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\nX-ray mission since the inception of X-ray astronomy. Among the persistent\nsources, LMC X-3 is uniquely both habitually soft and highly variable. Using a\nfully relativistic accretion-disk model, we analyze hundreds of spectra\ncollected during eight X-ray missions that span 26 years. For a selected sample\nof 391 RXTE spectra we find that to within ~2 percent the inner radius of the\naccretion disk is constant over time and unaffected by source variability. Even\nconsidering an ensemble of eight X-ray missions, we find consistent values of\nthe radius to within ~4-6 percent. Our results provide strong evidence for the\nexistence of a fixed inner-disk radius. The only reasonable inference is that\nthis radius is closely associated with the general relativistic innermost\nstable circular orbit (ISCO). Our findings establish a firm foundation for the\nmeasurement of black hole spin.\n