Reflection spectra of accretion disks illuminated by disk-like coronae

TL;DR

This study models the impact of a disk-shaped corona on the reflection spectra of black hole accretion disks, analyzing how coronal geometry influences spectral features and the measurement of black hole properties.

Contribution

It introduces a novel disk-like coronal geometry model and assesses its effects on spectral features and parameter estimation in black hole systems.

Findings

Astrophysical disk properties are reliably recovered with standard models.

Constraining the black hole's spacetime geometry is more challenging.

Disk-like coronae influence iron line shapes and emissivity profiles.

Abstract

Relativistic reflection features in the X-ray spectra of black hole binaries and AGNs are thought to be produced through illumination of a cold accretion disk by a hot corona. In this work, we assume that the corona has the shape of an infinitesimally thin disk with its central axis the same as the rotational axis of the black hole. The corona can either be static or corotate with the accretion disk. We calculate the disk's emissivity profiles and iron line shapes for a set of coronal radii and heights. We incorporate these emissivity profiles into RELXILL_NK and we simulate some observations of a black hole binary with NuSTAR to study the impact of a disk-like coronal geometry on the measurement of the properties of the system and, in particular, on the possibility of testing the Kerr nature of the source. We find that, in general, the astrophysical properties of the accretion disk are recovered well even if we fit the data with a model employing a broken power-law or a lamppost emissivity profile, while it is more challenging to constrain the geometric properties of the black hole spacetime.