What is the hard spectral state in X-ray binaries? Insights from GRRMHD accretion flows simulations and polarization of their X-ray emission

TL;DR

This paper investigates the hard spectral state in X-ray binaries by comparing GRRMHD simulations with IXPE polarization observations, providing new insights into accretion flow geometry and properties.

Contribution

It demonstrates that GRRMHD simulations can be consistent with polarization data, constraining the viewing angle of the hot accretion flow in X-ray binaries.

Findings

GRRMHD simulations align with IXPE polarization measurements.

Polarization constrains the viewing angle of the accretion flow.

Supports specific accretion geometries in the hard state.

Abstract

X-ray binaries are known to exhibit different spectral states which are often associated with different black hole accretion modes. The exact geometry and properties of these accretion modes is still uncertain. Recent IXPE measurements of linear polarization of X-ray emission in canonical X-ray binary system Cygnus X-1 allow us to test models for the hard spectral state of accretion in a unique way. We show that general relativistic radiative magnetohydrodynamic (GRRMHD) simulations of accreting stellar black hole in a hard X-ray state may be consistent with the new observational information. In the presented framework, where first-principle models have limited number of free parameters, the polarimetric X-ray observations put constraints on the viewing angle of the inner hot accretion flow.