drawn from the July data, is not easy to characterize. The image appears to exhibit a low-amplitude radial velocity modulation which reaches maximum redward excursion near phase 0.90. Figure 35 shows a gray scale image of H/9 made on September 6 . The phasing is that of the 12.4 minute spin period. There appears to be some hint of radial velocity modulation in the line, especially near line center. HORIZ RESOLUTION = 1 UERT RESOLUTION = 8 tt SPECTRA = 2 5 # WRAPS = 12 FIRST PIXEL = T BLACK LEUEL = 600 WHITE LEUEL = 450 ROLL OUER PIXELS? = N ONE MAX U1223 SQR ORBIT PHASINS/JULY ONLY 22 20 4 0 Figure 32. V1223 Sgr. Gray-scale image of the July spectra, on the 3.4 hour phasing. 00 oo HORIZ RESOLUTION = 5 VERT RESOLUTION = 8 « SPECTRA = 2 5 # WRAPS = 12 FIRST PIXEL = 693 BLACK LEVEL = 6 5 0 WHITE LEVEL = 4 5 0 ROLL OVER P IX E L S ? = N ONE P1AX V1223 S6R ORBIT PHASIN6/JULY ONLY 4800 4820 -48-40 48-60 4380 4900 4920 Figure 33. V1223 Sgr. Gray-scale image of the July 1988 spectra, in the neighborhood of IjiJ, on the 3.4 hour phasing. co VO HORIZ RESOLUTION = 5 VERT RESOLUTION = 8 # SPECTRA = 2 5 tt WRAPS = 12 FIRST PIXEL = 474 BLACK LEVEL = 7 0 0 WHITE LEVEL * 450 ROLL OVER P IX E L S ? = N ONE MAX Figure 34. V1223 Sgr. Gray-scale image of the July 1988 spectra, in the neighborhood of He II X4686, on the 3.4 hour phasing. VOo HORIZ RESOLUTION i VERT RESOLUTION = 1 6 » SPECTRA 10 8 WRAPS FIRST PIXEL = 693 BLACK LEVEL = 8 0 0 WHITE LEVEL * 3 00 ROLL OVER PIX ELS? ** N ONE MAX V 1 2 2 3 S G R 746-SEC PHASIN0/6-SEP-88 4800 4820 4840 4 8 6 0 4880 •4900 4920 ■ 1 ■ * I C — .00 — .20 — . 4 0 — . 6 0 P —. 8 0 P-. 00 — .20 — . 4 0 Figure 35. V1223 Sgr. Gray-scale image of the 6 September 1988 spectra, in the neighborhood of , on the 12.4 minute phasing. Approx. B—Magnitude 13.2 Figure 36. VI223 Sgr. Continuum light ContinuumcurvesforandJuly Sgr. VI223 36.Figure 1 I 1■1 . 1 1.5 1 0.5 0 □ ■ 1 □ ■ —i etme 19883.4Septembertheon phasing.hour ■ ---- □ □ 1 ■ ---- □ □ □ □ □ 1 ■ --- Coe: September Closed: ■ 1 .—or Phase 3.4—Hour --- □ 1 ■ --- □ 1 ■ --- ■ 1 □ --- □ July Open: ■ r --- □ 1 i — i i i i | i —I i — i ■ ■ ■ ■ 92 < I i - .. — 93 CONTINUUM Figure 36 depicts the continuum magnitude as a function of orbital phase for both July and September. The previously mentioned difference in flux between the two sets is apparent. It is also apparent that this modulation is less well defined in the September data than in the July data. In both cases there appears to be a peak near phase 0 .0 , defined by the ephemeris as the phase of maximum light. In both cases, the full amplitude (-0.15-0.20 mag) is comparable with results obtained elsewhere (e.g., Jablonski and Steiner 1987). The attentive reader will notice that there is no plotted point at phase 0.2 for July. In this case, the mean spectrum was found to have approximately one-half the signal-to-noise of the other phase-averaged spectra. It was found that the spectra composing this phase were biased towards observations at very large airmass, and so have been eliminated from consideration in constructing the plot. Figure 37 shows the 13.2 minute modulation for the September data. HYDROGEN LINES The H7 and H/J lines are expected to show modulations which reflect the various periods in the system. Here, as in the case with FO Aqr, only the H/? line is analyzed, as the IDS is less responsive blueward of H/3. In a search for flux modulation, the line was fitted with a Gauss fitting routine, and with a routine which simply determines the flux over a specified wavelength interval, without ass tuning any line profile shape. Both methods failed to find a persuasive flux Approx. B—Magnitude 13.1 ro Figure 37. V1223 Sgr. Continuum light curve for September 1988 ContinuumSeptemberfor lightcurve Sgr. V1223 37.Figure 05 1.5 1 0.5 0 nte 13.2theon minutephasing. 32Mnt Phase 13.2—Minute 94 Radial Velocity (km s ) 200 -100 0 100 Figure 38. V1223 Sgr. V1223 38. Figure 0 3.4hour phasing. 0.5 .—or Phase 3.4—Hour Ef the 1988on radialfor velocityJuly 95 1.5 96 modulation on the orbit cycle in any of the data. An orbit-modulated radial velocity variation was found, however, and is illustrated in Figure 38. It should be noted that the radial velocity modulation reaches maximum redshift near maximum light (Figure 36). An H/? radial velocity modulation at the spin period, which appears convincingly on only one night (6 September 1988), is depicted in Figure 39. In this case, the radial velocity was determined using the "RV" method, setting the input parameters such that the line core was measured. Attempts to find a modulation in the wings failed, but this could be due to distortion of the blue side of the profile of the kind seen in Figure 33, although this is not obvious in the gray scale image on this phasing (Figure 35). Or, it might simply be the case that the noise in the lines away from line center overwhelms the signal. HE II A4686 The He II A4686 line appears to be modulated only in radial velocity, and only at the spin period (Figure 40) and at the second lower sideband (Figure 41). In the former case the "RV" method was used to find the modulation in the line core. In the latter case, the "RV" method was again used, with parameters chosen to measure the wings of the line. Using a variety of search methods, this author was unable to produce plots which unambiguously exhibit modulations at any of the other known system frequencies. Radial Velocity (km s ) 200 0 200 Figure 39 0 ! _ -i 12 Sr H(3September6 1988forV1223 Sgr. radial velocity nte 12.4onthe minute phasing. ______----- I ______1 ----- I ______1 ----- 24Mnt Phase 12.4—Minute . 1 1.5 1 0.5 I ______1 ----- I ______1 ----- I ______1 ----- I ______1 ----- I ______1 ----- I ______[■ i 1 -i I ______I ______--- 97 I ______1 --- I ____ r Radial Velocity (km s Figure 40. V1223 Sgr. He 11^4686 radial velocity for 5 and 6 5and for radial velocity 11^4686 He Sgr. V1223 40.Figure 05 1.5 1 0.5 0 September 1988September 12.4onthephasing. minute 24 iue Phase Minute 12.4 98 Radial Velocity (km s o CM o - o o CM o Figure 41. V1223 Sgr. He II ^4686 radial velocity for 5 and 6 IIfor5and Heradial velocity ^4686 Sgr. V1223 41.Figure _J 05 1.5 1 0.5 0 ____ i i I i i i i September 1988,September 14.2 the on phasing. minute 1 Phase 4.2—Minute ______i 1 i i ------I 1 ____ i ' i ' i i i 99 100 PERIOD SEARCH This system has been searched for periodicities using the method of Scargle (1982) as modified by Horne and Baliunas (1986). The results of this search may be found in Table 3. The coherent modulations proved to be much less conspicuous here than in FO Aqr. The usual procedure in a period search is to measure some property of the spectrum in each of a series of spectra obtained over some time interval, then seach for periodicities in that property. In some cases here this procedure has been modified by searching for periodicities in spectra which were obtained by adding the original .pn98time series of spectra in groups of three. This method should have the effect of increasing the signal-to-noise ratio of the spectra, making it less likely that spectrum measurements will be overwhelmed by noise. The disadvantage, of course, is that the periods here are very short. So one is essentially looking for modulations in spectra averaged over a significant fraction of the period. In all cases, detections are claimed only when the false alarm probability is low, and only for specific nights. Figure 42 depicts the periodogram for magnitude data obtained on September 6 . The modulation at the first lower sideband is quite prominent. Figure 43 shows the periodogram for H/3 radial velocity obtained on September 6 . The peak occurs at the spin frequency. The periodogram for He II A4686 radial velocity on 5 and 6 September in Figure 44 shows prominent peaks at what appear to be the spin and second lower sideband. 101 Table 3 False Alarm Probabilities for Detected Frequencies for V1223 Sgr Source co- 2 0 CO-ft CO (14.2 min) (13.2 min) (12.4 min) Continuum 0.186 (6 Sep) 0.273 (7 Sep) Hy3 Radial Vel. 0.264 (6 Sep) He II A4686 Radial Vel. 0.200 (5+6 Sep) 0.353 (4 Jul) 0.277 (5+6 Sep) Power o LO o iue4. 12 g. Powerspectrum V1223Sgr. theof Figurecontinuum 42. magnitudefor rqec (rad/day) Frequency 500 6 September 1988. 1000 102 Power O l O iue4. 12 g. Powerspectrum V1223Sgr. ofFigureHp 43. radial velocity for 6 September 1988. rqec (rad/day) Frequency 500 1000 103 Power LO iue4. 12 g. Powerspectrum radial V1223 Sgr.ofHe 11^4686 Figure 44. 400 velocityfor 5 and rqec (rad/day) Frequency CM 6 September 1988. 800600 104 1000 105 ANALYSIS The Structure of the H8 Line It is difficult to reconcile the appearance of the H/J line on the orbit phasing (Figure 33) with the usual picture of CVs in which line emission originates in a disk rotating in a Keplerian fashion. Disk emission should result in lines showing strong symmetry. In this case, the lines are highly asymmetric in the sense that the blue side appears to be unmodulated in radial velocity. In an effort to determine the nature of this asymmetry, an attempt has been made to separate the modulated part of the line from that part which is unmodulated. Each of 10 spectra which were binned on the orbit phasing was fitted on the red side with a Gaussian function. This required including 1-2 A of the line blueward of the line peak in each fit. The fitted function was then subtracted from the whole emission line in each bin, resulting in a series of 10 residual profiles. A gray-scale image of these profiles is shown in Figure 45. This image represents the removal of most of the flux from already noisy spectra, which makes its interpretation difficult. The profile does appear to be modulated in flux, with maximum near phase 0.8. We note that this is near the maximum of the orbit continuum modulation. There may also be a modulation in radial velocity, but this impression could be due to a two component structure. The composite profile, created by co addition of 9 of 10 spectra (phase 0.2 was excluded, as this is an obvious over-subtraction) is shown in Figure 46. The radial velocity HORIZ RESOLUTION = 5 VERT RESOLUTION = IS H SPECTRA = 1 0 8 WRAPS = FIRST PIXEL = 693 BLACK LEVEL = 550 WHITE LEVEL = 475 ROLL OVER PIXELS? = N ONE MAX V1223 SGR ORBIT PHASING 4840 * 5: m m m. . 00 y m. ■> * 8 C — . 20 . 40 — . 60 K — . 80 f;1 3 S3 *3 n sax — . 0O SI •"J' jRBE — . 20 . 40 Figure 45. V1223 Sgr. Gray-scale image of the residual wind component on the 3.4 hour phasing. 106 107 CN I E o CN U) cn i_ 0 n* O CN CN x Z5 CN 4800 4850 4900 Wavelength (A) Figure 46. V1223 Sgr. The Hjf residual wind profile Radial Velocity (km s o CM o o Figure 47. V1223 Sgr. Radial velocity from Gaussian fits theto Radial fromGaussian velocity Sgr. V1223 47.Figure 0 e sideredof Hft ,the on 3.4phasing. hour 0.5 .- u Phase our 3.4-H 108 1.5 109 *1 of the centroid of this line is — 550 km s . The shape of this line does indeed suggest the presence of two components, located at — 400 1 1 km s-x and — 700 km s"-L respectively. The latter velocity is close to the -850 km s"^ reported by Watts et al. (1985) for the absorption dip blueward of H5. A radial velocity curve derived from the Gaussian line fits described above is shown in Figure 47. Comparison of this figure with Figure 38 shows that the blue component distorts the whole-line profile to the extext of giving it a negative gamma velocity. The Orbit Phasing of Radial Velocity and Continuum The relation between the phasing of the continuum (Figure 36) and that of the emission line radial velocity (Figure 47) can be used to determine the location of the continuum source. In this case, the continuum shows a reasonably well-defined modulation only for the July data. The latter light curve has been fitted with a sinusoid using a least squares minimization routine, with the result that maximum light occurs at phase 0.88 ±0.06. As has been noted above, this is at variance with the ephemeris, which places maximum at phase 0.0. The radial velocity curve for H/3 has also been fitted using the same routine used above, with the result that maximum redshift occurs at phase 0.73 ±0.03. It would appear that maximum light lags maximum redshift of H/? by only a small amount. 110 The Spin Modulation The relative phasing between the two radial velocity curves (Figures 39 and 40) phased on the spin period may yield information concerning the locale of the modulated light. Thus, each curve has been fitted with a sinusoid using a least squares minimization routine. It is found that H/3 radial velocity reaches maximum redshift at phase 0.93 ±0.01, while He II A4686 radial velocity reaches maximum redshift at phase 0.71 ±0.02. That is, He II A4686 maximum leads Hj9 maximum by 0.22 ±0.03 in phase. It would seem that these results are consistent with the coincidence of maximum He II A4686 radial velocity with the blue-to-red crossing of the H/? radial velocity curve. (Or, alternatively, He II A4686 minimum radial velocity is coincident with the red-to-blue crossing of the H/? radial velocity curve.) DISCUSSION The Orbit Modulation In the study of CVs it is often assumed that the orbit modulation of the continuum light is due to illumination (actually reprocessing) at the inner face of the secondary star by radiation originating at the white dwarf and/or the accretion disk. In this model, maximum orbital light occurs at superior conjunction of the secondary star. If one assumes that the lines originate in the disk, this can be proved explicitly by determining that the blue-to-red crossing of an emission line radial velocity occurs at maximum light. This is evidently not Ill the case here, as continuum maximum occurs much closer to the red-to- blue crossing of H/3 (which would occur at phase -0.98, based on the determination of the location of maximum redshift for this line given above). There are at least three sources of line emission in these objects: the inside face of the secondary star; the accretion disk; and the disk hotspot. As it is used here, "disk" should be taken to include contributions from both the classical disk and the white dwarf magnetosphere; the concern here is the identification of regions which will produce lines exhibiting distinctive o r b i t a l modulations. Assuming an origin for the lines in the secondary star implies a phasing illustrated in Figure 48a. In this case, continuum maximum is likely due to to viewing b o t h the inner face of the hotspot and inside face of the secondary most directly. After phase 0.88, while the continuum component due to the secondary star continues to increase, that due to reprocessing at the hotspot decreases, as its inner face is viewed at increasingly oblique angles. This scenario, however, does have a problem: The FWHM of H/J is -800 km s-^, a value which is rather large if the emission lines originate at the surface of the secondary. Figure 48b depicts the phasing based on the assumption that the lines originate in the disk. The phase of continuum maximum here strongly suggests the outer face of the hotspot as the source of the modulated continuum light. If this were the case, then the inner face of the hotspot would be viewed at continuum minimum light, and this 112 .73 (a) I — .98 (b) .98 _ 1 .73 Figure 48 V1223 Sgr. (a) Orbit phasing if the emission lines originate in the secondary star. (b) Orbit phasing if the emission lines originate in the disk. In either case, continuum maximum occurs at phase 0 .8 8 . 113 seems inconsistent with reprocessing as a major source of the continuum. Furthermore, the similarity of the continuum amplitudes on the orbital and 13.2 minute phasings does not support such a conclusion. Treating the case in which the lines originate in the hotspot is problematical, as we do not know its exact location. Nevertheless, its seems likely that maximum redshift due to a hotspot source should occur in the interval between phases 0.48 and 0.73 in Figure 48a. This would place the phase of maximum continuum light near quadrature, which would make sense in terms of a hotspot source for the continuum. In this case, reprocessing at the inner face of the secondary would evidently have little effect on the orbital continuum modulation. It finally must be admitted that the relative phasing between lines and continuum presented here may be incorrect. The light curve presented here is quite noisy, and indeed does not match the phasing expected from the ephemeris (but this also occurred in FO Aqr). If one were to assume that the maximum of continuum light does, indeed, fall at phase 0.0, then the phase of H/? radial velocity maximum (0.73) places the red-to-blue crossing very near continuum maximum. The conventional interpretation in this case is that the lines originate in the heated inner face of the secondary star, as it is assumed that the continuum originates there, too. 114 Evidence for a Wind As noted above, the Hy3 line on the orbit phasing shows strong asymmetry and evidence for a two component structure. The primary component appears to be modulated on the orbit phasing; in addition to a clear-cut red edge, close examination of Figure 33 reveals hints of a blue edge as well. The second component appears to fill in the blue side of the line. It would seem that this component is due to the presence of winds. The reader will recall that winds have been detected in FO Aqr as well. In that case, it was possible to determine that the winds originate at the hot spot and secondary star. The localization of the wind in this case is much more problematical. The phasing of maximum wind flux in the gray-scale image in Figure 45 suggests coincidence with orbital flux maximum. In the context of a reprocessing model, one thus has a wind originating either at the hot spot or the secondary star (or both). This result is at least in agreement with the result for FO Aqr. In addition, the centroid of the wind component (— 550 km s'^) is remarkably close to the radial velocity found for the weaker wind component found in FO Aqr. The Spin Modulation It seems that both H/? and He II A4686 (Figures 39 and 40) show modulations in radial velocity at the spin (12.4 minute) period. Repeated attempts to find other convincing modulations at this period in both Hj0 and He II A4686 in the binned spectra failed. As the modulations were, in both cases, found in the line core, it is 115 tempting to conclude that the emission is due to gas with relatively small velocity dispersion. However, the suggestion of the presence of a wind discussed above could give the false impression that no modulation exists in the wings. It should be noted that the significant phase shift between the two radial velocity curves differs from the result found for FO Aqr (Figures 19 and 21), where the radial velocity curves were found to be in phase. It would appear that the disk illumination model (Penning 1985) does not predict such a phase shift. Nor does the accretion curtain model of Mason, Rosen and Hellier (1988), unless one assumes that the Hj8 and He II A4686 emitting regions lie at different radial distances from the axis of rotation. In this scenario, one expects light due to gas closer to the axis of rotation to precede in phase light due to gas at a greater distance. This implies, in the case at hand, that the He II A4686 emitting region lies closer to the white dwarf than does the H/3 region. This is a reasonable result inasmuch as the former region must be hotter than the latter. The much simpler scenario, in which it is assumed that the changing aspect of the accretion funnel flow does not contribute to the spin-phased radial velocity modulation, implies that the two emitting regions must be separated by -80°. In any case, in the absence of a flux modulation in either H£ or He II A4686 at 12.4 minutes, it is likely that the radiating gas is optically thin. This is similar to the situation found in FO Aqr. CHAPTER V AO PISCIUM INTRODUCTION The intermediate polar AO Psc was first identified as the x-ray source H2252-035 by Marshall et al. (1979), and later as 3A2253-033 by McHardy et al. (1981). Griffiths et al. (1980) found an emission line object at the x-ray position. In view of the presence of prominent high-excitation emission lines in the spectrum and of a continuum slope in agreement with disk models, the latter authors assigned AO Psc to a class intermediate between the non-magnetic CVs and the strongly magnetic, but disk-less, polars. This turned out to be a very prescient choice of classification. Warner, O'Donoghue and Fairall (1981) and Patterson and Price (1981) first detected a high frequency photometric modulation with period 14.2 minutes. The latter authors also found photometric and spectroscopic variations having a period of 3.59 hr. The longer period was identified as the orbit period of the system. White and Marshall (1981) demonstrated that the x-ray flux from AO Psc is modulated with a period of 13.4 minutes, a value which evidently represents the rotational period of the white dwarf. Patterson and Price (1981) inferred that the 14.2 minute optical periodicity is due 116 117 to reprocessing of the x-ray beam at a non-axisymmetrie site outside the white dwarf magnetosphere. Fourier analysis of extensive unfiltered photometry obtained by Warner et al. (1981) produced the In the optical, the dominant modulation occurs at the first lower sideband (14.3 minutes). The amplitude of this signal is remarkably variable, Warner et al. (1981) finding full amplitudes as high as 0.35 mag. Clarke, Mason and Bowyer (1983) find the typical semiamplitude to be 0.06 mag (V-band) , while van der Woerd, de Kool and van Paradijs (1984) find the semiamplitude to be about 0.03 mag in both the V-band and the B-band. The semiamplitude of the 13.4 minute optical modulation is variable, but on average quite small (-0.013 mag in the V-band and -0.022 mag in the B-band; van der Woerd, de Kool and van Paradijs 1984). The latter authors also found that the amplitude of the 13.4 minute modulation increases toward shorter wavelengths, and that the 13.4 minute and 14.3 minute modulations are 180° out of phase at orbital maximum light. The orbital photometric modulation occurs with semiamplitude -0.08 mag in the V-band (Clarke, Mason and Bowyer 1983), and approximately 0.05 mag in the B-band (van der Woerd, de Kool and van Paradijs 1984). The locale in the system at which reprocessing occurs has not been unambiguously determined. Patterson and Price (1981) favor reprocessing at the secondary star surface; Hassall et al. (1981) prefer a model in which reprocessing occurs in the region of the disk hot spot. The former authors based their argument on the near- coincidence of the red-to-blue radial velocity crossing of the Balmer 118 lines and orbital continuum light maximum, which they associated with superior conjunction of the secondary star. The latter authors argued that emission lines of the width observed in this system (-1500 km s’^ FWZI) are unlikely to originate in a cool stellar atmosphere. The anti-phasing of the two high-frequency modulations at orbital maximum light is usually taken to imply that the light modulated at the shorter period originates in gas rotating with the white dwarf. In this model, the x-ray and optical light modulated at the spin period must be emitted in the same direction, a condition which evidently holds in AO Psc, as the x-ray and optical 13.4 minute modulations are in phase (Patterson cited in van Paradijs et al. 1985). But Motch and Pakull (1981) and van der Woerd, de Kool and van Paradijs (1984) have argued that there exists a second component of light modulated at 13.4 minutes, which is anti-phased with respect to the signal coming from the white dwarf, and must, therefore, originate in axisymmetric reprocessing in the accretion disk. Their arguments hinge upon their inability to match the continuous spectrum of the light pulsed at 13.4 minutes with that of a hot blackbody, the type of source expected to be found near the white dwarf's magnetic poles. Additional evidence supporting the existence of the second 13.4 minute component is provided by a run of optical photometry simultaneous with an x-ray observation (Pietsch et al. 1987) in which the 13.4 minute optical pulse exhibited a 180° phase shift with respect to the x-ray pulse between two nights. 119 AO Psc has been the subject of numerous spectroscopic studies: Griffiths et al. (1980); Patterson and Price (1981); Hassall et al. (1981); Warner, O'Donoghue and Fairall (1981); Wickramasinghe, Stobie and Bessell (1982); Clarke, Mason and Bowyer (1983); Cordova et al. (1983); Penning (1985); and Hutchings and Cote (1987). However, almost all of these studies suffer from shortcomings: A number of these works cover substantially less than one orbit cycle, and several use integration times that are inappropriate for examination of the high-frequency modulations. The Balmer lines (H/9, Hy) and He II A4686 vary in integrated flux, equivalent width and radial velocity as a function of orbit phase (Patterson and Price 1981; Wickramasinghe, Stobie and Bessell 1982; Clarke, Mason and Bowyer 1983; Hutchings and Cote 1987). Patterson and Price (1981) found the H/3 and He II A4686 lines to be in phase in radial velocity, with amplitude -145 km s"^, and a red-to-blue crossing near orbital photometric maximum. These facts were used to support the conjecture that the emission lines arise as a result of reprocessing of x-rays in the atmosphere of the secondary star. Clarke, Mason and Bowyer (1983) find flux modulations in the Balmer lines which reach maximum near orbital photometric phase 0 . 0 (maximum continuum light), a result which can be interpreted as support for reprocessing at either the secondary star or disk hot spot, so long as it is understood that, in the latter case, the line emission must be dominated by an s-wave. The He II A4686 flux on the orbit phasing was 120 found by Clarke, Mason and Bowyer (1983) to be retarded in phase by 0.17 with respect to the Balmer lines, a result which may imply a different source of emission for the helium lines. A further interesting result of the work of the latter authors is that the emission line fluxes vary with considerably greater amplitude than does the continuum flux (by a factor of 2-3) over the orbit cycle. Wickramasinge, Stobie and Bessell (1982) found that the Balmer lines showed a double-peaked structure in some orbit phases, indicative of the presence of an accretion disk. In addition, they claim to find several velocity components in the H/3 line, including an s-wave and an underlying disk component, the latter yielding a K- velocity for the white dwarf of 60 km s“^, a result distinctly different from that of Patterson and Price (1981). There is some evidence (Motch and Pakull 1981; Wickramasinghe, Stobie and Bessell 1982; Clarke, Mason and Bowyer 1983; Hutchings and Cote 1987) that there are emission line flux (and equivalent width) variations phased with the 14.3 minute period. This is entirely expected, as the reprocessing target, which is the source of the 14.3 minute light, is subject to irradiation by a time-varying x-ray beam. This model, however, makes it difficult to explain the He II A4686 radial velocity modulation at 14.3 minutes reported by Hutchings and Cote (1987). Their observation could be the result of line emission from the reprocessing site which adds a flux-modulated component displaced from the overall line-center, an idea suggested in this work 121 as an explanation for the peculiar behavior of the same line on the 2 1 minute phasing in FO Aqr. Evidence for variations in the emission lines phased on the 13.4 period is sparse. Clarke, Mason and Bowyer (1983) quote upper limits for Balmer line and He II A4686 flux amplitudes, and Penning (1985) finds evidence (via Fourier analysis) for radial velocity modulation in the same lines near the 13.4 minute period. Finally, it is worthwhile noting that, like many CVs, AO Psc is subject to non-periodic variations, including flickering and flares, and considerable instability in the amplitudes of the periodic photometric modulations (Griffiths et al. 1980; Warner, O'Donoghue and Fairall 1981). In particular, Warner et al. found flickering to be concentrated at the maxima of the 14.3 minute photometric modulation, a clue perhaps that this modulation is driven by reprocessing at the disk hot spot. OBSERVATIONS The data set for AO Psc consists of 1021 spectra obtained on the nights of 19 and 23 August 1988, and on 1-7 September 1988 (all times UT) . In all cases, the integration time was 60 seconds. As was the case for V1223 Sgr, some culling of the original data set was required in order to eliminate spectra of poor quality. The final working set consists of 973 spectra, representing more than 16 hours of integration on this object. 122 However, this set of spectra suffers from one of the problems encountered in the V1223 observations. That is, on three of the nights AO Psc was found to exhibit a roughly monotonic decrease in brightness through the course of that night's observation. In all cases where this problem occurs, AO Psc was observed, except near the beginning of the observation, at steadily increasing airmass. It therefore seems likely that there was on three nights a poor match between the mean extinction and the prevailing extinction. No attempt has been made to "fix" this problem. The unfortunate result is that the data show a dispersion in flux which is somewhat larger than the dispersion likely to be intrinsic to the object. It has been ascertained, however, that in the analysis which follows, none of the phase-binned data have been significantly distorted by biased selection of individual spectra. In the analysis which follows, the data have been lumped together into a single set. The orbit modulations (frequency O) are phased on a period due to Hutchings and Cote (1987) and an epoch of maximum light from Patterson and Price (1981). For the 14.3 minute (frequency w-fl) data, an ephemeris from Williams et al. (1984) has been employed which places maximum light at phase 0.0. The 13.4 minute (frequency w) phasing is based on a period derived from optical photometry (van Paradijs et al. 1985) with the epoch arbitrarily defined as that of the first observation of AO Psc on 2 September 1988. 123 SPECTRA Figure 49 depicts the composite spectrum of AO Psc. Again, the Balmer lines and He II A4686 lines are most prominent. Note that the He line differs greatly from the Balmer lines in shape. The former gives the impression of a two-component structure - narrow and broad; the latter do not. This is true of V1223 Sgr as well. Also present are He I lines at A4388, 4713, 4922 and 5016. There is no sign of the He II A4542 line. The gray-scale image shown in Figure 50, which represents all data binned on the orbit phasing, exhibits some astonishing structure. In the phase interval -0.45-0.80 the Balmer lines are significantly weakened and appear to show a double-peaked structure. An enlarged portion of Figure 50 in the neighborhood of H/J is shown in Figure 51. Wickramasinghe, Stobie and Bessell (1982) contend that this line is contaminated by an s-wave which lies on the red side of the line in the double-peaked phases. Such a structure is not immediately obvious in Figure 51. In particular, one might expect to find the red peak between phases -0.45-0.80 brighter than the blue peak, but such is not the case. A stronger impression is that of a line whose core has greatly weakened near phase 0.45, but which has maintained the wings intact. The second remarkable aspect of Figure 50 is the absorption feature located in the center of the He I A4471, which occurs approximately 124 odj CN I lO T cn a> 7 CM o x =3 4200 4400 4600 4800 5000 Wavelength (A) Figure 49. AO Psc. Composite spectrum formed by averaging all spectra. 125 between phases 0.55 and 0.75, an interval enclosed by the somewhat longer double-peaked episode in the Balmer lines. A composite spectrum constructed from those phases is shown in Figure 52 which, incidently, clearly demonstates the bifurcated structure of the Balmer lines. Note that the feature at A4471 is clearly an absorption feature. Though A4471 has never before been reported to be in absorption in this object, Wickramasinghe, Stobie and Bessell (1982) noticed that this emission line disappeared when the Balmer lines became double-peaked. In addition, Warner, O'Donoghue and Fairall (1981) have reported an absorption feature at A5090 in their mean spectrum. The He II A4686 line (Figures 50 and 53) shows complex structure. The line appears to exhibit a blueward extension, which peaks in intensity near phase 0.3, and a weaker redward extension near phase 0.9. Alternatively, it is possibly the case that the line has simply narrowed and weakened in a phase interval roughly coincident with that in which the Balmer lines become double-peaked. Figure 54, which shows a gray-scale image of the data binned on the 14.3 minute period, appears to show a narrowing of the He II A4686 line in the phase interval 0.6-0.0. The Balmer lines do not appear to be modulated in radial velocity. Figures 55a and 55b depict, respectively, the H/3 line and the He II A4686 line phased on the 13.4 minute cycle. H/3 is clearly modulated in radial velocity, although no flux modulation is apparent. He II HORIZ RESOLUTION = 1 UERT RESOLUTION 8 tt SPECTRA 25 # WRAPS = 12 FIRST PIXEL = 8LACK LEUEL = 600 WHITE LEUEL = 450 ROLL OUER PIXELS? = N ONE MAX AO PSC ORBIT PHASING/ALL FILES 4300 4400 -4500 -4600 4 700 4 8 0 0 4 9 0 0 5 0 0 0 E- • 20 'i'- r- . 40 E ~ . *0 E— . &0 v>:" . ■ i v y i ; E- . 20 "< 5?^-V s •••' -iv''-. . -. ■; ? i* *' . ** ej»- ■ V •* « * >j* »* ;--.c^ E - . 40 Figure 50. AO Psc. Gray-scale image of all spectra on the 3.6 hour phasing. to O' HORIZ RESOLUTION = 4 VERT RESOLUTION = 8 # SPECTRA = 2 5 # WRAPS = 12 FIRST PIXEL = 673 BLACK LEVEL = 650 WHITE LEVEL = 490 ROLL OVER PIXELS? * N ONE MAX AO PSC ORBIT PHASING/ALL PILES 4780 <4800 <4820 <48-40 4860 <4880 <4900 <4920 <4940 I a I a a k a I =-.20 E~ . 40 = - . 6 0 = - . 8 0 |— .20 = - . 4 0 Figure 51. AO Psc. Gray-scale image of all spectra, in the neighborhood of U p , on the 3.6 hour phasing. ho 128 0 CM ffl 4200 4400 4600 4800 5000 Wavelength (A) Figure 52. AO Psc. Composite spectrum formed by averaging spectra in the 3.6 hour phase interval 0.56-0.72. 129 A4686, too, is modulated in radial velocity, but the line profile here is much more complex. Once again (compare with Figures 50 and 53) , there appear to be extensions both to the blue and to the red which are not uniform in intensity through the cycle. CONTINUUM No continuum modulation was found on the orbit phasing. Continuum variations found on the spin and sideband periods appear to be marginal, at best. These results are consistent with the record of photometry of this object found in the literature, as discussed above. HYDROGEN LINES Again, as has been done throughout this work, only the H/3 line will be treated in detail, as the IDS gives relatively poor response blueward of H/J. Figure 56 depicts the H/J flux as a function of the orbit phase, as determined by a Gaussian fit to the whole line profile. Both the fall to minimum from maximum and the rise to maximum from minimum appear somewhat less dramatic here than they did in the gray scales. The radial velocity curve for H/? on the orbit phasing is shown in Figure 57. In this case, the velocities were determined by measuring the centroid of the line wings using the "RV" method. This method was chosen for a specific reason: In order to obtain a radial velocity curve over the whole orbit cycle, it is necessary to measure the line in the wings, as the wings appear to remain unaffected by the HORIZ RESOLUTION = 5 UERT RESOLUTION = 8 8 SPECTRA = 2 5 8 WRAPS = 12 FIRST PIXEL = 480 BLACK LEUEL = 600 WHITE LEUEL = 450 ROLL OUER PIXELS? = N ONE MAX AO RSC ORBIT PHASIN6/ALL FILES 4620 4640 4660 46>30 4700 4720 4740 Figure 53. AO Psc. Gray-scale image of all spectra, in the neighborhood of He II A4686, on the 3.6 hour phasing. HORIZ RESOLUTION = 1 UERT RESOLUTION = 1 6 8 SPECTRA = 1 0 8 WRAPS = 5 FIRST PIXEL = 1 BLACK LEUEL = 600 WHITE LEUEL = 450 ROLL OUEP. PIXELS? = N ONE MAX AO PSC 859-SEC PHASING/ALL FILES 4 6 0 0 4 7 0 0 4 S 0 0 4 9 0 0 5 0 0 0 ■ 4 3 0 0 4 4 0 0 4 5 0 0 l l I — . 0 0 V . ; ' : j . * 4 4 ; ■; i — . 20 . 4 :C44i^Ufc .4:4 (. ••• r - ---- . 40 .... ss . . i'i. ; v } ■ : i ■ ‘. 3 •:. :4 — . 60 4 4 4 4 4 - t i - L — . 80 4 . 4 - ;J 4. : — . 00 — . 20 4 4 / ^ i | Ri I — . 40 Figure 54. AO Psc. Gray-scale image of all spectra on the 14.3 minute phasing. — HORIZ RESOLUTION = 5 UERT RESOLUTION 16 H SPECTRA = 1 0 n WRAPS = 5 FIRST PIXEL BLACK LEUEL = 600 WHITE LEUEL = 453 ROLL OUER PIXELS'7 = N ONE MAX AO PSC 305—SEC PHASING/ALL FILES 4 8 0 0 4 8 2 0 4 9 0 0 4920 I « , 00 , 20 :>s , 40 60 , 80 00 , 20 . 40 HORIZ RESOLUTION = 5 UERT RESOLUTION = 16 *4 SPECTRA = 1 0 # WRAPS = FIRST PIXEL = 430 BLACK LEUEL = 600 WHITE LEUEL = 450 ROLL OUER PIXELS'7 = N ONE MAX AO PSC 305—SEC PHASINGALL FILES -4320 4340 4330 4 700 4 720 4740 i I 00 20 40 , 60 , 30 , 00 , 20 . 40 Figure 55. AO Psc. (a) (top) Gray-scale image, in the neighborhood of H/3, on the 13.4 minute phasing. (b) (bottom) Gray-scale image, in the neighborhood of He II A4686, on the 13.4 minute phasing. 133 appearance of the double peaks (see Figure 51). No persuasive modulations were found in H/3 flux or radial velocity on the 14.3 minute period. This finding was foreshadowed by the appearance of the gray-scale image. The radial velocity of the H/3 line is clearly modulated on the 13.4 minute period, as Figure 58 shows. In this case, the radial - velocities are derived from a Gaussian fit to the whole line profile. All attempts to find a flux modulation on this period failed. HE II A4686 The orbit-phased He II A4686 profiles have been fitted with a Gaussian function, despite the unusual appearance of the gray scale. The results are shown in Figures 59 and 60. Figure 59 is very noisy, yet it nevertheless implies that the He II A4686 flux is in phase with that of Hj8 . In view of the appearance of the gray-scale image, the odd behavior of the radial velocity curve (Figure 60) and its distinctly negative gamma velocity are not surprising. At 14.3 minutes, only the integrated flux of the He II A4686 line shows a persuasive modulation (Fig 61). The method for obtaining the flux is identical to that described in the previous paragraph. The radial velocity of the He II A4686 line, obtained from a Gaussian fitting routine, shows a remarkably well-defined modulation at 13.4 minutes (Figure 62), which appears to be in phase with the 134 CN I O co if) CD T _ X Li_ T 3 Q) CP 0 0 0.5 1 1.5 3.6—Hour Phase Figure 56. AO Psc. flux on the 3.6 hour phasing. Radial Velocity (km s o o CM o iue5. OPc H(3theinof wings radialPsc. AO measured velocity, 57.Figure 05 1.5 1 0.5 0 theline,the3.6 onhourphasing. ■ ■ ■ 3.6—Hour Phase ■ ■ ■ 135 Radial Velocity (km s in o o o in iue5. OPc H(J 13.4the minute AOradialPsc.on velocity 58.Figure 0 phasing. 13.4—Minute Period 0.5 136 137 radial velocity modulation of Hj9 on this period. However, unlike H/9, He II A4686 also is modulated both in flux (Figure 63) and in FWHM (Figure 64) at 13.4 minutes. ANALYSIS The Continuum Phasing on the Orbit Cycle On the orbit cycle, the phasing of U/3 radial velocity relative to the continuum modulation is not the expected result. The radial velocity plot on this phasing (Figure 57) has been fitted with a sinusoid with a least-squares fitting routine, resulting in location of the maximum at phase 0.55±0.01 (see Figure 67). Although there is no orbit-phased magnitude plot presented here, the orbit phasing is such that maximum continuum light occurs at phase 0.0. This means that maximum redshift of H/3 precedes maximum continuum light by -0.45 cycle, rather than the -0.25 cycle found by both Patterson and Price (1981) and Wickramasinghe, Stobie and Bessell (1982). That the phasing used here is correct has been verified by comparison with several times of maximum light found in the literature (e.g. van der Woerd, de Kool and van Paradijs 1984), and it seems unlikely that the phase of maximum light differs from phase 0 . 0 on the plots presented in this work by more than -0.05. It should be further noted that the Hy3 radial velocity was determined from the line wings, and, in the conventional interpretation, line emission in the wings should originate in light emitted from near the central parts of the disk. If this is the case, 138 E o CO CD X _=J Li_ "O 0 ■ + - > CD o i_ CJ> 0 -4-> c 0 0.5 1.5 3.6-Hour Phase Figure 59. AO Psc. He II A4686 flux on the 3.6 hour phasing. Radial Velocity (km CM o o o Figure 60. AO Psc. He IIthe HeradialonA46863.6 hour velocity Psc. AO 60.Figure 0 phasing. 0.5 .-or Phase 3.6-Hour 139 1.5 140 CN E o CO cn 0 O oo X _rs Lx_ C 0 0.5 1 4.3-Minute Phase Figure 61. AO Psc. He II \4686 flux on the 14.3 minute phasing. 141 O O o ■■6 2 0 0.5 1.5 13.4-Minute Phase Figure 62. AO Psc. He II )\4686 radial velocity on the 13.4 minute phasing. 142 CM I E o T <» (n o cn 0 to T o T - 0 0 w o X Li_ T> 0 A-> cns ^o -M0 _c 0 0.5 1 1.5 13.4-Minute Phase Figure 63. AO Psc. He II ^4686 flux on the 13.4 minute phasing. FWHM (km s_1) 600 800 Figure 64. AO Psc. He II He A 13.4theon 4686 FWHM phasing. minute Psc. AO 64.Figure 05 1.5 1 0.5 0 13.4—Minute Phase 143 144 then the inference due to Patterson and Price (1981) that the continuum light originates in reprocessing at the inner face of the secondary is incorrect, as, at maximum light, the system is viewed in quadrature. The HB Orbital Flux Modulation and the Absorption Feature The appearance of Figure 50 gives the impression that the fall-off in H/J light is quite sudden near phase 0.45, though the light curve (Figure 56) is less dramatic. Nevertheless, the loss of light is -50%, and the light level remains near minimum for -0.30 of the orbit cycle. This is in contrast to the behavior of the continuum light, which shows no modulation in these data (although there is considerable scatter in the continuum magnitudes). A typical value for the continuum modulation from the literature is ±1 0 % (van der Woerd et al. 1984). The fact that the absorption feature at A4471 occurs nearly concurrently with H/3 minimum light is unlikely to be coincidental. The velocity width of the absorption is -400 km s-^, and it clearly removes the core of the A4471 feature, but not the wings (see Figures 50 and 52), suggesting that the absorption occurs in a gas having significantly smaller velocity dispersion than does the emitting gas. In an attempt to discover what is occurring in other lines during the phase interval of absorption, the ratio between line flux outside this interval (near H/9 maximum flux) and that within the interval has been calculated. The results for Hj8 , Hy, and He II A4686, respectively are 145 1.93, 2.05 and 1.31. The significantly lower value for He II A4686 suggests that the mechanism of line photon loss in this case differs from that in the Balmer lines. The Snin Modulation The spin period of the white dwarf in this system is evidently well established by x-ray observations as 13.4 minutes. Comparison of Figures 58 and 62 shows that the radial velocity modulations in the lines at the spin period are in phase with each other, indicative of a common locale of origin for these lines. Sinusoids have been fitted (using a least-squares fitting routine) to the He II A4686 radial velocity, flux and FWHM modulations (Figures 62, 63 and 64) with the result that the red-to-blue crossing of the radial velocity occurs at phase 0.78 ±0.01, flux maximum occurs at phase 0.76 ±0.03, and minimum FWHM occurs at phase 0.80 ±0.02. That is, the red-to-blue crossing of He II A4686, the maximum of the flux and FWHM minimum would appear to be coincident on the 13.4 minute phasing. Insofar as the light pulsed at 13.4 minutes represents an additional source of light in the system, it might seem worthwhile to examine the behavior of the line profiles created by binning data on this period. We might hope to uncover behavior indicative of motion of the underlying component which drives the line at the spin period. Illustrated in Figures 65 and 6 6 are, respectively, H/9 profiles at maximum blueshift and zero velocity, and at maximum redshift and zero 146 velocity. In each case, the unshifted line is drawn in vector mode (i.e., not as a histogram). The overall impression is that a narrow component is blue-shifted in Figure 65, and red-shifted in Figure 6 6 . It seems likely that the component remains approximately constant in flux, as the whole line is not modulated in flux on the 13.4 minute cycle. The notion of a moving component is given further creedence by the appearance of peaks to the red and blue of line center at maximum redshift and maximum blueshift respectively. If these peaks indicate the extreme positions of an underlying component pulsed at the spin period, then the true full-amplitude of the 13.4 minute Hj8 modulation is -300 km s"^. DISCUSSION The Orbit Modulation and System Phasing If, as suggested above, the wings of the H/3 line track the motion of the inner disk, then observed orbit phases occur as illustrated in Figure 67. That is, the disk is moving away from the observer at phase 0.55. This would give maximum continuum light when the observer sees the part of the hot spot which is most directly irradiated by the x-ray beam which originates at the white dwarf. This phasing also implies that maximum H/3 flux is observed under the same circumstances. According to this picture, extensive loss of light in the Balmer lines occurs approximately from just before quadrature to just after inferior conjunction of the secondary star (phases -.40-85). The 147 octj _ ^ LO I oi E o CO CO L_ Cl) (N O X _D Ll. 4820 48404860 4880 4900 Wavelength (A) Figure 65. AO Psc. Hp at zero velocity (vector) and maximum blueshift (histogram), on the 13.4 minute phasing. 148 I CN I E o CO CD O 4820 4840 48604880 4900 Wavelength (A) Figure 66. AO Psc. Hp at zero velocity (vector) and maximum redshift (histogram), on the 13.4 minute phasing. 149 A4471 absorption episode is of somewhat shorter duration, and centered near phase 0.65, which suggests the hot spot region, where the disk rim is likely to be at its highest, as the locale of the absorbing gas. The absorption is indeed an absorption of photons from a continuum source, possibly located near the white dwarf (e.g., in the inner disk). If the He I A4471 line emission is also due primarily to the disk, then we have a natural explanation for the disparity in widths between the emission and absorption lines: The Keplerian velocity of the inner disk is likely to be much higher than the turbulent velocity in the hotspot region. Absorption of the continuum could also explain the loss of flux in the Balmer lines, but it would be necessary to invoke both a longer span of phase over which the effect occurs, and a cooler temperature in the absorbing gas. Again, in terms of the picture sketched above, Balmer line absorption must begin well before inferior conjunction of the hot spot, which could well account for the lower temperature, as the material at the disk edge must be cooler far from the hot spot. But we now have the constraint that the system inclination must be very large indeed, and this immediately raises the problem of why we don't observe this absorption at all orbit phases - as would be the case if the absorption occurs in the disk rim. Furthermore, one would also expect to observe an occultation of the central disk by the secondary, accompanied by a disk rotational disturbance. None of these effects are in fact seen. It might be possible to "save" this model by locating the continuum source low on the inside of the 150 .05 I .80 — ~ .30 1 .55 Figure 67 AO Psc. Orbital phasing based on the assumption that the wings of originate in the disk. H/ff minimum occurs between phases 0.45 and 0.85. The He I >4471 absorption feature occurs between phase 0.55 and 0.75. 151 hotspot. Especially if the continuum source is small, one could end up viewing the source through disk gas for a rather extended interval of orbit phase. If the geometry can be made "right", this does make sense in terms of classical disk models, in which the edge of the disk is taken to be optically thin in the continuum but optically thick in the lines. It should finally be noted that there is evidence for absorption concurrent with the A4471 absorption in another part of the spectrum - namely in the x-rays. Pietsch et al. (1987) report an x-ray "dip" having duration and phasing approximately the same as that of the optical A4471 absorption. Furthermore, the loss of x-ray flux in this case is a true absorption, rather than an occultation of the x-ray source, in that the depth of the absorption is energy-dependent. That is, the absorption is deeper at low energies than at high energies, indicating photoelectric absorption as the cause. The Spin Modulation It would appear that the results presented here can shed some light on the validity of models of the spin modulation. The fact that the H/9 and He II A4686 radial velocity modulations appear to be in phase (Figures 58 and 62) implies an origin in a common emission region. This is consistent with a model in which the light is due to axisymmetric disk illumination (e.g., Penning 1985). In this case all line emission derives from x-ray illumination of axisymmetric parts of the disk, forcing the radial velocity modulations on the spin cycle to 152 be in phase (though not necessarily having the same amplitude). Further evidence in support of this picture is provided by the behavior of the He II A4686 flux on the spin cycle (Figure 63). In this case, He II A4686 flux maximum occurs at the red-to-blue crossing, which is exactly what is predicted by the disk illumination model. A competing idea, in which one assumes that the line emission is due to optically thick gas entrained in the white dwarf magnetosphere near the magnetic pole, predicts m i n i m u m line flux at the red-to-blue crossing, if one assumes that the radial velocity modulation is driven primarily by rotational motion of the radiating gas. This is contrary to the situation which exists here. Still another category of model depends on spin-phased change in aspect of the radiating gas. The latter category describes the sort of model favored by Mason, Rosen and Hellier (1988), who predict that maximum c o n t i n u u m flux should coincide with maximum blueshift of the lines (i.e., flux and radial velocity are anti-phased). Although this study has turned-up no persuasive continuum modulation on the 13.4 minute period, it seems reasonable to assume that the model of the latter authors would predict maximum line flux at maximum blueshift as well. If this is the case, then their model is contrary to the result presented here with respect to the behavior of He II A4686. There is some difficulty in using the disk illumination model to explain the phasing of the He II A4686 FWHM. One might expect to find the FWHM peaking twice per cycle, at maximum blueshift and again at 153 maximum redshift. That is, FWHM is maximized when the spot is 90° either side of superior conjunction with respect to the white dwarf. At these locations the observed velocity dispersion due to the Keplerian flow across the disk will be largest. The absolute value of the dispersion will of course depend on the radial extent of the spot. The results presented here indicate only one FWHM maximum per spin cycle, and the maximum would appear to be displaced one-quarter cycle in phase from the expected location. The phasing of a FWHM modulation in the context of the accretion curtain model of Mason, Rosen and Hellier (1988) is not clear, although one might again expect to observe two maxima per spin cycle: once at flux maximum (and maximum blueshift), and again at flux minimum (and maximum redshift) . With the exception of the fact that the results reported here show FWHM maximum at flux minimum, the model of the latter authors evidently cannot explain the FWHM behavior of He II A4686. Little can be said about the behavior of H/? in the context of these models, as this line does not show a flux modulation. It is also not possible, based on this data set, to make a judgement regarding the possibility of there being two components in the 13.4 minute modulated light (Motch and Pakull 1981). CHAPTER VI SUMMARY SPIN MODULATIONS The nature of the modulation of the emission lines on the spin period in each of the systems discussed in this work is summarized in Table 4. The values for amplitude and y quoted in the table were derived from a least-squares minimization routine which fitted a sinusoid to each of the modulations. In addition, AO Psc (Warner et al. 1981) and FO Aqr (Chiapetti et al. 1989) are known to be modulated in the continuum at their spin periods. It should be apparent that optical emission modulated on the white dwarf spin period can originate in either the white dwarf magnetosphere, or in axisymmetric parts of the disk. In the former case, the observed radial velocity variations could be driven by two effects. In the first place, radiating matter in the magnetosphere will rotate at the white dwarf angular rotational velocity, thus driving a radial velocity modulation. Secondly, the angle between the line of sight and the direction of gas flow in the emitting region is likely to be a function of rotational phase, again causing a modulation in radial velocity on the white dwarf rotational phasing. In these models, flux modulations could arise if the emitting gas is 154 155 Table 4 Summary of Spin Modulations Obj ect Period Line Parameter Amplitude* 7 (min) FO Aqr 19.5 Radial Vel. 39±2 1512 He II A4686 Radial Vel. 28±3 913 V1223 Sgr 12.4 Radial Vel. 140111 -4218 He II A4686 Radial Vel. 5619 -3516 AO Psc 13.4 Radial Vel. 5613 612 He II A4686 Radial Vel. 8614 -4713 He II A4686 Flux .911.15 8 .1 1 . 1 He II A4686 FWHM 108113 79919 *Units: Radial Velocity km s-^ Flux 1 0 erg s-^ cm"^ FWHM km s' 1 156 optically thick. Or, if the gas is optically thin, occultation by the white dwarf or nearby optically thick gas (e.g., the accretion funnel) will cause a flux modulation. The radial velocity variation in axisymmetric reprocessing models is simply due to the time-varying component along the line of sight of the local Keplerian flow. A flux modulation will occur here only if the rotating x-ray beam excites gas in a disk which shows significant back-front asymmetry (e.g., Penning 1985). The latter condition is expected in thin disks; such disks will be concave (Shakura and Sunyaev 1973). However, it should be obvious that little or no flux modulation will occur at low inclination. The results presented here give only scant evidence for flux modulation at the spin period. However, all systems showed significant radial velocity modulations in the emission lines. If taken at face value, the amplitudes found in Table 4 could imply, in the context of rotation-driven magnetospheric emission, sources near the white dwarf: For a typical white dwarf radius (r - 10^ cm) and the given spin periods, equatorial velocites of -50-80 km s“^ are found. There is, of course, no reason to believe that the radiating gas will be located near the white dwarf equator. In fact, gas which is really quite close to the white dwarf will be found near the magnetic poles, requiring the poles to be located at large colatitude in order to produce the quoted velocities. The latter circumstance is suggested in this study for FO Aqr. However, the streaming velocity of the gas at the poles will be very large (-5000 km s''*" for a 1 Mg 157 white dwarf) as the gas is expected to be essentially in free fall. It is difficult to reconcile velocities of this magnitude with the low amplitude exhibited by the FO Aqr radial velocity curves. In the model favored by Mason, Rosen and Hellier (1988), the modulation in radial velocity at the spin period results from viewing the accretion stream at different aspect angles through the spin cycle. In the case of FO Aqr, Hellier, Mason and Cropper (1990) claim that the anti-correlation between radial velocity and continuum flux (in the sense that continuum flux maximum occurs at radial velocity minimum) demonstrates that the observed magnetic pole is inclined away from the observer at flux maximum, just when the gas in the accretion stream is moving t o w a r d the observer. It is to be observed that these authors are not comparing radial velocity with line flux, but rather with continuum flux. It is possible that the line and continuum light originate in different regions, in which case their argument is not persuasive. In any case, this model is not supported by the results of this study. Hellier, Mason and Cropper differ from this work in the identification of the FO Aqr spin period; no persuasive line or continuum flux modulations were found at the 19.5 minute period presented here. This was the case as well for the H/? and He II A4686 lines in V1223 Sgr, and for H/? in AO Psc. The only instance presented here in which both line flux and radial velocity have been determined (Figures 62 and 63) certainly contradicts the picture of Mason, Rosen and Hellier (1988). As has 158 been observed earlier, the coincidence of He II 4686 flux maximum with the red-to-blue crossing of the line in AO Psc can be explained by the axisymmetric disk illumination model. It seems unlikely that this phasing can be accounted for by emission from the rotating magnetosphere. Furthermore, the FWHM modulation (Figure 64) on the AO Psc spin phasing seems to contradict the simple disk illumination model. The results presented here for FO Aqr and AO Psc do imply at least that the H/? and He II A4686 light originate in close proximity to each other in their respective systems as their radial velocity modulations are in phase and very similar in amplitude. However, this appears not to hold for V1223 Sgr, for which the phase difference between H/J and He II A4686 radial velocity is considerable. And what is one to make of the values of these amplitudes? They seem incompatible with both emission from entrained gas, in which the streaming velocity is expected to be large, and emission from a disk in the disk illumination model. In the latter case, even from the edge of the disk, one would expect amplitudes of several hundred kilometers per second. What is most likely at work here is the fact that the emission line light modulated at the spin frequency is perhaps only a small fraction of the total emission line light produced by the system. In AO Psc, for example, the fraction pulsed at 13.4 minutes amounts to only a few percent of the mean light of the system. Even large velocity variations in a small component of a line might have 159 only a small effect on the location of the line centroid. A suggested example occurs in AO Psc, in which H/J appears to show a component passing back and forth between the red and blue sides of the profile with full amplitude ~300 km s~^. A similar effect in He II A4686 in this system could "save" the disk illumination explanation for the behavior of this line. Of course, some of these worries might be made to disappear in systems at low inclination. However, at least in the case of AO Psc, it seems likely that the system inclination is not small. In sum, the results of the this study have produced some clarification of the nature of the spin modulations in these systems. But many questions remain. ORBIT MODULATIONS Suggested orbit phasings have been presented elsewhere in this work for V1223 Sgr (Figure 48) and AO Psc (Figure 67). In the first case, the picture is ambiguous, as the locale of origin of the emission lines is not explicitly known. In the second case, the phasing depends on the assumption that the wings of the H)9 line originate in the accretion disk, and futhermore that the wings have been successfully measured. This picture of the AO Psc phasing is strengthened by the coincidence of the He I A4471 absorption feature with the phases where the disk hotspot would be viewed from the side opposite the white dwarf. The orbital phasing for FO Aqr was implied 160 by the conjecture that orbital light occurs near inferior conjunction of the secondary, a conventional choice. A diagram of the FO Aqr system may be found in Figure 6 8 . The He II A4686 line flux modulation on the orbit phasing in this system appears to be dominated by an s-wave. This is suggested by both the appearance of the gray scale images (Figure 3) and the apparent coincidence of the minimum of the continuum flux modulation (Figure 5) and the blue-to-red crossing of He II A4686 (Figure 13), although the latter event might be expected to occur somewhat earlier in phase. In any case, the orbital phasing depicted in Figure 6 8 is plausible in light of the observations. The attention of the reader is now drawn to the orbital H/3 flux curves for FO Aqr (Figure 7) and for AO Psc (Figure 56). The similarities are striking: In both cases, the decline to minimum light is somewhat steeper than the recovery to maximum; in both, the minimum is quite flat, extending through -1/3 of the orbit cycle; and, the fractional flux losses between maximum and minimum are comparable (-40% for FO Aqr; -50% for AO Psc). The impression in both cases is that of an extended partial occultation of the source of H/3 emission, rather than, say, the result of viewing the changing aspect of a disk hotspot region through the orbit cycle. (Compare, for example, the flux modulation of He II A4686 on the orbital phasing of FO Aqr in Figure 12.) The modulation of H/3 flux in FO Aqr could be the result of occultation by the "wind sheet" discussed in Chapter 3, and the same explanation could apply to AO Psc. This idea is reinforced by the 161 .25 .00 - 1 .75 Figure 6 8 FO Aqr. Orbital phasing based on the assumption that the continuum light originates at the inner face of the secondary star. ¥ flux minimum occurs between phases 0.65 and 0.00. 162 fact that both H/9 occultation episodes evidently occur in the same interval of their respective orbit cycles (Figures 67 and 6 8 ). Furthermore, it is to be noted that the interval of H/3 flux minimum in FO Aqr coincides with what appears to be an especially chaotic and noisy part of the Hj8 orbital radial velocity curve (Figure 8 ) . It is hard to say whether such is the case for AO Psc (Figure 57). REPROCESSING AND WINDS It was argued in Chapter 3 that the 21 minute period observed in FO Aqr, alleged to be the white dwarf spin period, is actually the period for radiation reprocessed at a fixed target in the rotating system frame (the first lower sideband, w-fi). It is worthwhile noting that in three of the four IPs known to show optical continuum modulations at w-fi (FO Aqr, AO Psc, V1223 Sgr and H0542-407), only FO Aqr exhibits the largest amplitude at what, before this study, was identified as the spin period. In the case of FO Aqr, the reprocessing evidently occurs in winds which originate in the hotspot and/or at the inner surface of the secondary star. V1223 Sgr is apparently windy as well, and although there is some indication that the wind may be localized at a potential reprocessing site, the evidence really needed, in the form of modulations in the lines at w-n, is lacking. The only hint of the presence of winds in AO Psc is provided by the peculiar appearance of the He II A4686 line (Figure 53). The presence of winds is often 163 detected in the ultraviolet; e.g., lines exhibit unusual asymmetries and/or P Cygni profiles. Winds have been discovered in a number of CVs in this way (e.g., Hutchings 1980; Guinan and Sion 1982; Greenstein and Oke 1982; Cordova and Mason 1985). The latter authors suggest that a wind may be present in the intermediate polar DQ Her. Unfortunately, the uv literature concerning the objects in this study is scant, and the subject of the presence of winds has not been addressed. A FINAL WORD The results of this thesis demonstrate a rich variety of emission line behavior in the intermediate polars studied here. Evidently, two of the objects (FO Aqr and V1223 Sgr) exhibit winds which may be localized in regions where reprocessing of the spin modulated radiation occurs. The spin modulations themselves evidently vary in character from one object to the next, and from line to line; only in the case of He II A4686 in AO Psc do we find a full range of line variation. The sources of light modulated on the orbit periods remain in question, but the H/3 line in AO Psc appears to be remarkably similar to that found in FO Aqr. A complete comparison of line behavior among these objects has not been possible; not all possible modulations occur in all objects. In this regard, V1223 Sgr turned out to be especially disappointing. 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