A NuSTAR and Swift View of the Hard State of MAXI J1813-095

TL;DR

This study analyzes NuSTAR and Swift spectra of MAXI J1813-095 during a failed outburst, revealing a close-to-innermost accretion disc and spectral variability driven by Comptonisation photon index changes.

Contribution

First detailed reflection modeling of MAXI J1813-095 during a failed outburst, constraining disc inner radius and accretion geometry.

Findings

Disc inner radius constrained to less than 7 gravitational radii.

Inner accretion geometry remains stable during observations.

Spectral variability mainly due to changes in Comptonisation photon index.

Abstract

We present an analysis of the NuSTAR and Swift spectra of the black hole candidate MAXI J1813-095 in a failed-transition outburst in 2018. The NuSTAR observations show evidence of reflected emission from the inner region of the accretion disc. By modelling the reflection component in the spectra, we find a disc inner radius of $R_{\rm in}<7$ $r_{\rm g}$. This result suggests that either a slightly truncated disc or a non-truncated disc forms at a few percent of the Eddington limit in MAXI J1813-095. Our best-fit reflection models indicate that the geometry of the innermost accretion remains consistent during the period of NuSTAR observations. The spectral variability of MAXI J1813-095 from multi-epoch observations is dominated by the variable photon index of the Comptonisation emission.