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

Abstract

X-ray binaries (XRB) are known to exhibit different spectral states which are associated with different black hole accretion modes. Recent measurments of linear polarization of X-ray emission in X-ray binary Cygnus X-1 allow us to test models for the hard 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 are consisitent with the new observational information. The state-of-the-art models of the hard state show that the X-ray emission is predominantly produced by extended jets, away from the central black hole with some contribution from hot corona near the black hole. Our modeling results are supporting the idea that the strong correlations between synchrotron and X-ray emission observed in many XRBs can be attributed to the jet emission. In the presented framework, where first-principle models have limited number of free parameters, the X-ray polarimetric observations put constraints on the viewing angle of the accreting black hole system.Comment: 7 pages, 4 figures, invited contribution to a special issue of a Springer Nature journal, comments are very welcom

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