A reflection model with a radial disk density profile

TL;DR

This paper introduces RELXILLDGRAD_NK, a relativistic reflection model with a radial electron density profile, improving spectral fits and highlighting the importance of density gradients in testing black hole spacetime metrics.

Contribution

The paper presents a new reflection model incorporating a radial electron density profile and demonstrates its impact on spectral fitting and spacetime metric testing.

Findings

The model fits NuSTAR data better than previous models.

Density gradients are crucial for accurate spacetime tests around black holes.

Model uncertainties can lead to false indications of deviations from Kerr geometry.

Abstract

In this paper we present RELXILLDGRAD_NK, a relativistic reflection model in which the electron density of the accretion disk is allowed to have a radial power-law profile. The ionization parameter has also a non-constant radial profile and is calculated self-consistently from the electron density and the emissivity. We show the impact of the implementation of the electron density gradient in our model by analyzing a NuSTAR spectrum of the Galactic black hole in EXO 1846-031 during its last outburst in 2019 and a putative future observation of the same source with Athena and eXTP. For the NuSTAR spectrum, we find that the new model provides a better fit, but there is no significant difference in the estimation of the model parameters. For the Athena+eXTP simulation, we find that a model without a disk density profile is unsuitable to test the spacetime metric around the compact object, in the sense that modeling uncertainties can incorrectly lead to finding a non-vanishing deformation from the Kerr solution.