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Optical investigations of X-ray binary systems

Observations in the optical range of the spectrum are critically important in the investigation of X-ray binary systems. X-ray binary systems contain relativistic objects (neutron stars and black holes) which accrete a substance of a satelliteÐa normal star. Optical investigations permit studying the motion of `probe bodies' (stars, gas disks, etc.) in the gravitational field of a relativistic object, and thereby make it possible to measure the masses of neutron stars (NSs) and black holes (BHs).Mass is the most important parameter determining whether the relativistic object belongs to either the class of NSs or the class of BHs. According to modern notions [1], with general relativity (GR) effects taken into account, when the core mass of a star which underwent chemical evolution due to thermonuclear reactions exceeds a value of 3M , it eventually evolves into a BH; when the mass of the stellar core is less than 3M , the stellar evolution results in the formation of a white dwarf or a neutron star. The possibility of measuring the masses of relativistic objects makes X-ray binary systems a powerful tool in the quest for BH stellar masses. This year will observe the 40th anniversary of the launch of American dedicated X-ray satellite Uhuru into a circumterrestrial orbit, which opened up the era of systematic sky observations in the X-ray region and marked the beginning of observational BH research. In this paper, we describe the results of the 40-year-long quest for and investigations of BHs in X-ray binary systems by means of optical astronomy techniques. It should be noted that it is quite sufficient to apply Newton's law of gravitation when determining BH masses by optical techniques, because the dimensions of the orbits of X-ray binary systems are far greater than the BH gravitational (Schwarzschild) radius rg  2GM=c ; for a BH of mass M  10M , this radius is equal to 30 km. We emphasize that, since the masses of BHs in X-ray binary systems are determined using Newton's theory of gravitation, BH masses evaluated in this way are independent of the type of the relativistic theory of gravitation, because all of these theories, including those that are alternatives to GR, cross over to Newton's theory at the distance from the gravitating center.