Bremsstrahlung in GRMHD Models of Accreting Black Holes

Ricardo Yarza, George N. Wong, Benjamin R. Ryan, Charles F. Gammie

Research output: Contribution to journalArticlepeer-review


The role of bremsstrahlung in the emission from hot accretion flows around slowly accreting supermassive black holes is not thoroughly understood. In order to appraise the importance of bremsstrahlung relative to other radiative processes, we compute spectral energy distributions (SEDs) of accretion disks around slowly accreting supermassive black holes including synchrotron radiation, inverse Compton scattering, and bremsstrahlung. We compute SEDs for (i) four axisymmetric radiative general relativistic magnetohydrodynamics (RadGRMHD) simulations of 108 M o˙ black holes with accretion rates between 10-8˙ MEdd and 10-5˙ MEdd, (ii) four axisymmetric RadGRMHD simulations of M87∗ with varying dimensionless spin a ∗ and black hole mass, and (iii) a 3D GRMHD simulation scaled for Sgr A∗. At 10-8˙MEdd, most of the luminosity is synchrotron radiation, while at 10-5˙ MEdd the three radiative processes have similar luminosities. In most models, bremsstrahlung dominates the SED near 512 keV. In the M87∗ models, bremsstrahlung dominates this part of the SED if a ∗ = 0.5, but inverse Compton scattering dominates if a ∗ = 0.9375. Since scattering is more variable than bremsstrahlung, this result suggests that 512 keV variability could be a diagnostic of black hole spin. In the Appendix, we compare some bremsstrahlung formulae found in the literature.

Original languageEnglish (US)
Article number50
JournalAstrophysical Journal
Issue number1
StatePublished - Jul 20 2020

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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