On the Prospect of Constraining Black-Hole Spin Through X-ray Spectroscopy of Hotspots

TL;DR

This paper explores how future X-ray observations of hotspots near black holes could improve measurements of black-hole spin by supplementing traditional spectral methods, despite observational challenges.

Contribution

It introduces a method to use localized hotspot observations to enhance black-hole spin constraints, supplementing existing spectral analysis techniques.

Findings

Hotspot observations can recover information lost in integrated line profiles.

Feasibility depends on measurement uncertainties and observational capabilities.

Potential to significantly improve black-hole spin measurements.

Abstract

Future X-ray instrumentation is expected to allow us to significantly improve the constraints derivedfrom the Fe K lines in AGN, such as the black-hole angular momentum (spin) and the inclination angle of the putative accretion disk. We consider the possibility that measurements of the persistent, time-averaged Fe K line emission from the disk could be supplemented by the observation of a localized flare, or "hotspot", orbiting close to the black hole. Although observationally challenging, such measurements would recover some of the information loss that is inherent to the radially-integrated line profiles. We present calculations for this scenario to assess the extent to which, in principle, black-hole spin may be measured. We quantify the feasibility of this approach using realistic assumptions about likely measurement uncertainties.