XMM-Newton and Broad Iron Lines

TL;DR

This paper discusses how broad and variable iron lines in X-ray spectra from accreting black holes serve as diagnostics for the inner accretion flow, enabling measurements of black hole spin and flow dynamics.

Contribution

It reviews the use of XMM-Newton observations of iron lines and reflection spectra to probe the innermost regions of accretion flows around black holes, highlighting the potential for future progress.

Findings

Broad iron lines indicate emission close to the black hole.

Reflection spectra can diagnose flow radius and velocity.

High-quality spectra require long exposure times.

Abstract

Iron line emission is common in the X-ray spectra of accreting black holes. When the line emission is broad or variable then it is likely to originate from close to the black hole. X-ray irradiation of the accretion flow by the power-law X-ray continuum produces the X-ray 'reflection' spectrum which includes the iron line. The shape and variability of the iron lines and reflection can be used as a diagnostic of the radius, velocity and nature of the flow. The inner radius of the dense flow corresponds to the innermost stable circular orbit and thus can be used to determine the spin of the black hole. Studies of broad iron lines and reflection spectra offer much promise for understanding how the inner parts of accretion flows (and outflows) around black holes operate. There remains great potential for XMM-Newton to continue to make significant progress in this work. The need for high quality spectra and thus for long exposure times is paramount.