The constant inner-disk radius of lmc X-3: A basis for measuring black hole spin

Abstract

The black hole binary system LMC X-3 has been observed by virtually every X-ray mission since the inception of X-ray astronomy. Amongthe persistent sources, LMC X-3 is uniquely both habitually soft and highly variable. Using a fully relativistic accretion diskmodel, we analyze hundreds of spectra collected during eight X-raymissions that span 26 years. For a selected sample of 391 RXTE spectra, we find that to within ≈2% the inner radius of the accretion disk is constantover time and unaffected by source variability. Even considering an ensemble of eight X-ray missions, we find consistent values of the radius to within ≈4%-6%. Our results provide strong evidence for the existence of a fixed inner-disk radius. The only reasonable inferenceis that this radius is closely associated with the general relativistic innermost stable circular orbit. Our findings establish a firm foundation for the measurement of black hole spin. © 2010 The American Astronomical Society.