A study for testing the Kerr metric with AGN iron line eclipses

TL;DR

This paper investigates whether AGN iron line eclipses can improve constraints on the Kerr metric, finding that unlike reverberation mapping, eclipses do not enhance relativistic effect detection.

Contribution

The study compares the constraining power of AGN iron line eclipses versus reverberation mapping for testing the Kerr metric, highlighting key differences.

Findings

Eclipses do not improve constraints on the Kerr metric compared to reverberation mapping.

Reverberation mapping provides better time-resolved information for testing black hole metrics.

The conclusions are robust across a broad class of non-Kerr metrics.

Abstract

Recently, two of us have studied iron line reverberation mapping to test black hole candidates, showing that the time information in reverberation mapping can better constrain the Kerr metric than the time-integrated approach. Motivated by this finding, here we explore the constraining power of another time-dependent measurement: an AGN iron line eclipse. An obscuring cloud passes between the AGN and the distant observer, covering different parts of the accretion disk at different times. Similar to the reverberation measurement, an eclipse might help to better identify the relativistic effects affecting the X-ray photons. However, this is not what we find. In our study, we employ the Johannsen-Psaltis parametrisation, but we argue that our conclusions hold in a large class of non-Kerr metrics. We explain our results pointing out an important difference between reverberation and eclipse measurements.