Black Hole Binaries as Ultraluminous X-Ray Sources
Saul Rappaport (MIT) - Apr 18 at 12:00 noon
Ultraluminous X-ray sources (ULXs) with Lx > 10^{39} ergs/sec have been discovered in
great numbers in external galaxies with ROSAT, Chandra, and XMM-Newton. The central
question regarding this important class of sources is whether they represent an
extension in the luminosity function of binary X-ray sources containing neutron stars
and stellar-mass black holes, or a new class of objects, e.g., systems containing
intermediate-mass black holes (100-1000 solar masses). We discuss a theoretical
study to test whether a large fraction of the ULXs, especially those in galaxies with
recent star formation activity, can be explained with binary systems containing
stellar-mass black holes. We find that for donor stars with initial masses greater
than ~10 solar masses the mass transfer driven by the normal nuclear evolution of the
donor star is sufficient to potentially power most ULXs. We show that with only a
modest violation of the Eddington limit, e.g., a factor of ~10, both the numbers and
properties of the majority of the ULXs can be reproduced. Finally, we discuss the
formation and evolution of ULXs as binary systems containing an intermediate-mass
black hole. This involves the capture of a companion star, possibly via exchange
encounters with a passing binary system; the evolution of the captured star with
tidal heating as the orbit circularizes; and interactions among the entourage of
stars captured by the IMBH.
The seminar will be held in 544 Campbell Hall.
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