Black hole spin and X-ray reflection

Overview

X-rays from the corona reflect off the inner accretion disk and come back stamped with the marks of extreme gravity: a broadened iron line and a Compton hump, shaped by how close the disk reaches to the black hole. That distance depends on the spin, which makes relativistic reflection one of the main ways we measure how fast a supermassive black hole turns. Getting the spin right, though, depends on understanding the geometry of the corona and the ionization of the disk that together produce the reflected light.

Approach

I use simulations and physical modeling to test how faithfully reflection-based methods recover the true properties of a system, and to find what can throw them off. This includes blind recovery tests, where spectra are simulated by one person and analyzed by another, and studies of how the ionization of the disk varies with radius.

Key results

• In a blind test, the spin came out right most reliably when the corona sits low above the black hole and the spin is high, recovered in every such case, while high signal-to-noise on its own was not enough to guarantee a good measurement.
• A disk whose ionization falls off with radius can imitate a very steep emissivity profile, an effect that is easy to mistake for an extremely compact corona and that can bias the inferred spin.
• These results help mark out when relativistic reflection gives a trustworthy read on the innermost accretion flow, and they feed directly into my later XRISM and polarimetry work on the same systems.