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Published July 10, 2023 | Published
Journal Article Open

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


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.

Additional Information

© 2023. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. This work was supported by the National Natural Science Foundation of China (NSFC), grant No. 12250610185 and grant No. 11973019, the Natural Science Foundation of Shanghai, grant No. 22ZR1403400, the Shanghai Municipal Education Commission, grant No. 2019-01-07-00-07-E00035, and Fudan University, grant No. JIH1512604. J.J. acknowledges support from the Leverhulme Trust, the Isaac Newton Trust, and St Edmund's College. J.A.T. acknowledges partial support from NASA under Astrophysics Data Analysis Program (ADAP) grant 80NSSC19K0586.

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August 22, 2023
October 20, 2023