Speaker
Description
The launch of the Imaging X-ray Polarimetry Explorer (IXPE) marked a major advance in X-ray astronomy by enabling measurements of X-ray polarization from compact astrophysical objects. In accreting black holes, polarization provides a powerful tool for probing the geometry of the emitting region and constraining the black hole spin.
IXPE measured a polarization degree of about 2% in Cygnus X-1 during its soft state, when the emission is dominated by thermal radiation from the accretion disc. However, the observed polarization angle was found to be aligned with the jet direction, showing no evidence of rotation with energy. This result is in tension with classical models of polarization produced in electron-scattering-dominated atmospheres of accretion discs around Kerr black holes. It has recently been proposed that the combined effects of returning radiation, which is polarized parallel to the disc axis, and coronal radiation reflected from the disc may explain the observations, provided that the dimensionless black hole spin is close to unity.
These models, however, assume that the coronal emission is produced by thermal Comptonization, an assumption that is inconsistent with the observed spectrum, particularly the power-law-like tail extending to MeV energies in Cygnus X-1. The data are instead better described by Comptonization of disc photons in a corona containing a hybrid thermal/non-thermal electron population, a natural consequence of particle acceleration by magnetic reconnection or shocks followed by cooling and thermalization through Coulomb collisions and synchrotron self-absorption.
Here I will describe the polarimetric properties of such models and show that Comptonization by the low-temperature component of the electron distribution produces polarization consistent with the IXPE measurements. This model also alleviates the need to invoke a rapidly spinning black hole in Cygnus X-1, a scenario that remains in tension with several observational constraints and theoretical expectations.