A Determination of the Spin of the Black Hole Primary in LMC X-1

Abstract

The first extragalactic X-ray binary, LMC X-1, was discovered in 1969. In the 1980s, its compact primary was established as the fourth dynamical black hole candidate. Recently, we published accurate values for the mass of the black hole and the orbital inclination angle of the binary system. Building on these results, we have analyzed 53 X-ray spectra obtained by RXTE and, using a selected sample of 18 of these spectra, we have determined the dimensionless spin parameter of the black hole to be a∗ = 0.92+0.05 −0.07. This result takes into account all sources of observational and model-parameter uncertainties. The standard deviation around the mean value of a∗ for these 18 X-ray spectra, which were obtained over a span of several years, is only Δa∗ = 0.02. When we consider our complete sample of 53 RXTE spectra, we find a somewhat higher value of the spin parameter and a larger standard deviation. Finally, we show that our results based on RXTE data are confirmed by our analyses of selected X-ray spectra obtained by the XMM-Newton, BeppoSAX, and Ginga missions.