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Session 59 - Relativistic Astrophysics.
Oral session, Thursday, January 08
Monroe,
It has recently been shown that some of the Galactic X-ray binaries habor most possibly a black hole (BH) whose specific angular momentum is near the maximal possible value, while the BHs in some others may be slowly or not rotating (Zhang, Cui, amp; Chen 1997; Cui, Zhang, amp; Chen 1997). In this paper we discuss the possible spin-up or spin-down history of the BHs due to accretion from its companion star. If the accreted mass carries the specific angular momentum at the last stable orbit, it can spin up the BH. For A 0620-00 and GRO J1655-40, we have derived the spin-up time. The results show that given the low average mass transfer rate for A 0620-00 the BH spin is hardly affected over the Hubble time, while the relatively high rate of GRO J1655-40 might allow a significant spin-up of the BH over a time \le10^9 yrs. We argue, however, that the short lifetime of the active Roche-lobe overflow episode and the limited supply of mass from the secondary make it unlikely that the high BH spin of GRO J1655-40 is the consequence of mass accretion. Assuming various initial distributions of BH binary birth rate, as well as the initial BH mass, BH spin, and mass transfer rate from the secondary, we compute the evolution of the BH spin distributions. The relevance of the results to observations will also be discussed.
