Temporal and Spectral Variations of 2S 0921-630 and LMC X-2

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X Ray Binaries, X Rays, Variability, Spectrum Analysis, Spontaneous Emission, Periodic Variations, Emission Spectra, X Ray Sources, Ginga Satellite, Eclipses, Light Curve, Temporal Distribution

Scientific paper

The goal of this study is to use data from the Ginga archive to study the variabilty of X-ray emission from low-mass X-ray binaries (LMXB's). Most of our effort has been directed towards an investigation of 2S 0921-630. Only limited data are available on a similar X-ray source called LMC X-2. Since these targets are members of the very small subclass of LMXB's having long orbital periods, a comparison will be made with the much more common short-period LMXB's. The X-ray light curve of an eclipse of 2S 0921-630 (observed in May 1989) has been been extracted from the Ginga archive. The time of mid-eclipse and its duration agree with the prediction based on two X-ray eclipses recorded with the EXOSAT satellite in 1983 and 1985. Optical photometry that we have obtained during 1988-1994 confirms the 9-day orbital period for this system and refutes a 13-day periodicity proposed by some ground-based observers. The Ginga data show that the depth of eclipse is greater at higher energies, implying a vertical temperature structure to the X-ray emission region that is consistent with an Accretion Disk Corona (ADC) model. In an ADC model the X-rays seen during eclipse come from cool material that is high above the accretion disk. Finally, we note that for the highest X-ray energies recorded by Ginga, the eclipse light curve is subject to large systematic errors in background subtraction. Hence, the investigation is restricted to X-rays with energies below about 14-15 keV. This problem of background subtraction also affects the determination of short time scale variability since the satellite operated in a low-altitude orbit.

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