X-ray spectra of black hole X-ray binaries with returning radiation

TL;DR

This paper presents an improved model for the X-ray spectra of black hole X-ray binaries, incorporating returning radiation effects and self-consistent ionization calculations to better understand spectral components across different states.

Contribution

It introduces a refined model that accounts for returning radiation and self-consistent ionization, enhancing previous spectral calculations for black hole X-ray binaries.

Findings

Reflection spectrum can be dominated by returning radiation in soft states.

Model predicts spectral features across different accretion regimes.

Self-consistent ionization improves spectral accuracy.

Abstract

In the disk-corona model, the X-ray spectrum of a stellar-mass black hole in an X-ray binary is characterized by three components: a thermal component from a thin and cold accretion disk, a Comptonized component from a hot corona, and a reflection component produced by illumination of the cold disk by the hot corona. In this paper, we assume a lamppost corona and we improve previous calculations of the X-ray spectrum of black hole X-ray binaries. The reflection spectrum is produced by the direct radiation from the corona as well as by the returning radiation of the thermal and reflection components and is calculated considering the actual spectrum illuminating the disk. If we turn the corona off, the reflection spectrum is completely generated by the returning radiation of the thermal component, as it may happen for some sources in soft spectral states. After choosing the radial density profile of the accretion disk, the ionization parameter is calculated self-consistently at any radial coordinate of the disk from the illuminating X-ray flux and the local electron density. We show the predictions of our model in different regimes and we discuss its current limitations as well as the next steps to improve it.