Title: Stellar-Mass Black Holes in Binary Systems
Speaker: Alex Filippenko
Abstract: An important class of binary systems has been identified in which a low-mass secondary (companion) star orbits a probable black hole. In all cases they were first observed in outburst as "X-ray novae." Their X-ray spectra are generally characterized by a prominent, "soft" thermal component (kT ~ 1 keV) as well as a "hard" power-law tail extending to very high energies (0.1-1 MeV). During outburst, the radiation is emitted predominantly by the accretion disk surrounding the compact primary. A lower limit to the mass of the primary in a given X-ray nova can be measured when it returns to quiescence; at that time, light from the secondary star contributes significantly to the visible spectrum, and the secondary's radial-velocity curve can be determined with a series of time-resolved spectra. I will describe Keck observations that have led to the determination of the minimum mass of the primary in four X-ray novae, about half of the current sample of low-mass X-ray binaries having compelling evidence for black holes. In some cases, we have also been able to constrain the mass ratio and the inclination of the system. The derived distribution of black-hole masses peaks sharply at 7 solar masses, although exceptions do exist. Evidence suggesting the presence of an event horizon in X-ray novae will also be discussed.
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