Strong Polarization Signatures from Magnetically Stabilized Luminous Thin Accretion Disks

RESUMO

We utilize the Monte Carlo radiation transport capabilities of Cosmos++ to explore the polarization and Faraday rotation of radiation emitted from a set of general relativistic radiation magnetohydrodynamic simulations of magnetically stabilized, thin, black hole accretion disks. The guiding question is whether or not the Faraday rotation depolarizes the radiation to such a degree as to be inconsistent with the relatively high polarization measurements coming from the Imaging X-ray Polarimetry Explorer (IXPE). After first confirming that our code reproduces expected polarization results for electron clouds, planar scattering atmospheres, and AGN disks, we demonstrate the polarization and Faraday rotation capabilities using a Novikov-Thorne disk threaded with a purely toroidal magnetic field. We then analyze temporal snapshots from three different simulations of luminous, thin accretion disks threaded with different magnetic field configurations. We find that the effects of Faraday rotation are generally modest over the energy range of interest, since the strongest fields present in these simulations ($\gtrsim 10^8$ G) are mostly hidden beneath the photosphere. We can easily produce polarization degrees $\ge 4$\% for sources seen mostly edge-on. As one example of a relevant target, we compare our results to an IXPE observation of Cyg X-1, finding that we can match the polarization degree quite well, though our polarization angle is rotated approximately $90^\circ$ with respect to the observed data. We speculate on a few possible explanations for this.