Abstract It has long been thought that black hole accretion flows are driven by magnetohydrodynamic turbulence, and there are now many general relativistic global simulations illustrating the dynamics of this process. However, many challenges must be overcome in order to predict observed spectra from luminous systems. Ensuring energy conservation, local thermal balance, and local ionization equilibrium, our post-processing method incorporates all the most relevant radiation mechanisms: relativistic Compton scattering, bremsstrahlung, and lines and edges for 30 elements and all their ions. Previous work with this method was restricted to black holes of 10 M ⊙ ; here, for the first time, we extend it to 10 8 M ⊙ and present results for two sub-Eddington accretion rates and black hole spin parameter 0.9. The spectral shape predicted for stellar-mass black holes matches the low-hard state for the lower accretion rate and the steep power law state for the higher accretion rate. For high-mass black holes, both accretion rates yield power-law continua from ∼0.5 to 50 keV whose X-ray slopes agree well with observations. For intermediate-mass black holes, we find a soft X-ray excess created by inverse Compton scattering of low-energy photons produced in the thermal part of the disk; this mechanism may be relevant to the soft X-ray excess commonly seen in massive black holes. Thus, our results show that standard radiation physics applied to general relativistic magnetohydrodynamical simulation data can yield spectra reproducing a number of the observed properties of accreting black holes across the mass spectrum.
Nagele et al. (2026) studied this question.