Masses of Neutron Stars and Black Holes in X-Ray Binaries

Abstract

The determination of masses for compact stars in X-ray binaries is useful for at least four reasons. First, the observed values of the masses of neutron stars provide a unique test of the combined predictions of theories of nuclear matter and of general relativity (or of alternative theories of gravity). Many important recent theoretical investigations have been devoted to sharpening these predictions. Second, the observed masses provide information about the final stages of stellar evolution. The determination of the masses of X-ray sources enables one to learn something about the cores of stars that have passed through the final stages of stellar evolution when neutron stars and black holes are formed. Third, the values of the masses are important for answering questions about the nature of the X-ray source itself. eyg X-l is a good example of this kind of application since the mass inferred for the X-ray star exceeds the standard theoretical limits for the masses of neutron stars and of white dwarfs. Finally, the numerical values of the masses are required for understanding specific phenomena, such as the spin-up rate of the pulsar period, various other processes that depend on mass transfer, and the applicability of the Eddington limit. This review presents a coherent summary of the methods used to determine the masses of X-ray sources in binary systems and describes the presently available results. I have paid special attention to some questions that have not been emphasized in the general literature such as the importance of the assumed geometry (or equipotentials) and the nature of possible systematic distortions that may affect observational parameters.