RICE UNIVERSITY

A MEASUREMENT OP

HARD X AND GAMMA RADIATION

PROM SCO X-l

DONALD BAKER TWIEG

A THESIS SUBMITTED

IN PARTIAL FULFILLMENT OP THE

REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE

IN SPACE SCIENCE

Thesis Director's Signature:

Houston, Texas

April, 1971 TABLE OF CONTENTS

I. INTRODUCTION 1

A. Experimental Findings 1

B. X-Ray Production Mechanisms 9

C. The Need for Hard X-ray Investigations

of Sco X-l 11

II. EXPERIMENT 69-2 13 A. Detector System 13

B. Experimental Procedure 16

C. Data Reduction 17

III. RESULTS AND CONCLUSION 27

IV. APPENDIX 32 A. General Considerations and Models 32

B. The Synchrotron Models 36

C. The Close Binary Model 37

V. ACKNOWLEDGMENTS 54

VI. REFERENCES 55 ABSTRACT

A MEASUREMENT OP HARD X AND GAMMA RADIATION

PROM SCO X-l by Donald Baker Twieg

Sco X-l is the brightest extrasolar X-ray source in the sky. Additional knowledge of its output in the hard X~ray

and gamma-ray regions could be particularly valuable in

judging the validity of hypotheses intended to explain its

unusual emission characteristics. On November 26, 1969, a balloon-borne scintillation

detector was launched from Parana, Argentina to measure the output of Sco X-l above *10 keV. Despite complications

caused by a partial battery failure and a solar X-ray flare,

a flux above 75 keV was measured. The instrument and experimental procedures are dis¬

cussed briefly, and the reduction procedures and calculated

fluxes are presented and discussed. Because of a possible

solar contribution, the measured fluxes are interpreted as upper limits, but the results appear to verify the existence of a previously observed possibly non-thermal flux component departing at about *10 keV from the softer thermal component.

Observations of Sco X-l in the X-ray, visual, infrared,

and microwave regions are reviewed. Derived source param- eters and proposed models for the source are discussed In an appendix.

No hypothesis has thus far been completely successful in explaining the many unusual characteristics of the electromagnetic emission of Sco X-l, but the close-binary hypothesis may be the most satisfactory. I. INTRODUCTION

Sco X-l was discovered in 1962 in an X-ray survey of the sky (Giacconi et al. 1962); it is therefore histor¬

ically the first extra-solar system discrete source of

X-radiation to be found. Its X-ray emission has been

studied on numerous occasions since then. In the low-energy

X-ray region, it is the brightest source in the sky, bright¬ er even than the quiet sun.

In this section we survey the present knowledge of the

electromagnetic fluxes from Sco X-l. We then discuss briefly the emission mechanisms which are likely to be responsible for the X-radiation from Sco X-l, and the rea¬

sons that hard X-ray investigations are needed. Sections

II and III are concerned with the experiment and its re¬

sults, respectively.

The theoretical models proposed for Sco X-l are dis¬

cussed in an appendix; the close-binary hypothesis is dis¬ cussed at some length, not because it seems to be verified by the results of this experiment, but because it seems the most valid on the basis of the general body of evidence.

A. EXPERIMENTAL FINDINGS

1. X-rav Observations The findings in the rocket energy range (~1 keV:£E;£ ^0 keV) have been almost invariably consistent with a single exponential spectrum, and thus, presumably, with emission due to thermal bremsstrahlung (however, see Appendix). The nature of these rocket-borne experiments and their results should be mentioned here. Most of the Investiga¬ tions in this energy range use gas-filled proportional

counters as detectors. Because of K- and L-shell edges in the detector absorption function, there are discontinuities

in the detector response kernel, so that the process of un¬ folding the incident photon spectrum becomes a non-trivial matter. Thus the data must be interpreted by testing hypo¬

thetical input spectra, by folding them through the detector response, and then comparing the resulting pulse-height

spectrum with the observed one. The most successful hypoth¬

esis tested in the case of Sco X-l has been invariably a

single exponential spectrum, though there is obviously no

guarantee that this is a unique solution. The point here is

that experimental complications make it necessary to present

proportional counter data in a model-dependent form. A list of X-ray investigations of Sco X-l is given in Table 1. The presumably exponential emission in this range has

exhibited at various times different integrated intensities

and has had e-folding energies varying from kT = 3 keV to kT = 8 keV. Apparently the softer spectra coincide with

periods of greater total intensity. In a balloon-borne ex¬

periment, Lewin et al. (1968) observed a flare above 20 keV

for which the e-folding energy was kT — 3 keV; the duration -3- of the flare was about 30 minutes. The presence of X-ray line structure has not been con¬ clusively demonstrated, though Holt et al. (1969) and Acton et al. (1970) have reported detection of an iron line near

7 keV. More observations are needed to verify this. The high-energy X-ray observations of Sco X-l indicate the presence of another, possibly non-thermal, continuum component (see Figures 1 and 2). The flux of photons with hV>40 keV is apparently variable, but is apparently always in excess of that expected from the kT=*6 keV thin-source bremsstrahlung spectrum observed at lower energies.

The findings of Hudson and Peterson (1966) first sug¬ gested the presence of a harder spectral component for pho¬ tons with hi>S:i|0 keV. Agrawal et al. (1969) and Riegler et al. (1970) report a flux nearly an order of magnitude great¬ er than that seen by Buselli et al. (1969) near 60 keV (see

Figure l). The data of Buselli et al. may be fitted equally well by an exponential with kT = 1^.8 keV or a power law with n = 3.79. The corresponding parameters for the data of Agrawal et al. are kT = 30 keV and n = ^J.5. This dis¬ parity is probably a real temporal variation; similar changes have been observed by various groups in the JjO-50 keV region, where the harder high-energy component first departs from the low-energy exponential. The flare observed by Lewin ot al. (1968) apparently involved flux increases

in both spectral components. TABLE 1 m § m !H m & o rH O w A I sfe m § § a m & CS K iss > o co H ■=? t>j x H Jz EH o ClJ ft \> fxj ei EH <3! EH M ft 0 /—s vo vo \ vp vo m s 03 rH CO rH 0 1? P rH in 0 CM p-J CO CQ O Cd P 3 1 •* • • • ✓—N 00 VO K vo S X. o rH -=r CM o\ rH P H m 0% CO f\l Cd 0 P O O >> I •* • • #»—> 00 VO X. vo i s vp vo \ ■=T o rH CM P 03 lA H H w rH P m 0 O l>3 0 1 •i • • ✓-N 10 vo PM o cc; \ X. M3 o ^r CM CM 03 rH P rH PH *d O •H rH H CO P 0 m 0 I • CO vo vo CO \ vo pq vo X. 00 o rH o\ IS- tn PH h> rH p p 'd in rH 0 d cd 0 P m d d 0 0 0 0 cd 0 i •* * N vo P-i pc; vo \ vo O O X 03 CM rH P CM rH X O P *d •rH rH rH in CM cd 0 O 1 n « /—\ 00 00 vo vo P-i vo « 00 \ O o P rH 03 O rH 0 rH in *d 0 cd P P 0 0 I t • N ^ vo VO VO CO \ X o w O H O P rH PC; 03 X- rH »rl rH rH CM rH in CM 0 cd 0 bO P 0 1 •i % ft w0COpq ft watopqp * P dK054 r *i1••.. PH *Hd10 r4O-POdd)>0 C> 54dWd *4 HOP10 ftd MKDHii5HCd O(dO0^OKlH d P1O54>> d ft054 54 0)0HGd •• s 54 O0Cd(0OHCti O P0HftJ4*H«Hr4 0*H*r4pH OCOP 00 0(xjMd P d * CO pq Pq EH pq pq PS PS pq PQ PS PS pq pq pq PS PS PS PS •> [Y| pq *s n CO •» n O O o O O O o O O O O o O O O g CO CO pq pq co CO pq pq •* CO CO CO pq pq pq pq O O

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w LT\ IC\ rH rH H O o o o O O o O O CO o VO CVJ CVJ o o CVJ § rH H rH CVJ CVJ rH rH in rH •^r rH rH rH -=T 00 rH CVJ s 1 I l I 1 1 1 ! 1 ! l 1 1 1 1 1 1 >H o o VO CVJ CVJ VO VO CV1 CVJ E- E- O O co rH CVJ CVJ CVJ « • CVJ CVJ • PS VO VO VO o E- o w H rH rH

CO CO 00 VO VO VO cn cn cn rH rH rH v~'' v— s s £ o d d d E— o 0 , , 0 cd /—s 00 cn cn cn E'¬ /—s R 00 co R R ^ N /-N o vo vo vo O rH en S3 E- o d VO VO d d t- 00 E'¬ cn cn cn E'¬ rH PS vo E'¬ 0 cn cn 0 0 vo vo en rH rH rH o en o o\ en d rH rH d d cn cn rH '—«" E'¬ rH • EH rH rH 0 '—✓ cd 0 rH rH '—«• en 'w' rH « < EH EH EH • • • rH 0 rH O « • • rH rH rH 'w' * 0 H • • H H Td *0 • • rH 0 0 0 rH 4-3 EH rH rH d 0 0 d d rH rH 0 • 0 0 4-3 co Cd 0 0 cd 0 0 0 4-3 4-3 4-3 rH 0 4-3 4-3 4-3 0 0 0 0 4-> U p> 4-> 4-3 0 0 R R 4-3 4-3 0 0 O 0 0 0 o o O 0 0 •H rH rH 4-3 hO U FH 0 H rH 0 0 d r—1 0 0 0 U 0 d d d R •H •H R R 0 N o rH £ 0 jq £ •H •H U *0 *4 R 4-3 w 0 0 0 U *0 cd 0 0 Q O 0 0 O •H *0 CO U O 0 o 4-> 0 0 > rC R > > O U d d hO 50 O u i—0« •H R R O o o o o o ft PS pq < EH f—«

E- E- E— E— E- o- E- E- E- E— VO 00 00 CO 00 GO 00 cn vo VO VO vo vo vo vo vo vo \ vo vo vo vo vo vo vo w \ \ \ \ \ \ \ \ \ vo \ \ \ \ \ \ \ EH CO in in 00 cn •^r vo o E— o^ a) CVJ CVJ in oo E- rH rH rH rH CVJ CVJ CVJ rH o CVJ CVJ CVJ CVJ rH \ \ R \ \ \ \ \ \ \ \ ov 4*3 \ \ \ \ \ rH 00 CVJ O in m o^ in vo CVJ CVJ oo in rH rH—i Lf\ H DATE INVESTIGATORS ENERGY RANGE (keV) COMMENTS* \ 00 \ VO CO W 4-> •P cr\ o\ O rH rH cu t* OJ CVJ o CVJ cd £ U Cd 2 0 cd 1 • \ Vp \ CO ov m •P rH O iH •H 0- H ov in ts- o on pc; o £ 2 P a 0 cd bO o I • • V \ \ VO AT CO O'v w •P o in •H rH rH cn tv o o< CVJ CO IP o c P £ o 0 cd £ bO >> 1 • o VO \ vo VO \ < H rH •p rH ov rH o\ ov o o CO H o CVJ cvi m 0 cd P cd bO cd 1 • /-N \ \ vo ffi OV rH *P o CO 4-> rH rH o\ CO in PH o »—1 I—1 CO o fp 0 cd O 1 • V / ^, \ vo vo 00 \ «=q o rH rH *p «P rH rH CT\ O £"- in in CO £ cd 0 o o t • 2. Optical 'Observations Until 1966, Sco X-l was undetected at optical wave¬ lengths, but due to increasingly accurate position determi¬ nations by X-ray investigations (particularly that of Gorenstein et al. 1966), Sandage et al. (1966) were able to

tentatively identify a peculiar blue object of mv = 12.5 with Sco X-l. Though the X-ray object has not been located with sufficient accuracy to guarantee this identification, several facts make it likely to be correct: a) The X-ray object has been shown to subtend an angle of less than 7 arc seconds (Oda et al. 1965), implying an object of stellar dimensions. The optical object is the most peculiar one in the area (Sandage et al. 1966). b) The optical continuum of the object can be predict¬ ed accurately by an extrapolation of the X-ray bremsstrah- lung emission assuming self-absorption (Chodil et al. 1968). c) Simultaneous temporal variations in the optical and X-ray regions have been observed (Hudson et al. 1970). The optical object exhibits irregular variations of the order of 1 magnitude over a time scale of hours (Hiltner and Mook 1967). Also observed is a flickering of amplitude ~0.02 mag. and "flares" of amplitude ~0.2 mag. with dura¬ tions of the order of minutes and tens of minutes respec¬ tively (Hiltner and Mook 1967, Westphal, Sandage and Kristian 1968). These variations occur at irregular inter¬ vals with no apparent periodicity and no immediately appar- ent characteristic structure. A typical photometric record Is given in Figure 3. Optical flares have been observed by various Investigators (Sandage et al. 1966, Mumford 1966, Hardle 1967, Mook 1967, Hiltner and Mook 1967). Hiltner and Mook find that Sco X-l is flaring about 12$ of the time. Attempts have been made to detect a periodicity in the optical output of Sco X-l, but with doubtful success. Rao et al. (1969) claim to have found a periodicity of about three hours by applying autocorrelation analysis to their own photometric data, but as Wilson and Twigg (1970) point out, the periodicity is probably illusory. Feldman et al. (1970) claim to have found harmonic power spectrum peaks

3 3 1 at 5.5 X 10~ an(j 8.5 X 10“ sec"*- - in data taken just after a flare. These peaks might represent the fundamental and first harmonic frequencies of a post-flare stellar pulsa¬ tion. Seeds (1970) finds no significant features between 2.8 X 10"3 and 0.1 sec"-1-, although brief periods of post¬ flare pulsation could have been buried in his relatively long sampling period. Lampton et al. (1970) report no os¬ cillations with periods between 2 X 10"3 and 5 X! 10^ sec"-1-. There appears to be a characteristic, if non-periodic, form of variation (Wilson and Twigg 1970), illustrated in Figure 4a. This clearly suggests a recurring process of some sort. Several UBV photometric studies have been made (Mumford 1966, Ilardie 1967, and Stepien 1968). Spectrographlc and spectrometric studies have been made by Sandage et al. -6-

(1966), Johnson et al. (1967), Neugebauer et al. (1969), Mook (1970), and Mool-c et al. (1971). B-V appears to remain fairly constant at about + 0.20, and TJ-B remains near - 0.75. Both of these quantities tend to be larger during periods of greater X-ray and optical intensity (Chodil et al. 1968). No long-term optical variations are indicated. The object is present on blue-sensitive plates made at Harvard as long ago as 1896; the value of B was apparently the same at that time (Sandage et al. 1966), and no long-term changes are indicated by any plates since then (Johnson and Stephenson 1966). Polarization is apparently small (~0.015 mag.; Hiltner et al. 1967). Sandage et al. (1966) found emission spectral lines from the Balmer series of hydrogen, Hell lines, and a com¬ plex of high-excitation lines which they attributed to CIII, NHI, and perhaps Oil. They also found the K line of inter¬ stellar Call in absorption. Large variations in the Balmer line strengths were observed, apparently independent of changes in the continuum and the Hell lines. It was noted that the line features, the continuum, and the temporal vari¬ ations in both were all characteristic of old novae and U Geminorum variables. Sandage et al. noted a "suggestion" of an S-wave distortion of spectral trails on plates which were moved laterally during exposure, indicating possible periodic changes in radial velocity. Westphal, Sandage and Kristian -7

(1968) find the Hell (*l686 A) and hydrogen (Hr and He) lines moving oppositely In wavelength with an Irregular period of about 3 hours. However, the I-Iell lines alone exhibited ad¬ ditional systematic changes over a period of several days, and the conclusion is that something more complex than simple binary motion is involved. Line emission was found indicating H, Hell, Hel, Oil, and possibly Fell. The pres¬ ence of both high and low-excitation lines Indicates that the source is not nearly isothermal. As the continuum var¬ ied (B varies typically between 12.5 and 13.7)* the line emission was observed to remain constant in intensity to within 20$.

The Hoc equivalent widths vary markedly with B, while Hell (ll686 A) widths remain fairly constant (see Hiltner and Mook 1970 and Mook et al. 1971). 3. Infrared and Radio Observations Neugebauer et al. (1969) measured the infrared flux from Sco X-l at I.65 and 2.2 microns. Their findings are consistent with the picture of thermal bremsstrahlung from a source becoming thick at infrared wavelengths. Apparently a "rather extreme set of conditions" would be required to explain the V dependence observed by means of the syn¬ chrotron mechanism. Andrew and Purton (1965) first detected radio emission from Sco X-l at *1.6 cm. The flux intensity observed was 0.021 ± .007 f.u. (one flux unit = 1 X 10~26 Watt3/m2 Hz), 8-

which is much lower than would be expected from thermal bremsstrahlung optically thin at these wavelengths (see

Figure 5).

Abies (1969) detected a flux density at 6 cm varying between .003 and .075 f.u. The mean value of the flux was .022 ± .002 f.u. Variations were over the time scale of hours, but variations of shorter time scale would not have been detected due to the detection procedure.

Interferometric studies (at 2695 and 8085 MHz) by

Hjellming and Wade (1971) show the existence of three dis¬ tinct sources near Sco X-l. One source coincides with the

X-ray object, and the other two are 1.3 and 2.0 arc minutes away from the first, and in opposite directions. The flux level from the central source was observed to fluctuate over a time scale of hours, and varied by a factor of 60 during the experiment. The central source alone was re¬ sponsible for the radio flares observed; no changes were detected in the flux from the other two sources. Further investigations are needed to establish that the sources are physically associated.

In a program of simultaneous optical and radio monitor¬ ing of Sco X-l, Lampton et al. (1971) observed variations in both the radio and optical emissions, but they report no correlation between the two. -9-

B. X-RAY PRODUCTION MECHANISMS Of the many production mechanisms for high-energy

X-rays (cf. Fazio 1967), the two that most likely account for the continuum X-ray emission from Sco X-l are synchro¬ tron radiation, or magnetobremsstrahlung, and thermal bremsstrahlung.

X-rays are produced by the synchrotron process when relativistic electrons enter regions containing magnetic

2 fields. If an electron with relativistic energy E = ymec moves in a magnetic field with perpendicular component Bx, it will radiate primarily at a characteristic frequency GJc, where

The actual time-averaged spectrum of the radiation produced by the electron will be

(1) where K5/3(x) is a modified Bessel function of the second kind (Jackson 1962). Because electrons accelerated by cosmic mechanisms commonly have an energy distribution proportional to E~n (where n is a constant termed the spec¬ tral index), folding such a distribution through the above photon frequency spectrum will give the photon spectrum one might expect from a cosmic X-ray source. The result of this operation is a power-law dependence, with -10-

where E Is In this case the photon energy and o< = . 2

Electron-proton Coulomb scattering Is the dominant source of bremsstrahlung for non-relatlvlstlc electron energies (Tucker and Gould 1966), and the resulting dif¬ ferential emission spectrum has the form

^ _ nenz ^ /2rftn| 2 e**hV/lcT

dtdVd^ 3^m2c3 'id? ' for electrons with Maxwellian distribution at temperature T encountering ions of charge z. g(v>,T) is the Gaunt factor, which is approximately unity for h>b»kT. A more exact ex¬ pression is given by Chodil et al. (1968);

15 2 l(hv)d(hv») = 1.68 X 10“ Z Nene ^

hv 2k3, X e~ / Ko(hi//21d?) (ld?)“'^ d(hv), where Ko(x) is a modified Bessel function of the second kind, ne is the electron number density, and Ne is the total number of electrons in the plasma. This expression is ap¬ plicable to low Z and all photon energies. Chodil et al.

(1968) show that for kT = 5 keV, the flux at 100 keV is

smaller by a factor of four, according to this expression,

than the flux according to the cruder exponential approxi¬

mation, if both fluxes are equal at 3 keV. A significant amount of energy may be emitted in X-ray

lines from a hot tenuous plasma. Such line emission occurs

by radiative de-excitation follovjing inelastic electron-ion collisions; it is significant only at temperatures such

that the typical atom of a species is highly, but not com- -11-

pletely, Ionized. Of course, the plasma must also be opti¬ cally thin at the line frequency.

X-ray line emission, as well as bremsstrahlung emission from a hot low-density plasma, have been Investigated by Tucker and Gould (1966). Tucker (1967) has examined the nature of the line, bremsstrahlung, and synchrotron radi¬ ations to be expected from such a plasma.

C. THE NEED FOR HARD X-RAY INVESTIGATIONS OF SCO X-l Our present understanding of Sco X-l is not limited by a lack of potentially satisfactory theoretical models (see

Appendix.), but by a lack of new experimental evidence which might be used to judge the validity of the present models, and to develop a more quantitative, detailed picture of this source. For the reasons discussed below, the hard

X-ray region appears to be one of the most promising areas of investigation in the advancement of our knowledge of

Sco X-l. In comparison with lower-energy investigations, mea¬ surements of the hard X-ray flux from Sco X-l have been sparse and inconclusive (see Figure l). Only upper limits have been established above 75 keV, but beginning with the findings of Peterson et al. (1966), all hard X-ray inves¬ tigations have suggested the presence of a spectral com¬ ponent with a frequency dependence different from that of the soft X-ray spectrum (h-g^lO keV; see Section I.A.I.).

This component apparently undergoes temporal variations, -12-

as does the lower-energy component, but more accurate simultaneous measurements of both need to be made to establish the correlation between them.

In summary, It would be highly desirable to obtain additional, accurate measurements of the X-ray and gamma- ray emission of Sco X-l In the region above 30 keV. For this reason, this Investigation was undertaken. -13-

II. EXPERIMENT 69-2

At 0630 local time, November 26, 1969, a 6 million cubic foot hydrogen-filled balloon was launched from Parana, Entre Rios, Argentina. The purpose of the experiment con¬ ducted with the balloon-borne instruments was to measure the flux of hard X-rays and gamma-rays from two sources: a new source in Centaurus, and Sco X-l. The instrument, the ex¬ perimental procedure, and the data reduction procedures used in the observation of Sco X~1 are discussed below. The payload rose to float altitude (^-130,000 feet) by 0820 local time. It then reversed its eastward drift, and floated westward until it was cut down by command from the ground about seven hours later. Figure 6 gives the altitude record for the flight, and the drift of the balloon is plot¬ ted in Figure 7. A. DETECTOR SYSTEM The detector system is described thoroughly in Craddock (1967), Ellis (1967), and various published articles (e.g., Haymes et al. 1968). The Nal(Tl) detector crystal is 10 cm p in diameter and 5 cm thick, with a sensitive area of 75 cm . The central crystal is surrounded by an anticoincidence shield of Nal(Tl) (See Figure 8). The phototube viewing the central crystal is connected via anticoincidence circuitry to a 128-channel pulse-height analyzer, while the phototubes viewing the guard crystal supply the anticoincidence pulses. -Hi-

Fi gure 9 is a block diagram of the detector system. Non-rejected pulses from the central phototube are pulse-height analyzed, and the resulting seven-bit word which represents the pulse’s channel number is put into a register for immediate transmission to the ground station. Two additional registers provide temporary storage when the first is occupied. The purpose of the active shielding surrounding the central crystal is threefold: to provide directionality of response, to suppress the Compton continuum and X-ray es¬ cape peaks (which would otherwise appear in the pulse-height spectrum), and to prevent contamination of the data by un¬ wanted radiation types, such as charged particles. There is a 4- inch thick shield of plastic scintillator in front of the central crystal to detect charged particles entering the detector and prevent their being counted as gamma rays; thus k-r\ charged-particle rejection is provided. Three Schonstedt flux-gate magnetometers were used on the payload to determine the orientation of the detector in the horizontal plane. One was used to drive a servo sys¬ tem for pointing the system in azimuth, and the outputs of the other two were transmitted to the ground. These two were called the "parallel" and "perpendicular" magnetometers according to their orientations with respect to the servo magnetometer. All were mounted horizontally, and as far from ferrous materials as was practically possible. (See -15-

Figure 10) A three-channel FM/FM telemetry system with a carrier frequency of 2^0.2 MHz was used; the subcarrier frequencies and information carried on the three channels were:

a) 3.9 kHz — a commutator, monitoring various on-board systems b) 30 kHz -- the output of the parallel magnetometer c) 165 kHz — the PCM output of the pulse-height, ana¬ lyzer. Included in the housekeeping information monitored by the commutator were: a) the rate of occurrence of rejected events in the central crystal, by means of which the detector dead-time may be estimated, b) the output of a thermal-conductivity pressure trans¬ ducer, c) voltage-supply monitors, d) the output of the perpendicular magnetometer, and e) monitors of the thermostats used to maintain con¬ stant detector temperature. -16-

B. EXPERIMENTAL PROCEDURE Since the declination of the new Centaurus source cor¬ responds to the latitude of Parana ( -31.5°), the detector was mounted in a zenith-pointing configuration for the ob¬

servation of the new source, and was later tilted by command from the ground to a zenith angle appropriate to observation of Sco X-l. A magnetometer-driven servo system was to have pointed the detector alternately north and south during the

Sco X-l portion of the flight, but failed to do so for rea¬ sons discussed below.

The initial energy calibration was accomplished in

Houston. Five gamma-ray sources were used: Na^(5H keV),

Cs137(66l keV), Mn5^(835 keV), Cd109(88 keV), and Am2^1

(60 keV)0 The calibration of the magnetometers, and the alignment of the servo magnetometer probe, were carried out

in Argentina.

During the early portion of the flight (about 1030 local time), a failure of the servo azimuth motor occurred, overloading the 28-volt battery pack. The resulting voltage drop eventually became great enough to cause the high-volt¬ age power supplies for the phototubes to drop out of regu¬ lation, thereby lowering the height of the phototube output pulses. In effect, this altered the energy gain of the pulse-height analyzer. The techniques employed to overcome this problem will be discussed in the section on data reduc¬ tion. -17-

As the voltage continued to drop, the 5 volt supply to the pulse-height analyzer (PIIA) dropped to the point that the PHA clock pulses were no longer gated by the shaped phototube pulse, and the PCM information was lost shortly after 1300 local time. By the time the detector was cut down, it had floated only 75 miles from Parana.

The telemetry data was recorded throughout the flight on f-inch magnetic tape, by means of a CDC recorder. Each of the three channels and the FM/FM signal itself were re¬ corded, as was a time signal from WWV. In addition, the commutator and parallel magnetometer channels were recorded on a Sanborn strip-chart recorder. C. DATA REDUCTION

1. Tape Playback

Upon playback of the data tape, each PCM word was de¬ coded and stored in the memory of a Nuclear Data pulse- height analyzer. During this procedure, the PCM signal had to be monitored on an oscilloscope so that noise was never interpreted by the decoder as a PCM word.

During the flight, WWV reception was sporadic, so a

"fake" WWV signal was recorded onto the tapes. This allowed extrapolation of time from periods when reception of the original WWV signal was good. This "fake" WWV signal was used to time accurately the segments as they were analyzed.

After the playback of a segment, the contents of the Nuclear

Data memory were printed out with a typewriter. -18-

At around 1030 local time, one or more of the cells In the 28-volt battery pack apparently dropped from the pla¬ teau voltage under heavy load, causing the pack voltage to drop to the point that the regulated high-voltage power supplies could no longer regulate their outputs (see Figure ll). There were two major effects of the failure. The azimuth pointing capabilities were of course invalidated, so that the detector rotated freely in azimuth; and the gain setting of the pulse-height analyzer was altered.

Descriptions of the procedures used to compensate for these problems will be followed by a description of the flux calcu lation procedures.

2. Determination of Pointing Direction

As was mentioned previously, the telemetry data in¬ cluded the outputs of the parallel and perpendicular magne¬ tometers; the latter was monitored for about fifteen seconds each five minutes, and the former continuously. From the records of these outputs, the azimuthal coordinate of the detector's pointing axis was determined with an estimated accuracy of between one and seven degrees, depending on the azimuth angle. After the tilt command, the zenith angle of the detector axis was fixed at 22.5° ± 1.0°. The strip- chart record of the magnetometer output verifies that the command was executed. The point representing the projection of the detector axis on the celestial sphere was located by plotting the zenith at Parana as a function of time on the -19-

celestlal map, and then finding the point at a zenith angle of 22.5° at the azimuth heading appropriate to the time. The resulting plot is Figure 12.

3. Energy Recalibration

Normally, temperature- and voltage-regulation assure the validity of a linear gain relation:

E(n) = a(n-l) + EQ 0) where n, is the integer denoting the n^*1 channel of the pulse- height analyzer, E0 is the threshold energy, E(n) is the energy corresponding to the lower boundary of the n*'*1 chan¬ nel, and is the gain constant. Because of the loss of high-voltage regulation, the central phototube had a reduced supply, so that the pulse height for a given scintillation event was also reduced. This had the effect of changing both a. and EQ.

Assuming continued phototube linearity (and this is believed to be a safe assumption on the basis of simulation tests), a gain relationship of the form of Equation ^ will still apply. The two conditions necessary to determine the two constants £ and EQ were obtained by assuming:

1) Constancy of the peak energy of a background feature which remained prominent at all times before, during, and after the gain change.

2) Constancy of total background flux over the energy interval between the center of this feature and the lower boundary of the revised channel 2. -20-

The feature, occurring near 510 keV in this case, may have been observed previously at lower energies (Glenn 1969*

Shafroth 1967). Its energy suggests pair annihilation.

The pre-gain-change data were taken from a background segment of 58 minutes, 51 seconds duration ending around

0930 local time, before the high voltage power supplies dropped out of regulation. The post-gain-change data were taken from the last two ten-minute sepments before the tilt command, when the high voltage had apparently stabilized so that the value of EQ and & were relatively constant. The record of the 28-volt monitor together with the fact that the 510 keV feature was stationary during and after this period, indicate that Equation 4 still applied.

Small variations in the constants probably occurred on a short time scale since the high voltage, though stable, was not constant. (That is, the voltage varied slightly about a fixed point.) The resulting smearing-out of the spectrum was a small effect, however, since the width of the feature was unchanged. The normalized count rate spec¬ tra for the pre-gain-change and post-gain-change periods are given in Figure 13.

The normalized count rate between the feature and the lower edge of the second channel after the gain change was compared to the normalized count rate below the feature in the pre-gain-change data, and the energy that made the count rates equal was taken to be the new lower edge to - 21-

channel 2. (Channel 1 has been eliminated from the anal¬ ysis entirely, because It has been shown to have small threshold variations.) The lcr statistical error In this process corresponds to a 2 keV uncertainty In the lower boundary of channel 2. The revised gain relationship had the constants EQ = 57.2 keV and a = 17.8. 4. Flux Calculation

An assumption implicit in the following calculations is that the background count rate during the source-pointing segments was identical to that of the background segments, so that the difference is purely a source contribution. The validity of this assumption depends upon the following premises: 1) absence of incident solar flare X-rays,

2) absence of azimuthal dependence in the background count rate,

3) absence of temporal background variations, and

A) absence of contamination of background or pointing segments from other X-ray sources.

Solar flare X-rays may have contaminated the pointing data; this is discussed in section II.C,5. No azimuthal dependence of X-ray flux has been demon¬ strated. Fishman (1969) gives 0.7$ as a 2cr upper limit on azimuthal variation at these energies.

The isotropic hard X-ray background at balloon alti¬ tudes has been shown to be constant, since both the diffuse -22-

cosmlc X-rays and the Isotropic cosmic rays have little temporal variation (Glenn 1969., Hooper 1964, Arnold et al. 1962). The X-ray fluxes of the sources Nor XR-2 and Sco XR-2, both of which were viewed by the detector during the back¬ ground segments, are less than 8% of the Sco X-l flux at low energies (Fisher et al. 1967) and probably represent an even smaller proportion at the energies observed here.

As can be seen in Figure 12, the pointing azimuth angle varied slowly after tilt, passing first through south and west to northwest, reversing twice, and passing close to

Sco X-l. The thirty-five minutes of observation time were divided into 5~minute segments, differentiated as "back¬ ground" segments (A, B, and C), "pointing" segments (F and G), and intermediate segments (D and E). The intermediate c segments were omitted from the background time because they represent a significant exposure to Sco X-l, and they were considered separately from the pointing time because their effective exposures to both Sco X-l and the sun were sub¬ stantially different from those of F and G.

Residual conting rates were calculated for each chan¬ nel by subtracting the sum of the background segments, nor¬ malized to five minutes, from the normalized sum of the pointing segments. This process was repeated for segments

D and E. The results are given in Figure 14.

The calculation of these residuals and their standard -23 deviations was carried out with the help of a Hewlett-

Packard 9100 electronic calculator, which was also used for the flux calculations described below. The photon flux (photons/cm2sec keV) corresponding to the normalized residual count rate Rn In channel n is given by B prn = C ^n n B e An(h,y) n( ) where I is the instrumental correction factor, including corrections for absorption and photofraction ratio; A(h,y) is the atmospheric absorption correction, a function of the atmospheric depth h and the source zenith angle y.

An(h,y), for small y, is given by

An = exp(-h/rncos y), where h and the optical depth rn for a photon in channel n

2 are in gm/cm . The optical depth rn was obtained from a graph given by Fillius (1965)* and h was determined from a profile of the payload’s altitude recorded from an NCAR aneroid device (see Figure 6). Bn(e), the off-axis cor¬ rection factor, compensates for attenuation due to the active shielding when the source is at a separation angle 9. This has been measured at several energies, and is given for the appropriate range of angles in Figure 15.

The instrumental factor I has also been experimentally determined, and is given in Figure 16. This factor com¬ pensates for absorption by the central phototube and asso- -24- ciated apparatus, and also corrects for losses due to K X-ray and Compton escapes from the central crystal. For hp>500 keV, values for the total correction were determined from an extrapolation of the measured absorption curve, and efficiency data from Neiler and Bell (1965). The constant C converts counts per five minutes per sensitive crystal area per channel width a. to photon flux (photons/cm^sec keV). The photon flux is multiplied by the median photon energy to obtain the energy flux (keV/cm2sec keV). The separation angle 0 is given in Figure 17 for the pointing segments. P/C was calculated for each channel, and for hypothetical pointing errors of 0° and ± 1°. The re¬ sulting energy fluxes for segments F and G are plotted in Figure 1; the estimated angular uncertainty of 1° would cause the changes in the measurement indicated by the top and bottom edges of the black rectangles. (The statistical error bars have been extended from the central point, not from the edges of the rectangles.) These data are also given in Table 2.

Although the data nominally extend from 75 keV to over 2,000 keV, gamma-ray fluxes were not calculated for energies beyond 1,000 keV. For energies greater than this, the photopeak efficiency of the detector and the collimating ability of the guard crystal decrease, so that it becomes impossible to determine accurately the energy or direction of detected photons. -25'

5. Possible Solar Flare Contamination The sun was within the detector's field of view dur¬ ing the pointing segments; Bp(9) varied between .O^J and .50 during this period. The solar separation angle Is plotted

In Figure 17. ESSA records show a normal-type solar flare In progress at the time of the experiment (ESSA Comprehensive Report,

May 1970). The solar X-ray fluxes measured by the SOLRAD

9 satellite (ESSA. Report, May 1970) Indicate that a solar X-ray flare coincided with the general event, and one might expect a detectable solar contribution to the residual count rate in the lower channels of this experiment. A discussion of this difficulty follows.

The solar X-ray flux measured by the SOLRAD experiment during the pointing segments is shown in Figure 18. The times of the pointing and intermediate segments are indi¬ cated by the shaded areas.

The flux of hard solar X-rays has been investigated by means of satellite-borne detectors (Hudson et al. 1969,

Kane 1969, Frost 1969, Kahler et al. 1970). The X-ray spectrum of a solar flare is apparently thermal below about

15 keV and either non-thermal, or thermal with a higher characteristic temperature, above this point. Hudson et al. find that the possibly non-thermal radiation dominates dur¬ ing the short beginning phase of a flare, and that the flare's longer decay period is dominated by softer thermal -26 emission. An example of a solar flare X-ray spectrum, ob¬ served by the OSO-III satellite, is given in Figure 19

(Hudson et al. 1969). Kane (1969) observed two types of flare, one with the two-component structure seen by Hudson et al., and another type with only the soft component. In the first type of flare observed by Kane, the hard compo¬ nent had a duration of only about 30 seconds, while the soft component lasted for minutes. If the flare observed by SOLRAD during this investigation was of this type, one would expect a negligible total contribution above 75 keV over the ten minute pointing periods. However, given the present state of knowledge of solar X-ray flares, it would probably be very unwise to attempt any sort of quantitative extrapolation from the SOLRAD data alone.

We can state with complete confidence only that the solar flare could have contributed either a major or a minor portion of the X-ray flux observed. However, it appears that most solar flares giving the low-energy flux observed during the experiment by SOLRAD 9 would not have produced significant contamination above 75 keV. Thus, although the data presented in Figures 1,2, and 5, and in Table 2 can be strictly interpreted as upper limits on the hard X-ray flux, from Sco X-l, in the following section we will consider the data to be true measurements. TABLE 2: ENERGY FLUXES

IN UNITS OP 1 X 10“2 keV/cm2sec keV O H CD W Pq O I k-w EH + ^ ft rH g 63 -H p b ft; iz> y a X *H ft «H H CM ft 0 ft u t>'O rHCMCO-=TLHVOb-CT\ 00 ft ^ O .*H COOOvCMVO0-=rOVVOCOrH CD rH H-l •P S Ctf -=rCMCOCO■TOq'COOHHO^rO] CO Oi—ICMt"1Tr—*^T-~T avCMHCOCM^TCMVOVOCOCMH-^rCO mt—tnrHVO !>-(OC0OAVOl/wcorH ino\rH LT\0-=J-OMP\cr\ro^rvoirw rH O LT\ COCO (OCOCMCMCYliHrHHCMCMCMCMCOCO CM COrHCMrHtHrHCMCMCMCM CM -^rOVCC-)LT\C'-CO-=TCOCOVO0\ CM LOCOVOCOOVrHC-OVCMmO'i CM COVO COCO CMrHCUrHrHiHCMCMCMCMCOCM C"-- OVC—rH00tCMOVVOIfW tncoLniHvo CMt^H-=rmi>-vomco CM COCOLTit^OVCMmavcooo(0(0(0-=J- • % CM VOCOI| I HrHCMCO-=TIT\LO rH CMCOCO-=TLfWOOO rH r-lCMCO^T O covoo-=Tow-=r

1001 -27“

III. RESULTS AND CONCLUSION

Table 2 and Figures 1 and 2 present the results of this experiment. While these data may represent partial contamination of the measurement by solar X-rays, as upper limits they are compatible with those of Agrawal et al. and

Buselli et al. (see Figures 1 and 2), and do not disprove the existence of the hard component above AO keV.

If, as is suspected, the solar contribution was neg¬ ligible, these results are the first positive measurements of fluxes from Sco X~1 at energies above 75 keV, and are very strong evidence for the existence of a hard component.

We note that neither a single exponential nor a single power law is a good fit to these data together with the low-energy (5 keV kO keV) data. The synchrotron spec¬ trum calculated by Manley and Olbert (1969) predicts at

100 keV an energy flux of about 10“3 keV/keV-cm^-sec. This is a factor of twenty and three standard deviations below the flux measured at 110 keV. As mentioned previously, the approximate dependence of the thermal bremsstrahlung and the Manley and Olbert synchrotron spectrum are exp(-hv»/kT) and exp(-C(hv')°*^) respectively, so that a bremsstrahlung fit to these same data would be even less satisfactory. Thus it seems probable that the data of this experiment, along with the data of Buselli et al. (1958),

Riegler et al. (1970), Agrawal et al. (1969), and Lewin et -28-

al. (1968) represent detection of a spectral component in¬ dependent of the component responsible for optical and lower energy X-ray emission, as proposed by Riegler and Ramaty (1969). We discuss below the status of the close-binary hy¬ pothesis (see Appendix), and how these results relate to it. One objection to the close-binary model of Sco X-l lias been the identification of the optical object with the

Scorpio-Centaurus association on the basis of its proper motion (Gatewood and Sofia 1968). Sco X-l has an apparent H-R position in the subdwarf region, which precludes the possibility of its having a main-sequence or giant compo¬ nent. Gatevjood and Sofia conclude that it is a white dwarf or a neutron star, possibly a supernova remnant.

Their analysis, however, does not take into account the self-absorption indicated by the findings of Chodil et al. (1968).

Another objection to the close-binary accretion hypoth¬ esis is that the amount of infalling matter necessary to explain the X-ray luminosity of the source should effective¬ ly attenuate the X-ray and optical continua, giving rise to a "thick-source" spectrum which would not resemble the one observed (Manley and Olbert 1969). This objection disre¬ gards the thin-ring nature of the accreting matter. The source sould be thick only in the equatorial plane of the

system. If Sco X-l is a close binary, the earth cannot be -29- near Its equatorial plane, since we do not observe the periodic light curve of an eclipsing binary.

Manley and Olbert (1969) also claim, correctly, that the accreting ring proposed by Prendergast and Burbldge (1968) fails to generate the high temperatures (10^-10^ °K) needed to account for Sco X-l by thermal bremsstrahlung.

However, as Prendergast and Burbidge pointed out, higher temperatures would be expected in the region near the surface of the star, where thermal energy contributed by the angular momentum transfer from the rotating disk would be large but very difficult to estimate.

The initial measurement of very soft X-rays (hv^l keV) (Friedman et al. 1966) was much higher than would be ex¬ pected from a thermal bremsstrahlung source. Subsequent

investigators (Grader et al. 1970) did not detect this ex¬ cess, and the original investigators have concluded that calibration error is responsible for the apparent excess

(Fritz et al. 1968). Additional soft X-ray measurements are needed to determine conclusively which of the mechanisms is responsible for the X-ray output.

The optical continuum of Sco X-l is almost certainly due to thln-source thermal bremsstrahlung, since there is negligible polarization (lliltner et al. 1967)* and the

source has had roughly the same luminosity for 70 years,

much longer than the expected lifetime of the synchrotron source proposed. -30-

There Is a striking similarity, in shape and duration, of the characteristic light-curve variations noted by Wilson and Twigg (1970) and the light-curve calculated by Bath

(1969) for a semidetached binary undergoing mass transfer. (See Figure One might conjecture that a burst of re¬ combination radiation, which Bath predicts but has not taken into his calculations, provides the short flare-like struc¬ ture superimposed on the otherwise smooth curve of Wilson and Twigg.) More sensitive spectrophotometric studies might prove or disprove this idea.

If this comparison of light-curves is a meaningful one, then it is additional independent evidence that Sco X-l is a semidetached binary emitting thermal bremsstrahlung. Of course this does not preclude the presence of synchrotron radiation mechanisms operating in the vicinity of Sco X-l, and as was discussed previously, there is increasing evidence that this is the case.

It is intriguing to speculate that Sco X-l may once have been a contact binary losing mass to an outer ring

(cf. Kuiper's model for (3 Lyrae), in which the radio and hard X-radiatlon originate by a synchrotron mechanism.

The results of this experiment seem to confirm that the X-ray emission spectrum of Sco X-l has two components.

These results do not imply or deny the validity of the close-binary hypothesis, but in the light of this evidence, it seems desirable to determine theoretically how a close- -31- binary system could produce such a two-component spectrum. -32-

IV. APPENDIX

A. GENERAL CONSIDERATIONS AND MODELS

Assuming that the low-energy X-ray and optical spectra of Sco X-l are due to bremsstrahlung radiation from an iso¬ thermal plasma, one may Infer the values of certain source parameters. Such an Interpretation of the spectrum Is not unanimous (cf. Manley 1966, Manley and Olbert 1969, and the section on the synchrotron model for Sco X-l, which fol¬ lows) .

The approach taken by Chodil et al. (1968) and others is to determine, for a given distance d. to the source (and an amount of interstellar reddening derived from this dis¬ tance), the amount of free-free absorption necessary to re¬ duce the optical continuum to the observed level, assuming that this optical continuum is due to the same thermal brems¬ strahlung mechanism as the X-rays (see Figures 2 and 20).

This self-absorption, with the temperature inferred from the form of the X-ray spectrum, fixes for an isothermal spheri¬ cal source the value of the plasma electron density ne and radius R.

Integrating the modified Bessel function approximation to the spectral intensity (Equation 3)* and setting the re¬ sult equal to the estimated total intensity inferred from the measured spectrum, one obtains n/R3 = C 1> (5) d2 -33- where is a constant (C^ = 101? for the Chodll et al. data.). The optical depth for the case of attenuation by free-free absorption and electron scattering takes the form

(Tucker 1967)

r(E) = (3ksckff)^R - C2n§R, (6) where C2 is calculable (see Chodil et al. 1968) and (E) must supply the attenuation necessary to match the observed optical continuum spectrum after interstellar reddening is taken into account. If one assumes a value for the dis¬ tance d., the two relations (5) and (6) constitute a set of

simultaneous equations with ne and R as the unknowns. An estimate of the distance to Sco X-l can be made by assuming its absolute magnitude to be that of a typical old nova; this places jd between 230 and 1000 pc. Estimates may also be based on the strength of the interstellar Call absorption line in its spectrum, on the amount of hydrogen in the line of sight, or on the estimated X-ray optical depth (for this, see Gorenstein et al. 1968). These esti¬ mates are discussed by Westphal, Sandage and Kristian (1968), and their conclusion is that 300 pc < d ^ 1000 pc. For distances in this range it follows that R - 109 cm,

- ne — lO^^cm ^^ 1 and M -10“ 3M@

where R is the radius, n0 the electron density, and M the -34- mass of the idealized source (see, for example, Chodil et al. 1968, Mark et al. 1969, Neugebauer et al. 1969, and

Kltamura et al. 1970). (A search for X-ray emission lines could provide a test for emission by thermal bremsstrahlung, since the two should coexist. Tucker and Gould (1966) have calculated fluxes expected from a hot, low-density plasma. Tucker

(1967) shows that X-ray emission lines may be expected and that their intensities should provide an observable indi¬ cation of elemental abundances in a thermal X-ray source.

He calculates X-ray spectra for three such sources with different abundances. Detection of these lines would in¬ dicate the presence of a gas hot enough to emit thermal bremsstrahlung, and thus constitutes a test for thermal bremsstrahlung when the shape of the continuum is uncertain.)

In the construction of a model for Sco X-l, all of the observed features of its electromagnetic spectrum must be

taken into account. Before we consider models for the

source, we list the most salient features: l) There is a variable, approximately exponential

spectral component which dominates the soft X-ray region (E ^ AO keV), and possibly also the optical region, and

which closely matches the spectral output expected from a

hot plasma (KT =* 7 keV) which is thin at X-ray energies. The X-ray and optical ranges of this component are at least

roughly correlated in intensity changes. -35-

2) The optical variations are apparently non-periodic, but there are several characteristic types of variations

(e.g., "flickers", "flares") which occur at irregular intervals.

3) Additional components may exist in the microwave and hard X-ray regions.

*0 The optical line emission appears to be like that of old novae and U Gem variables, indicates complex motions, and may originate in a low-temperature (T — 10^ °K) gas decoupled from the hot plasma presumably responsible for the varying X-ray output.

Manley (1966, Manley and Olbert 1969) and others

(Buselli et al. 1969, Hiegler et al. 1970) have suggested that synchrotron emission is responsible for at least part of the electromagnetic radiation from Sco X-l. If synchro¬ tron emission is not responsible for the X-rays from Sco

X-l, then bremsstrahlung emission probably is the cause. A simple black-body emission curve is a very poor fit to the X-ray data, although Meekins et al. (1969) report that a composite of bremsstrahlung and black-body spectra can be made that fits the data as well as does thermal bremsstrah¬ lung alone. A mechanism for accelerating electrons is nec¬ essary for a synchrotron model, and a method of plasma heating is necessary for a thermal bremsstrahlung model.

It has been proposed that the source of plasma heating may be shocks from stellar pulsations (Cameron 1966, Westphal -36- et al. 1968). Thermodynamic arguments indicate that such shock-heating is too inefficient to explain Sco X-l's

X-ray luminosity (Cameron and Mock 1967,, Manley and Olhert

1969, Burbldge 1969). Another possible energy source for plasma heating is the gravitational potential energy available in a close binary system undergoing mass transfer from one component to the other. This model will be dis¬

cussed in some detail below, following a brief discussion of the synchrotron model.

B. THE SYNCHROTRON MODELS Manley (1966) has shown that an approximately expo¬ nential spectrum can result from synchrotron radiation when

the relativistic electrons have a sharp energy cut-off.

Manley and Olbert (1969) postulate for Sco X-l a synchro¬

tron mechanism involving an optically thin dilute plasma

with large-scale magnetic fields, and a source of MHD

"noise". It is shown that plasma inhomogeneities may

develop and maintain themselves through radiative instabil¬

ity (Alikhanov 1967) and that through a non-trivial gener¬

alization of the Fermi mechanism, a steady-state distribu¬

tion of relativistic electrons is provided. The resulting

photon spectrum is approximately proportional to the factor

exp (-cOn))0-6), where C is a constant dependent only on source parameters. Over the X-ray range investigated, it

would be difficult to distinguish this spectral shape from

that due to thermal bremsstrahlung. In particular, more -37 very soft (hv> ^ 1 keV) X-ray measurements are needed. The investigators claim that their model can account for short-term fluctuations over time scales of minutes, as well as longer-term variations. The lifetime of such a source at the present rate of X-ray production, however, should be only about J]0 years. Riegler and Ramaty (1969) propose a hybrid model, with radio and hard X-radlation emanating by synchrotron emission from a region containing tenuous plasma. This region, of radius ~10J> cm, encloses a smaller, denser object (R ^ lO^cm) radiating thermally. The dense core is held responsible for the optical and soft X-radiation. The model was intended to reproduce the Sco X-l microwave radiation, which is much higher in intensity than would be expected from either bremsstrahlung or synchrotron emission chosen to fit the optical and soft X-ray spectra. The hard X-ray spectrum has not been investigated in the light of this model, however, and the characteristic times for synchrotron energy losses turn out to be much greater (~10^ sec) than the observed flux variation period of ~10^ sec. Riegler and Ramaty point out that shorter-period variations could be explained by a process other than synchrotron losses.

C. THE CLOSE BINARY MODEL Because a close binary system could apparently provide most, and perhaps all, of the observed characteristics of Sco X-l's electromagnetic emissions, we will consider the -38' close binary model at some length. We examine in the following sections a) the mechanics of close binary systems, b) the conditions under which mass transfer or ejection may occur, and c) the flow of the re¬ sulting gas streams. We further investigate the compati¬ bility of.obervations with the X-ray and optical output expected from such a binary model, and we make suggestions for further development of the theory, 1. The Mechanics of Close Binary Systems In a discussion of close binaries, it is convenient to consider the equipotential surfaces in the rotating refer¬ ence frame. Critical surfaces are those including the inner

Lagrangian point and the outer Lagrangian point L2 (see Figure 21). It is important to remember that the set of equipotential surfaces is determined only by the ratio of the components' masses and their separation. It is at L-^ that mass first escapes the potential well of an expanding star and falls into the well of its denser companion. It is

at Lg that mass first escapes the common potential well of a contact binary when its common envelope expands sufficient¬ ly, The portion of the Lagrangian surface through which surrounds a single star is its Roche lobe. These curves are sketched in the x-y plane in Figure 21 for 0j\. A con¬ tact binary is defined as one in which both components fill their Roche lobes; a semidetached system is one in which only one star has Its Roche lobe filled. Close binary -39-

systems were first treated by Moulton (191^) and, more ex¬ tensively, by Kuiper (l9*ll), who attempted to explain the light-curve and spectral characteristics of the eclipsing binary yOLyrae, of which we shall speak in more detail later. Chandrasekhar (1933) demonstrated that a superposition principle may be applied to rotational and tidal distortion for a polytrope with n = 3 in a binary system, and found expressions in terms of Lane-Emden functions for the den¬

sity distribution under those conditions. Since

Chandrasekhar’s results are based on the approximation

that the sixth and higher orders in V = (where R n is the separation of the components) may be neglected, they seem to have limited applicability to close binary

systems. Nonetheless, Kuiper applied them to contact bi¬

naries in his work, reasoning that since ninety per cent of

a star's mass lies within a sphere with half its radius Chandrasekhar's condition is satisfied.

Consider a binary system, the larger component of which

we designate A and the smaller component B. Let us take a rotating coordinate system with the origin at the binary

system's center of mass, the x-axls extending through the

mass centers of each component, and the z-axis parallel to

the rotation axis of the system. Then Lj and L2 may be determined by the equation for the potential energy in the

rotating frame, ~/|0-

-a~ lit + JtL + x yg r l r2 2 together with the conditions

/dA\ fdja\ Q where x^ s x(L^) and

\dx )Xl“ °’ (dx / x 2 x2 = X(L2) ,

Kuiper concludes from Kepler's Law,

2 2 ^ (l-|>i) a = constant, where ^=MB and MA + MB= 1, that as mass is transferred from one component to the other,

(as increases) a must decrease. For instance, if at the beginning of the mass transfer process, JA = 0.25, and a = 1, then at the end of the process (i.e., - 0.5), a = 9/16. Furthermore, increased rotational angular momentum should decrease the orbital angular momentum (and separation) to an even greater extent than that implied above. It seems obvious that such a decrease in separation would cause a decrease in the values ofifl(L^) andl^Lg). This would in¬ crease the rate of mass transfer, and, in Kuiper's model of

|3Lyrae, would lead to mass loss through L2 before the pre¬ sumably stable situation of j^~ 0.5 were achieved.

Kuiper argues that for constant angular velocity u>on a given equipotential, where Jl is constant, P and must also be constant. It is clear that P and p cannot be con¬

stant on the same level surface in both stars. Thus OJ can¬ not be constant, and flows of mass must occur.

We have shown that mass transfer between components of ->n-

semldetached and contact binaries can occur, and that mass

may be ejected from a contact binary system. It remains to be seen whether the necessary instabilities can occur, and

over what time scales transfer may be sustained.

2. Instabilities Over Thermal and Dynamic Time Scales

The question of the sort of instabilities that are re¬

quired to sustain mass transfer was apparently left unan¬

swered until i960, when Morton investigated evolutionary

mass exchange. His investigation employed computer-evolved stellar models.

Spectroscopic evidence indicates that in binaries with

a Roche lobe filled, it is the less massive component that

has its lobe filled. Because one would expect, from the

standard notions of stellar evolution, that the primary

component would fill its lobe first, Morton concluded that

the primary has already undergone mass loss to the secon¬

dary, and that the process of mass transfer is so brief as

to be unobserved.

If it can be shown that while a star fills its Roche

lobe, a decrease in mass will lead to an increase in radius,

then the situation is obviously an unstable one, so that

mass loss through will persist at least as long as

Mass loss will actually last longer than this condition

holds, since the Roche lobe is shrinking during the process, but this condition is a convenient criterion for instability to mass loss.

The time scales over which such an instability might be expected to occur correspond to the thermal and dynam¬ ic (or hydrostatic) relaxation times, which have the fol¬ lowing values for a star with M = 7M0:

"Kelvin time" = TK c; 1C)5 years

"Pulsation time" ~ \ ~ 8 X ltT^ year.

Morton discounted the possibility of such an instabil¬ ity to mass loss in a zero-age main-sequence star, since a) a homogeneous star has

> 0 dM and b) a homogeneous star could not expand to fill its

Roche lobe initially. Consequently, he attempted to find the appropriate instability by examining linearly-perturbed inhomogeneous stellar models with M = 1OM0. His models indicated that instabilities could not occur over a dynamic time scale, but that they could over a thermal time scale, which was a sufficiently small time to explain the dearth of observed binaries with the primary's lobe filled.

Among Morton's approximations, however, were a) spher¬ ical symmetry, which clearly cannot apply to the pertinent outer layers, and b) y = 5/3 throughout. Paczynski (1965) and Paczynski, Zialkowski, and Zytkow (1969) have demon¬ strated that for outer convective layers of polytropic Index n = 1.5, dynamic instabilities could exist. Bath

(1969) suggests that surface layers of partially ionized

H and He could be ejected over a time of xp, and he has calculated, for a number of initial models with incom¬ pletely ionized surface shells, the mass loss and luminosity curves. The luminosity curves he obtains fall Into two categories: those greatly resembling the light curves of U Geminorum variables, and those greatly resembling the light curves of novae. The H-R tracks of Bath’s models during their mass loss are shown in Figure 23, and the approximate light curves for two of the models in Figure 2k

(Figure l*Jb includes an estimated time scale). Figure 22 gives the positions in the Hertzsprung gap that Morton has determined must be occupied by stars of various masses in order for those stars to overflow their Roche lobes.

Bath points out that, considering the small amount of mass ejected due to an Instability in one of his Type I models (the ones giving rise to U Gem type outbursts), and the inevitable restoration of the surface to its initial condition after mass loss, that the process could repeat

itself for a large number of cycles. This makes his model even more appealing as a candidate for a U Gem variable.

Stellar evolution in a close binary system could

clearly be a highly complex affair. Kippenhahn, Kohl, and

Welgert (196?) consider the evolution of a binary in which

the primary transfers mas3 to the secondary as it reaches -Ml-

the red giant phase, after which It becomes a white dwarf with a hydrogen-rich envelope. The secondary then evolves

to the giant stage and transfers mass back. Presumably, If the components of a binary system have

similar masses, they may evolve at roughly the same rate, giving rise to a contact binary. Lucy (1968), In his

study of contact binaries, points out that despite Kuiper's argument to the contrary, zero-age binaries with unequal

components may exist. He uses such a system as a model for

W Ursae Majoris stars, meeting with limited success.

3. das Streams in Close Binary Systems At this point, it is necessary to investigate the dynamics of appropriate gas streams so that we may a) es¬ timate the spectral characteristics and the intensity of

X-ray production from a semidetached system, b) determine the effects gas streams may have upon optical emission, and c) assess the effects of accretion on the evolution of component B.

Kuiper, Kopal (1956, 1957), Gould (1957, 1959), and

Prendergast (i960) have studied the problem; the first three

investigators relied on particle mechanics to determine orbits, but Prendergast showed that the mean free path appropriate to the situation is only 1 to 10 km. The

studies of Crawford and Kraft (1956) and Kraft (1958, 1959) indicate that p — 10 10 cm and Prendergast takes a col¬ lision cross-3ectlon of lO”^ to 10" ^ cm^. He computes the flow pattern for close detached systems, but the results are consistent with Kuiper's flow pattern near the smaller component of a semidetached system. A summary of Kulper's treatment follows.

Consider the Jacobi Integral:

where v. is the velocity of a particle ejected through the

2 Lagrangian point Lg. For small vQ, vQ /2 will be small, so that 0o is only slightly less than the C corresponding to v = 0. Since v2>- 0, we have + 2i±if r2 2 which defines forbidden zones for ejected particles. In

Figure 25 the forbidden and allowed regions are indicated for both types of mass loss; loss of type A, through in a semidetached system, and loss of type B, through Lg in a contact system. The dotted lines indicate the larger region available to faster particles, for which the above approximate treatment is less satisfactory.

Kuiper finds that type B ejection will result in a

"tail" spiraling out to form a disk about the system.

This "tail" could easily explain the assymetric light curve of p Lyrae. A disk about the companion star should be formed at a radius of about 0.2R for 0.4 In the case of type A mass los3. Kulper concluded that the particle orbits, perturbed by component A, would have precesslng lines of apsides, leading to collisions and averaging of orbits into circles. Prendergast and Burbidge (1968) verify this result by solving coupled hydrodynamic and radiative- 88 transfer equations for a star of mass M = 10-^gm and radius

R = 1,5 X 10-^ cm, with a mass flux of 2 X 10^9 gm-sec“-*-.

Their results are as follows: T = l.ij X 106 x"1,05 °K

cr = 10^ x~°*9 gm cm“2

vtan = 6.7 x 1q7 x~°'5 cm sec-1 1 1 1 Total flux = 3.47 X loS^x^ *^ -X2“ ^5) ergs gee" , where x is the distance to the center of the star in units of 1,5 X IcA^cm, and x^ and Xg the distances from the stel¬

lar center to the ring's inner and outer edges respectively.

Prendergast and Burbidge make a crude estimate that half

the gas lost from the A component will accrete onto B, the

other half returning to A or escaping the system entirely.

4. Close Binaries As X-Ray Sources Shklovsky (1967), Cameron and Mock (1967), and others

have suggested that Sco X-l is a semidetached system in

which mass is transferred from a giant or main-sequence star to a compact companion, possibly a white dwarf or

neutron star. That accretion is capable of supplying an

X-ray luminosity of 4 X lO^erg sec-'*' is easily seen. An upper limit to the rate of mass accretion is given by the -'-ir¬ radiation stress condition,

NO2:T L = GM_ c l\rtR2 R2 ^ where yue is the mean mass per electron, a; is the Thomson scattering cross-section, and L is the luminosity due to mass accretion. For a star with M = 1M@, this yields a

^ O *1 maximum luminosity of 1.7 X lCP0erg sec" . Shklovsky's neutron star/giant system was shown by Cameron and Mock to emit a thin-bremsstrahlung spectrum of higher characteristic temperature than that observed from Sco X-l, and they proposed a model consisting of a white dwarf and a main-sequence star. Though Cameron and Mock carefully chose a white dwarf of 0.2 solar mass, Prendergast and Burbidge (1968) point out that the correct temperature may be obtained through any binary model with the correct primary mass-radius ratio, since the maximum temperature that can be reached by gas falling onto a star is

Te ^ 10? ^ R M0 . The model of Prendergast and Burbidge, intended specif¬ ically to fit the source Cyg X-2, predicts optical emission from the outer regions of the disk encircling the B compo¬ nent, and X-ray emission from the inner regions of the disk. The disk undergoes a continuous transition from opacity domination by absorption to domination by electron scattering, the transition occurring roughly at the radius where the product of the optical depths is unity.

This model cannot predict the X-ray flux quantitative¬ ly, since the equatorial velocity of the primary's surface

is unknown, and knowledge of the angular momentum transfer at the star-disk interface is essential to such a pre¬ diction,

5. Conclusion

We have examined the mechanisms necessary for X-rays to be emitted by close binary systems. It has been demon¬

strated that a) mass will be transferred from one star of a close binary to the other if the first component fills its Roche lobe, b) such transfer can be sustained over short or long time periods when the dynamics of the star's

surface satisfy dR/dM < 0,

and c) in semidetached systems, bremsstrahlung X-ray emis¬

sion will result from the interaction of the smaller star's

atmosphere with the gas falling into it. FIGURE CAPTIONS

Figure 1. Hard X-ray energy fluxes observed from Sco X-l.

Note wide variation in flux levels, and evidence for

hard component beyond AO keV. The solid line is ex¬

trapolated from low-energy measurements.

Figure 2. Representative Sco X-l X-ray and gamma-ray

energy fluxes observed between 5 and 1000 keV.

Figure 3. Typical photometric record of B (from Mark et

al. 1969).

Figure A. (a) Sections of photometric records of B for two

nights, superimposed to show an apparently character¬

istic feature of the Sco X-l light curve (from Wilson

and Twigg, 1970). (b) Light curve calculated by Bath (1969) for a

close binary with MA = 1M@. Note similarity in time scale with Figure A(a).

Figure 5. Representative fluxes from Sco X-l from all

spectral regions. Range of apparent temporal vari¬

ations are indicated by open rectangles. For explana¬

tion of boxes in optical region, see caption to Figure 20.

Figure 6. Altitude record of Flight 69-2, from NCAR

aneroid altimeter. -50-

Figure 7. Path of balloon during Flight 69-2.

Figure 8. Cross-section of detector. (Phototubes viewing Nal guard crystal and plastic shield are not shown.)

Figure 9. Block diagram of detector system.

Figure 10. Cross-sectional view of experimental payload, tilted for observation of Sco X-l.

Figure 11. Record of the 28-volt battery pack monitor, Indicating the decline in pack voltage between 0920 and 1110 local time.

Figure 12. The path of the central axis of the detector as projected onto the celestial sphere. The scale in the center represents the position of the zenith at Parana at the local times indicated, and the bracketed points represent the pointing direction of the detec¬ tor. Bars indicate uncertainty in azimuth.

Figure 13. Normalized background count rate plotted against channel number for a 58 minute period prior to gain change, and for a 20 minute period just prior to the tilt command. Constancy in energy of the feature indicated by the arrows was assumed for the energy recalibration.

Figure 1*1. (a) Residual count rates for segments (D -f E) -51-

(b) Residual count rates for segments (F + G). The five-minute normalized count rates for pointing

and background segments are subtracted to determine

residuals.' The count rates are uncorrected for at¬ tenuation.

Figure 15. Angular response of detector (fraction of pho¬

tons detected which are incident at angle 0 from cen¬

tral axis of detector).

Figure 16. Instrumental correction factor.

Figure 17. Separation angle 0 for sun and Sco X-l during pointing segments.

Figure 18. Record of solar X-ray flux as measured by the

SOLRAD 9 satellite during this experiment. The times

of the ten-minute segments (D + E) and (F + G) are indicated by the shaded regions. The energy ranges are 0.6-1.5 keV (8-20 A), 1.5-i2.il keV (l-8 A), and 4.1-24.8 keV (0.5-3 A).

Figure 19. Record of solar X-ray flare observed by 0S0-III. (Hudson et al. 1969). This event, associated with a flare of importance 3", produced a much larger X-ray flux than the event which took place during this investigation. The apparent flux decrease between 0030

and 0130 in lower channels is a spurious effect due to saturation of the detector

Figure 20. X-ray and optical data obtained during the May

9, 1968, observation. Solid line passing through the X-ray data points is thermal bremsstrahlung emission

according to eq. (l) of Chodil et al. (1968) from a

plasma with no self-absorption and a temperature Id? = 7 keV. Dashed box indicates limits of the cal¬

culated emission spectrum from the plasma when free-

free absorption occurs in the plasma. Solid box

indicates limits of the calculated optical intensity

after the emission spectrum with free-free absorp¬

tion has been modified by interstellar extinction.

(From Mark et al. 19

Figure 21. The critical Lagrangian surfaces in a close

binary system. (Rotation is about the z-axis, which

leaves paper through the center of mass.)

Figure 22. A star of the given mass in a system with the indicated separation will overflow its Roche lobe'

only when it has evolved to the right of the zig-zag

line. (from Morton i960)

Figure 23. The H-R tracks of Bath’s models for perturbed

dynamically unstable stars in semidetached systems, (from Bath 1969) -53-

Figure 2^1. Change of bolometric magnitude with mass loss

m, for two 1 M0 models of Bath (1969).

Figure 25. Cross-section of Lagrangian surfaces of close

binary system in plane of components and rotation axis. ENERGY hLUX (keV/cm2 sec keV ) PHOTON ENERGY(keV) Figure I BUSELLI ETAL(1968)2/68^ ENERGY FLUX (keV/keV-crrr - sec) 10 10 io‘ 10 10 r I -2 i I "III| 10 I0‘ Figure 2 hz/(ke V) J It-J-Ll- i— iir —i HUDSON ETAL6/67. HUDSON ETAL6/67' PRESENT EXPERIMENT-: LEWIN ETAL3/69 AGRAWAL ETAL4/68' AGRAWAL ETAL12/68- CHODIL ETAL9/67; PETERSON AND BUSELLI ETAL2/68. AGRAVVAL ETAL4/69 JACOBSON 6/65 (NON-FLARING) (FLARING) i l i r 11/69 - JL_L 10^ f96Q MAY 19 UT 0

0-2 ;*»- v C» > 9 JH J,* •* i /*<

* •* mm* 0-4

*A.> *...... ' V; •*. vArV^4A 'V : V*%* X 0G

0 004 0-08 0-12 016 0 20 At N X LO

o

o Figure

r

("3) xmd A9d3N3 903 ' LOCAL TIME, NOVEMBER 26,1969 0520 0600 0800 1000 1200 1500

'id £oi x danuriv o CO UJ X o z

00 Figure

—kr T

CL co

CD k- 3 U> L TORQUE MOTOR

MAGNETOMETER PROBES

THERMAL INSULATION DETECTOR PHA ELECTRONICS TRANSMITTER NCAR PAYLOAD BATTERY PACK

BALLAST HOPPER TELEMETRY ANTENNA

Figure 10 LOCAL TIME AT PARANA 0900 1000 1100 1200 1300

SIIND Ayvaiisyv RIGHT ASCENSION (HOURS)

(S33H93Q) N0I1VNTO3CI 180 (SBinNiw s/siNnoo) 3ivaiNnoo

CHANNEL NUMBER RESIDUAL COUNT RATE (COUNTS / 5 MIN.) RESIDUAL COUNT RATE (COUNTS / 5 MIN. CHANNEL NUMBER Figure 14 a b FRACTIONAL RESPONSE OFF-AXIS ANGLE(DEGREES) Figure 15 PHOTON ENERGY (kev)

(yoiovj N0liD3!jy03) I UT, NOVEMBER 20,1971

S33U93Q Nl 319NV NOI±VdVcl3S -8 FLUX

0-3 FLUX ergs/cm2sec 8-20 FLUX Figure 19 Figure 20

(D8S-zuiD-Ao>1/Ao>i) AilSN3iNi Figure 21 o sj" Figure 22 LOG Te 4.6 4.4 4.2 LOG L/L © Figure 23 O INFLECTIONPOINTIN. + MAXIMUMLUMINOSITY MODELS NOVALIKE (CLASSII) Figure 24 CM

v V, ACKNOWLEDGMENTS

I would like to express my appreciation to my thesis advisor, Dr, R. C. Haymes, for his assistance and advice in the writing of this thesis, and for his work in the conception, preparation, and launch of the experiment.

I would like to thank A1 Heath for his contributions in every phase of the experiment, from construction and testing to launch and telemetry duty.

My sincere thanks go also to Neil Johnson, who assisted in data reduction, to Rick Harnden, Martha Ann Fenner, and Luiz Dias, who helped prepare the instrument, and to my wife, Kathy, who helped type the manuscript.

I am indebted to Mr. A1 Shipley of NCAR, who super¬ vised the launch procedure, to the U. S. Air Force and NCAR personnel who transported the instrument between

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