IXPE Observations of Stellar-Mass Black Holes in the Thermal, Steep Power Law, and Hard States

X-ray polarimetry offers a unique insight into black hole X-ray binary systems found in soft, intermediate, and hard states of accretion. Every state is dominated by a combination of thermal disk emission and coronal emission—each type containing different information about the space-time around the black hole that can be understood through polarization. The Imaging X-ray Polarimetry Explorer (IXPE) recently measured the polarization signature of seven stellar-mass black holes finding unexpectedly high polarization degrees. Cygnus X-1 and Cygnus X-3 were observed in the hard state where black hole coronal emission tends to dominate. For this state, the polarization of the coronal emission (Comptonized seed photons) can constrain the geometry of the corona, its viewing angle, and the origin of soft X-ray seed photons. 4U 1630-47, LMC X-1, LMC X-3, and 4U 1957+115 were observed in the soft state where thermal emission from the optically thick accretion disk dominates the signal. In this state, the polarization signature of a source can be used to rectify the degeneracies between black hole mass, spin, and inclination. Additionally, Cygnus X-1 and 4U 1630-47 were observed a second time during their intermediate and steep power-law states where the emission consists of competing thermal and coronal components. We discuss conclusions drawn from the polarization measurements of these six stellar-mass black holes and their implications for accretion disks and their coronae.