http://arxiv.org/abs/1408.2661
Galactic field black hole (BH) low-mass X-ray binaries (LMXBs) are believed to form in situ via the evolution of isolated binaries. In the standard formation channel, these systems survived a common envelope phase, after which the remaining helium core of the primary star and the subsequently formed BH are not expected to be highly spinning. However, the measured spins of BHs in LMXBs cover the whole range of spin parameters from a*~0 to a*~1. We propose here that the BH spin in LMXBs is acquired through accretion onto the BH during their long and stable accretion phase. In order to test this hypothesis, we calculated extensive grids of binary evolutionary sequences in which a BH accretes matter from a close companion. For each evolutionary sequence, we examined whether, at any point in time, the calculated binary properties are in agreement with their observationally inferred counterparts of 16 observed Galactic LMXBs. Mass-transfer sequences that simultaneously satisfy all observational constraints represent possible progenitors of the considered LMXBs and thus give estimates of the mass that the BH has accreted since the onset of Roche-Lobe overflow. We find that in all Galactic LMXBs with measured BH spin, the origin of the spin can be accounted by the accreted matter, and we make predictions about the maximum BH spin in LMXBs where no measurement is yet available. Furthermore, based on this hypothesis, we derive limits on the maximum spin that a BH can have depending on the orbital period of the binary it resides in and the effective temperature of the companion star. Finally we discuss the implication that our findings have on the BH birth-mass distribution, which is shifted by ~1.3 Msolar towards lower masses, compared to the currently observed one.
T. Fragos and J. McClintock
Wed, 13 Aug 14
5/57
Comments: 15 pages, 8 Figures, 5 Tables, submitted to ApJ
You must be logged in to post a comment.