title: A 15.65-solar-mass black hole in an eclipsing binary in the nearby spiral galaxy M 33.
abstract: Stellar-mass black holes are found in X-ray-emitting binary systems, where their mass can be determined from the dynamics of their companion stars. Models of stellar evolution have difficulty producing black holes in close binaries with masses more than ten times that of the Sun (>10; ref. 4), which is consistent with the fact that the most massive stellar black holes known so far all have masses within one standard deviation of 10. Here we report a mass of (15.65 +/- 1.45) for the black hole in the recently discovered system M 33 X-7, which is located in the nearby galaxy Messier 33 (M 33) and is the only known black hole that is in an eclipsing binary. To produce such a massive black hole, the progenitor star must have retained much of its outer envelope until after helium fusion in the core was completed. On the other hand, in order for the black hole to be in its present 3.45-day orbit about its (70.0 +/- 6.9) companion, there must have been a 'common envelope' phase of evolution in which a significant amount of mass was lost from the system. We find that the common envelope phase could not have occurred in M 33 X-7 unless the amount of mass lost from the progenitor during its evolution was an order of magnitude less than what is usually assumed in evolutionary models of massive stars.
authors.affiliation: Department of Astronomy, San Diego State University, 5500 Campanile Drive, San Diego, California 92182-1221, USA. orosz@sciences.sdsu.edu
authors.forename: Jerome A
authors.initials: JA
authors.lastname: Orosz
authors.forename: Jeffrey E
authors.initials: JE
authors.lastname: McClintock
authors.forename: Ramesh
authors.initials: R
authors.lastname: Narayan
authors.forename: Charles D
authors.initials: CD
authors.lastname: Bailyn
authors.forename: Joel D
authors.initials: JD
authors.lastname: Hartman
authors.forename: Lucas
authors.initials: L
authors.lastname: Macri
authors.forename: Jiefeng
authors.initials: J
authors.lastname: Liu
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Pietsch
authors.forename: Ronald A
authors.initials: RA
authors.lastname: Remillard
authors.forename: Avi
authors.initials: A
authors.lastname: Shporer
authors.forename: Tsevi
authors.initials: T
authors.lastname: Mazeh