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 1015~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 1015~cm−3, the disk ionization parameter would be overestimated while the inner disk radius is unaffected.
@article{arxiv.2303.10593,
title = {High-density reflection spectroscopy of black hole X-ray binaries in the hard state},
author = {Honghui Liu and Jiachen Jiang and Zuobin Zhang and Cosimo Bambi and Andrew C. Fabian and Javier A. Garcia and Adam Ingram and Erin Kara and James F. Steiner and John A. Tomsick and Dominic J. Walton and Andrew J. Young},
journal= {arXiv preprint arXiv:2303.10593},
year = {2023}
}