The Eclipsing Supersoft X‐Ray Binary CAL 87

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Abstract
We present and discuss 25 spectra obtained in 1996 November, covering all phases of the CAL 87 binary system. These spectra are superior both in signal-to-noise ratio and wavelength coverage to previously published data, so additional spectral features can be measured. Photometry obtained on the same nights is used to confirm the ephemeris and for comparison with light curves from previous years. Analysis of the color variation through the orbital cycle has been carried out using archival MACHO data. When a barely resolved red field star is accounted for, there is no (V-R) color variation, even through eclipse. There have been substantial changes in the depth of minimum light since 1988; it has decreased more than 0.5 mag in the last several years. The spectral features and radial velocities are also found to vary not only through the 0.44 day orbit but also over timescales of a year or more. Possible interpretations of these long-term changes are discussed. The 1996 spectra contain phase-modulated Balmer absorption lines not previously seen, apparently arising in gas flowing from the region of the compact star. The changes in emission-line strengths with orbital phase indicate that there are azimuthal variations in the accretion disk structures. Radial velocities of several lines give different amplitudes and phasing, making determination of the stellar masses difficult. All solutions for the stellar masses indicate that the companion star is considerably less massive than the degenerate star. The Balmer absorption-line velocities correspond to masses of ~1.4 M☉ for the degenerate star and ~0.4 M☉ for the mass donor. However, the strong He II emission lines indicate a much more massive accreting star, with MX > 4 M☉.
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