The Spin Of The Black Hole Primaries In LMC X-1 And A0620-00

We determined the dimensionless spin parameter for LMC X-1 and A0620-00 by fitting the thermal X-ray continuum spectrum with our fully relativistic model of a thin accretion disk. For LMC X-1, we have selected a sample of 18 RXTE PCA spectra and the spin is shown to be a*=0.92 (-0.07, +0.05) (68% confidence range). The high spin is confirmed by our analysis of selected X-ray spectra obtained by the XMM-Newton, BeppoSAX and Ginga missions, as well as by our complete sample of 53 RXTE PCA spectra. For A0620-00, we used the only-existing spectra from OSO-8 and obtain a spin of a*=0.25(-0.17, +0.16) (68% confidence range). The large difference in the spin between these two objects may motivate different scenarios for the formation and evolution of these binaries.