OBSERVATION AND INTERPRETATION OF THE CYGNUS X-1 SYSTEM by ZORAN NINKOV M.Sc., Monash University, 1981 B.Sc.(Honours), University of Western Australia, 1978 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Department of Geophysics and Astronomy We accept this thesis as conforming to the req^r^o" standard THE UNIVERSITY OF BRITISH COLUMBIA July 1985 £ © Zoran Ninkov, 1985 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the The University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Geophysics and Astronomy The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date: July 1985 ABSTRACT The results of a long term monitoring program on the massive X-ray binary Cygnus X-1, whose constituents are believed to consist of a normal 0 star primary and a black hole companion, are presented. Spectra of this system were collected between 1980 and 1984 using a Reticon detector. The resulting absorption line radial velocity (RV) curve is characteristic of a single line spectroscopic binary. These velocities were combined with those available in the literature to determine an orbital period of 5.59977 ± 0.00001 days. A P/P =* 10"5 day"1 was found from analysis of all available velocity measures. This change in the period is larger than that expected as a result of mass loss from the primary or from- models of the system in which large mass transfer rates occur between the components. A fit of the orbital motion of the primary to the RV curve gives a K = 75.0 ± 1 km/s and no significant eccentricity. The vsini of the primary was found, using the fourier transform technique, to be 94.3 km/sec. This is substantially smaller than the literature value of vsini = 140 km/sec. The value of the K and vsini allow the ratio m /m to be determined as p x * 2.0 . The equivalent width of H7 allows the absolute magnitude -ol the primary to be estimated at -6.5 ± 0.2 . A comparison of the spectrum of the primary to those of an array of standards allows the spectral type to be given as between 09.5 and 09.7 I . This spectral type is consistent with the absolute magnitude obtained and it is thus likely that the primary is a normal star of mass « 20 MQ. The mass of the secondary is therefore 10 ± 3 MQ. Measurement of the interstellar lines to obtain an independent E(B-V) reveals that the interstellar line strength per unit E(B-V) is lower than in any other direction in the sky. Stars for which velocity-excitation slopes and mass loss estimates, from UV line profile modeling and/or radio free-free emission measures, are available in the literature were collated. An empirical fit to this material allowed the mass loss rate for HDE 226868 (the primary of Cygnus X-1) to be estimated at 5.7 ± 2 x 10~6 M/year. The He II X4686 and Ha lines are found in emission. After removal of the contribution to the line profile from the primary the radial velocity curve of the residual He II X4686 line is found to have small scatter from a smooth fit ( ± 10 km/sec ) with no significant eccentricity. No sizeable variation in the K amplitude at different epochs was found contrary to a previous investigation and the origin of the emission is thus apparently fixed and stable. A phase lag of 130° is measured between the absorption and emission velocity curves and thus the simple interpretation of the emmision originating near the secondary can not be correct. The He II emission equivalent width, corrected for the underlying primary absorption, shows strong modulation (30%) over the 5.6 day orbital period. This variation is probably the result of the profile of the primary varying with which face of the star is directed towards the observer. During two separate observing sessions in 1982 the He II equivalent widths were found to be 40% and 15% larger than the mean of all other observations while still showing the same variation with orbital phase. Such a change has been seen once before and may be associated with transitions to the X-ray high state. The H7 and H/3 lines show a 20% variation on the 294 day X-ray period in the sense of largest equvalent widths at X-ray minimum ( 0 phase ). The Balmer lines are a composite of an absorption component from the primary and a weak emission component. This is best explained by variations in the outflow from the star, which is the source of both the emission component and the X-ray flux via accretion. Such variations may be the result of pulsation of the primary. The Ha line profile has been decomposed into three components; the absorption component from the primary, emission from a shell with an inner radius 1.4 times that of the primary, arid a component with properties similar to the He II X4686 line. The great width of the Ha line, previously explained as being the result of rotation of the disc, is instead shown to be the result of superposition of these components. The origin of the He II X4686 emission is explained by assuming that a stellar wind enhanced in the direction of the . secondary is completely ionized within a volume surrounding the secondary. The He II between the edge of this volume and the surface of the primary is enhanced as a result of X-ray heating and ionization. Model profiles appear in reasonable agreement with high dispersion spectra. The obvious explanation for the orbital variation in the He II line is that X-ray heating of the side of the primary facing the secondary produces a change in the effective temperature. Calculation of the size of this effect reveals that it is too small to explain the changes observed. X-ray observations made with EXOSAT with excellent time resolution allowed timing of the X-ray absorption features seen near orbital phase zero. Simultaneous X-ray spectra allowed an estimate of their column density as 2.0 x 1023 cm"2. Two scale lengths of dips were found of 10s and 1011 cm. These values are in good agreement with theoretical predictions for the sizes of inhomogeneties in high mass loss stellar winds. The location of the material producing the absorption dips was calculated as being * 4-8 R@ from the X-ray source. v Table of Contents Abstract ii Table of Contents vi List of Tables viii List of Figures x Acknowledgements xiv 1 . Introduction 1 1.1 Early History of X-ray Binaries 1 1.2 History of Cygnus X-1 6 2. Data Collection and Analysis 11 2.1 The Observations 11 2.2 The Reticon Detector 12 2.3 The Data Reduction 16 2.4 Problems with Darks ". 19 2.5 Cosmic Ray Events 23 2.6 The Data 36 2.7 Digital Subtraction 36 3. The Absorption Line Spectrum 48 3.1 Rationale for the Study 48 3.2 The Spectrum Itself 50 3.3 Equivalent Width Determination 58 3.4 The Spectral Type 64 3.5 The Rotational Velocity 80 3.6 The Radial Velocity Curve 92 3.7 Masses of the Components 131 3.8 E(B-V) and the Interstellar Lines 136 3.9 Mass Loss Rate 138 vi 3.10 Equivalent Width Variations 146 3.11 Line Halfwidth Variations 160 4. The Emission Line Spectrum 165 4.1 Rationale for the Study 165 4.2 Analysis of the He II 74686 Data 166 4.2.1 Finding a Good Reference Star 166 4.2.2 Correction of the He II Profiles 172 4.2.3 The CFHT He II 74686 Profiles 181 4.3 The He II Emission Radial Velocity Curve 181 4.4 Equivalent Width Measures 192 4.5 The Ha Data 199 4.6 The Ha Profile of the Primary 206 4.7 The Hydrogen Absorption Lines 216 5. Emission Mechanisms ..222 5.1 X-Ray heating of the Primary 222 5.2 Overflow onto the Secondary 223 5.3 Trailing Shock Model 231 5.4 Emission from the Disc 233 5.5 Enhanced Stellar Wind 234 5.6 The Model 238 5.7 Variation of He II Absorption Line 255 6. X-Ray Observations 266 7. Conclusions 281 Bibliography 285 Appendix A 30.2 vii List of Tables 1.01 Type I vs Type II X-ray System Characteristics 5 1.02 Observed Properties of the Cygnus X-1 System 8 2.01 Ret icon Systems 16 2.02 Details of occurrence of spikes 32 2.03 Details of observations of Cygnus X-1 37 2.04 Details of observations of Comparison Stars 40 3.01a Line List for Early Type Stars (blue) 56 3.01b Line List for Early Type Stars (Red) ....57 3.02 Standard Star Characteristics 67 3.03 Standard Star Ratios of Spectrally Sensitive Lines 78 3.04 Line weights for least squares fitting radial velocity curve and excitation potential' for line groups 97 3.05 Calculated velocities for spectra of HDE 226868 ....98 3.06 Fitted ?T? at different epochs ...123 3.07 Weights for least squares fitting radial velocity curve by year 130 3.08 Orbit Solution for Cygnus X-1 130 3.09 E(B-V) as determined from the Interstellar lines ..137 3.10 Mass loss rates from different techniques ...142 3.11 Normalized equivalent width variations with phase .158 3.12 Normalized phase variations of He I halfwidth 162 4.01 Line List for CFHT 169 4.02 Equivalent Widths for lines from CFHT Data 174 vi i i 4.03 He II X4686 emission radial velocities and equivalent widths ..187 4.04 Orbit parameters for He II X4686 emission radial velocity curve 191 4.05 He II X4686 equivalent widths and halfwidths as function of orbital phase 194 4.06 Equivalent Widths Binned on the 294 day Period ....217 5.01 Assumed Basic Parameters of Cygnus X-1 253 5.02