X-ray Spectra from General Relativistic RMHD Simulations of Thin Disks

TL;DR

This study compares X-ray spectra from general relativistic radiation magnetohydrodynamic simulations of thin black hole accretion disks, revealing how spin and accretion rate influence spectral features and emission mechanisms.

Contribution

It introduces a detailed post-processing Monte Carlo method to enhance spectral fidelity and compares it with the M1 closure scheme, demonstrating good agreement and insights into spectral hardening and emission components.

Findings

Spectral peaks around 2-2.5 keV match between methods.

Inverse Compton scattering dominates at higher energies with increasing spin.

Faint free-free emission observed near the horizon at high energies.

Abstract

We compare X-ray emission from several general relativistic, multi-frequency, radiation magnetohydrodynamic simulations of thin black hole accretion disks with different accretion rates and spins. The simulations were performed using the M1 closure scheme, resolved with twelve frequency (energy) bins logarithmically spaced from $5 \times 10^{-3}$ to $5 \times 10^3$ keV. We apply a general relativistic Monte Carlo transport code to post-process the simulation data with greater fidelity in frequency resolution and Compton scattering treatment. Despite the relatively few energy bins and Kompaneets approximation to Compton scattering utilized in the M1 method, we find generally good agreement between the methods. Both produce prominent thermal profiles with peaks around 2 - 2.5 keV, where agreement is particularly strong and representative of the soft state. Both also find weaker (lower luminosity) thermally sourced emission extending out to 100 keV due to the hotter innermost 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 upscattered component becomes comparable to the thermal flux, a phenomenon typically associated with intermediate states. Beyond $10^4$ keV, we observe faint, free-free emission from hot, optically thin coronal regions developing near the horizon, common to both spinning and nonspinning black holes.