The Constant Inner-Disk Radius of LMC X-3: A Basis for Measuring Black Hole Spin

TL;DR

This study demonstrates that the inner radius of the accretion disk in LMC X-3 remains constant over time and across different X-ray missions, supporting its association with the innermost stable circular orbit, thus enabling black hole spin measurements.

Contribution

The paper provides the first extensive observational evidence that the accretion disk's inner radius is constant and linked to the ISCO, facilitating accurate black hole spin determination.

Findings

Inner disk radius is constant within ~2% over 26 years.

Consistent radius measurements across eight X-ray missions within ~4-6%.

Supports the association of the inner radius with the ISCO.

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. Among the persistent sources, LMC X-3 is uniquely both habitually soft and highly variable. Using a fully relativistic accretion-disk model, we analyze hundreds of spectra collected during eight X-ray missions that span 26 years. For a selected sample of 391 RXTE spectra we find that to within ~2 percent the inner radius of the accretion disk is constant over 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 percent. Our results provide strong evidence for the existence of a fixed inner-disk radius. The only reasonable inference is that this radius is closely associated with the general relativistic innermost stable circular orbit (ISCO). Our findings establish a firm foundation for the measurement of black hole spin.