Polarized x-ray spectra from general relativistic radiation-MHD simulations of thin black hole accretion disks

We compare polarized X-ray emission from several general relativistic, radiation magneto-hydrodynamic simulations of thin black hole accretion disks with different accretion rates, spins, tilts, and truncations. The simulations were performed using either single or multi-frequency M1 closure schemes. We additionally apply a Monte Carlo transport code to postprocess the simulation data with greater fidelity in frequency resolution and Compton scattering treatment. Both approaches produce prominent thermal profiles with peaks around 2 keV, where agreement is particularly strong and representative of the soft state. Both methods also find weaker (relatively low-luminosity) thermally sourced emission extending out to 100 keV due to the hotter inner-most regions of the disks. Inverse Compton scattering becomes increasingly effective at hardening spectral outputs with increasing black hole spin and becomes the dominant mechanism for photons that escape with energies between 10 to several hundred keV. At very high rates of spin, the radiation flux in this up-scattered component becomes comparable to the thermal flux, a phenomenon typically associated with intermediate states. Beyond 10 MeV, we observe faint, free–free emission from hot, optically thin coronal regions developing near the horizon, common to both spinning and non-spinning black holes. All models exhibit modest amounts of polarization (< 5%) in the thermal part of the spectrum, consistent with expectations from thin accretion disks.

LLNL-ABS-2008955