High-density reflection spectroscopy of black hole X-ray binaries in the hard state

TL;DR

This study analyzes high-density relativistic reflection spectra of six black hole X-ray binaries in the hard state, revealing higher disk densities than previously assumed and providing insights into disk structure and plasma conditions.

Contribution

It introduces a high-density reflection analysis for black hole binaries, showing most require densities above 10^{15} cm^{-3} and comparing these to active galactic nuclei.

Findings

76% of observations need higher disk density than 10^{15} cm^{-3}

Inner disk radius is close to the innermost stable orbit

Coronal temperatures are lower than purely thermal plasma predictions

Abstract

We present a high-density relativistic reflection analysis of 21 spectra of six black hole X-ray binaries in the hard state with data from \textit{NuSTAR} and \textit{Swift}. We find that 76\% of the observations in our sample require a disk density higher than the 10$^{15}$~cm$^{-3}$ assumed in the previous reflection analysis. Compared with the measurements from active galactic nuclei, stellar mass black holes have higher disk densities. Our fits indicate that the inner disk radius is close to the innermost stable circular orbit in the hard state. The coronal temperatures are significantly lower than the prediction of a purely thermal plasma, which can be explained with a hybrid plasma model. If the disk density is fixed at 10$^{15}$~cm$^{-3}$, the disk ionization parameter would be overestimated while the inner disk radius is unaffected.