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Session 10 - Studies of Black Holes.
Display session, Monday, June 09
South Main Hall,

[10.01] The Distribution of Black Hole Masses in X-ray Transients

C. D. Bailyn, R. K. Jain, P. Coppi (Yale U.), J. A. Orosz (PSU)

Measuring the mass functions in soft X-ray transient binary systems has provided some of the strongest evidence for the existence of black holes in nature. This evidence comes in the form of a lower limit on the mass of the compact object, which in six cases is at or above the maximum mass of a neutron star. To determine the true mass (rather than a lower limit) of the black hole, one needs to determine the orbital inclination, and either the mass ratio or the mass of the secondary star. A variety of methods have been employed to determine these parameters, in particular modelling of ellipsoidal variability of the secondary star.

Here we assess the results of these efforts, and employ Baysian statistical techniques to explore the mass distribution of the black holes in these systems. We find that the mass distribution encompasses a surprisingly small range of masses, from 6-8 times solar. There is one exception, namely V404 Cyg, which has a black hole with M>10M_ødot . V404 Cyg is also unique in having a highly evolved secondary star. We find that it is statistically improbable that the mass of the black hole in V404 Cyg is drawn from the same distribution as the other systems. The concentration of black hole masses near 7M_ødot may pose interesting constraints on the supernova events which produced them.

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