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Register a new userA Revised Ephemeris and FUSE Observations of the Supersoft X-ray Source CAL 83
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A new ephemeris has been determined for the supersoft X-ray binary CAL 83 using MACHO photometry. With an improved orbital period of 1.047568 days, it is now possible to phase together photometric and spectroscopic data obtained over the past two decades with new far ultraviolet spectra taken with FUSE. We discuss the properties of the orbital and longterm optical light curves as well as the colors of CAL 83. In the far ultraviolet the only well-detected stellar feature is emission from the O VI resonance doublet. The radial velocity of this emission appears to differ from that of HeII in the optical region, although we only have partial phase coverage for the O VI line. The FUSE continuum variations are similar to the optical light curve in phase and amplitude.
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We have studied the long-term (~ years) temporal variability of the prototype supersoft X-ray source (SSS) CAL 83 in the LMC, using data from the MACHO and OGLE projects. The CAL 83 light curve exhibits dramatic brightness changes of ~1 mag on timescales of ~450 days, and spends typically ~200 days in the optical low state. Combined with archival XMM-Newton X-ray observations these represent the most extensive X-ray/optical study to date of this system, and reveal in much greater detail that the X-ray light curve is anti-correlated with the optical behaviour. This is remarkably similar to the behaviour of the transient SSS, RX J0513.9-6951, where the SSS outbursts recur on a timescale of ~168 days, and also anti-correlate with the optical flux. We performed simple blackbody fits to both high and low state X-ray spectra, and find that the blackbody temperature and luminosity decrease when the optical counterpart brightens. We interpret these long-term variations in terms of the limit-cycle model of Hachisu & Kato (2003a), which provides further support for these systems containing massive (~1.3 Msun) white dwarfs. In addition, we have refined their orbital periods in the MACHO and OGLE-III light curves to values of 1.047529(1) days and 0.762956(5) days for CAL 83 and RX J0513.9-6951, respectively.
Cal 87 was observed with XMM-Newton in April of 2003. The source shows a rich emission line spectrum, where lines can be identified if they are red-shifted by 700-1200 km/s. These lines seem to have been emitted in a wind from the system. The eclipse is observed to be shifted in phase by 0.03 phi(orb), where phi(orb) is the phase of the optical light curve.
We present and discuss 25 spectra obtained in November 1996, covering all phases of the CAL 87 binary system. These spectra are superior both in signal-to-noise and wavelength coverage to previously published data so that additional spectral features can be measured. Photometry obtained on the same nights is used to confirm the ephemeris and to compare 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 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 ~1.4Msun for the degenerate star and ~0.4Msun for the mass donor. However, the strong He II emission lines indicate a much more massive accreting star, with Mx>4Msun.
Nova Vel 1999 (V382 Vel) was observed with BeppoSAX twice, 15 days and 6 months after the optical maximum. A hard X-ray source was detected in the first observation, while the second time also a very luminous supersoft X-ray source was detected. The continuum observed in the supersoft range with the BeppoSAX LECS cannot be fitted with atmospheric models of hot hydrogen burning while dwarfs. We suggest that we are observing instead mainly a ``pseudocontinuum, namely a blend of very strong emission lines in the supersoft X-ray range.
Compact binary supersoft X-ray sources (CBSS) are explained as being associated with hydrostatic nuclear burning on the surface of a white dwarf with high accretion rate. This high mass transfer rate has been suggested to be caused by dynamical instability, expected when the donor star is more massive than the accreting object. When the orbital period is smaller than ~6 hours, this mechanism does not work and the CBSS with such periods are believed to be fed by a distinct mechanism: the wind-driven accretion. Such a mechanism has been proposed to explain the properties of objects like SMC 13, T Pyx and V617 Sgr. One observational property that offers a critical test for discriminating between the above two possibilities is the orbital period change. As systems with wind-driven accretion evolve with increasing periods, some of them may reach quite long orbital periods. The above critical test may, therefore, also be applied to orbital periods longer than 6 hours. CAL 87 is an eclipsing system in the LMC with an orbital period of 10.6 hours that could provide the opportunity for testing the hypothesis of the system being powered by wind-driven accretion. We obtained eclipse timings for this system and show that its orbital period increases with a rate of P/Pdot = +7.2(+/-1.3) X 10^{6} years. Contrary to the common belief, we conclude that CAL 87 is the first confirmed case of a wind-driven CBSS with an orbital period longer than 6 hours. The system is probably an evolved object that had an initial secondary mass of M2i=0.63 solar masses but is currently reduced to about M2=0.34 solar masses. We discuss evidence that other CBSS, like CAL 83 and V Sge stars, like WX Cen, are probably also wind-driven systems. This may in fact be the rule, and systems with inverted mass ratio, the exception.
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