The Effects of Magnetic Fields and Inhomogeneities on Accretion Disk Spectra and Polarization

TL;DR

This study uses radiative transfer calculations on magnetohydrodynamical simulations of black hole accretion disks to analyze how magnetic fields and inhomogeneities influence spectra and polarization, with implications for X-ray polarimetry diagnostics.

Contribution

It provides new insights into how magnetic support and density inhomogeneities affect spectral color correction factors and polarization in accretion disks, linking simulations to observable signatures.

Findings

Magnetic support increases the color correction factor by about 10%.

Inhomogeneities soften the spectrum, offsetting magnetic effects.

Magnetic fields cause Faraday depolarization near the spectral peak.

Abstract

We present the results of one and three-dimensional radiative transfer calculations of polarized spectra emerging from snapshots of radiation magnetohydrodynamical simulations of the local vertical structure of black hole accretion disks. The simulations cover a wide range of physical regimes relevant for the high/soft state of black hole X-ray binaries. We constrain the uncertainties in theoretical spectral color correction factors due to the presence of magnetic support of the disk surface layers and strong density inhomogeneities. For the radiation dominated simulation, magnetic support increases the color correction factor by about ten percent, but this is largely compensated by a ten percent softening due to inhomogeneities. We also compute the effects of inhomogeneities and Faraday rotation on the resulting polarization. Magnetic fields in the simulations are just strong enough to produce significant Faraday depolarization near the spectral peak of the radiation field. X-ray polarimetry may therefore be a valuable diagnostic of accretion disk magnetic fields, being able to directly test simulations of magnetorotational turbulence.