High-density Reflection Spectroscopy of Black Hole X-Ray Binaries in the Hard State

Honghui Liu, Jiachen Jiang, Zuobin Zhang, Cosimo Bambi, Andrew C. Fabian, Javier Alonso-García, Adam Ingram, Erin Kara, James F. Steiner, John A. Tomsick, Dominic J. Walton, Andrew J. Young

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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 NuSTAR and 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 luminous 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 is overestimated while the inner disk radius is unaffected.
Original languageEnglish
Article number145
Pages (from-to)1-17
Number of pages17
JournalThe Astrophysical Journal
Issue number2
Early online date10 Jul 2023
Publication statusPublished - 12 Jul 2023


  • X-ray binary stars
  • Stellar-mass black holes
  • High-energy astrophysics
  • Accretion


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