Estimating the spins of stellar-mass black holes

Summary

We describe a program that we have embarked on to estimate the spins of stellar-mass black holes in x-ray binaries. We fit the continuum x-ray spectrum of the radiation from the accretion disk using the standard thin disk model, and extract the dimensionless spin parameter a_* = a/M of the black hole as a parameter of the fit. We have obtained results on three systems, 4U 1543–47 (a_* = 0.7−0.85), GRO J1655−40 (0.65−0.8), and GRS 1915+105 (0.98−1), and have nearly completed the analysis of two additional systems. We anticipate expanding the sample of spin estimates to about a dozen over the next several years.

Introduction

The first black hole (BH), Cygnus X−1, was identified and its mass estimated in 1972. We now know of about 40 stellar-mass black holes in x-ray binaries in the Milky Way and neighboring galaxies. The masses of 21 of these, which range from ~5−15 M_⊙, have been measured by observing the dynamics of their binary companion stars (Remillard & McClintock 2006; Orosz et al. 2007). In addition, it has become clear that virtually every galaxy has a supermassive black hole with M ~ 10⁶−10¹⁰M_⊙ in its nucleus. A few dozen of these supermassive BHs have reliable mass estimates, which have been obtained via dynamical observations of stars and gas in their vicinity (Begelman 2003).

With many mass measurements now in hand, the next logical step is to measure spin.