Bremsstrahlung in GRMHD models of accreting black holes

R. Yarza, G. N. Wong, B. R. Ryan, C. F. Gammie

Published 2020-06-01Version 1

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 $10^{8}M_{\odot}$ black holes with accretion rates between $10^{-8}\dot{M}_{\text{Edd}}$ and $10^{-5}\dot{M}_{\text{Edd}}$, (ii) four axisymmetric RadGRMHD simulations of M87$^\ast$ with varying dimensionless spin $a_\ast$ and black hole mass, and (iii) a 3D GRMHD simulation scaled for Sgr A$^\ast$. At $10^{-8}\dot{M}_{\text{Edd}}$, most of the luminosity is synchrotron radiation, while at $10^{-5}\dot{M}_{\text{Edd}}$ the three radiative processes have similar luminosities. In most models, bremsstrahlung dominates the SED near $512\text{ keV}$. In the M87$^\ast$ models, bremsstrahlung dominates this part of the SED if $a_{\ast} = 0.5$, but inverse Compton scattering dominates if $a_{\ast}= 0.9375$. Since scattering is more variable than bremsstrahlung, this result suggests that $512\text{ keV}$ variability could be a diagnostic of black hole spin. In the appendix, we compare some bremsstrahlung formulae found in the literature.