The polarimetric properties of black holes in X-ray binaries

The accreting stellar-mass black holes in X-ray binaries (BH-XRBs) provide unique opportunities to study matter and radiation processes under the influence of extreme gravity. These luminous galactic and extra-galactic X-ray sources are highly variable and exhibit multiple components in the energy spectrum as they undergo different accretion states. Recently, after several spectral and timing studies with many space observatories, spanning over last few decades, the Imaging X-ray polarization explorer (IXPE) mission has revealed the polarimetric properties of BH-XRBs. The IXPE has observed about a dozen BH-XRBs with different binary plane inclinations and in various spectral states in energy range of 2-8 keV. The polarization measurement in a low inclination (~ 30°) system, LMC X-1 is found to be below MDP of 1.1% in soft state whereas high inclination (70°-75°) systems LMC X-3 and 4U 1957+115 display a higher degree of polarization (PD) of about 2%-4% in their soft states, as anticipated. The most intriguing and an exceptionally high PD is measured in the soft state of 4U 1630-47 of about 8% with a statistically significant increase of the PD with energy. From observations of Cyg X-1 and Swift J1727.8-1613, it is observed that soft state display weaker polarization levels than the respective hard states. The angle of polarized emission is found to be constant over the energy range of IXPE for all the BH-XRBs and in case of Cyg X-1 is found to be aligned with the jet emission direction. The polarization properties of semi-infinite electron-scattering atmosphere of the standard geometrically-thin optically-thick accretion disk model combined with relativistic effects are largely consistent with the polarimetric results of BH-XRBs except for 4U 1630-47.