Optically Variable Close Binary Sources of Strong, Variable X-Ray Radiation (X-Ray Sources)

Abstract

X-ray variable stars! Surely, you jest? In fact, many X-ray variable stars have an optical component that is within the observational reach of visual observers. Without a doubt, you will fail to detect the X-ray portion of these star’s electromagnetic energy spectrum but that’s not what you want anyway. And if you’re not new to variable star observing, you have probably already viewed stars similar to X-ray variables. Both low-mass X-ray binaries (LMXB) and cataclysmic variables (CV) have similar orbital periods, low-mass donor stars, mass-transfer rates, variability, and eruptions. The essential difference between CVs and LMXBs is that the accretion is onto a white dwarf for cataclysmic variables and in the case of low-mass X-ray binaries, it is onto a more compact neutron star, or possibly a black hole. In a sense, X-ray variables may provide you with an opportunity to study, in an indirect manner of course, neutron stars and perhaps black holes Figure 8.1.

Close binary systems that are sources of strong, variable X-ray emission and which do not belong to or are not yet attributed to any of the other types of variable stars. One of the components of the system is a hot compact object (white dwarf, neutron star, or possibly a black hole). X-ray emission originates from the infall of matter onto the compact object or onto an accretion disk surrounding the compact object. In turn, the X-ray emission is incident upon the atmosphere of the cooler companion of the compact object and is reradiated in the form of optical high-temperature radiation (reflection effect), thus making the area of the cooler companion’s surface an earlier spectral type. These effects lead to quite a peculiar complex character of optical variability in such systems.

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