Revealing the spectro-polarimetric insights of black hole X-ray binaries with IXPE and Prospects of XPoSat

We present the results of a comprehensive spectro-polarimetric analysis of nine black hole X-ray binaries namely Cyg X−1, 4U 1630−47, 4U 1957+115, Cyg X−3, LMC X−1, LMC X−3, Swift J1727.8−1613, Swift J151857.0−572147 and GX 339−4 using quasi-simultaneous IXPE, NICER and NuSTAR observations. Polarimetric measurements with IXPE confirm the detection of a significant degree of polarization varying in the range of 1.44 − 21.41% (2 − 8 keV) for all the sources in different spectral states (LHS, HIMS, SIMS, HSS) except LMC X−1. The energy-dependent polarization study imparts a significant (> 4σ) increase of polarization degree with energy up to ∼ 11% (25%) for 4U 1630−47 (Cyg X−3) whereas marginal (< 2σ) variation is observed for rest of the sources. A detailed spectro-polarimetric study (2 − 8 keV) and broad-band spectral analysis (0.7 − 60 keV) using the high quality IXPE, NICER and NuSTAR observations further corroborates the detection of polarization from model-dependent approach and reveals the morphologies of distinct spectral states, respectively. In addition, we find a significant correlation between the spectro-polarimetric parameters, tightly connected to the spectral states of the sources. Based on the obtained results, we infer the disc-jet-corona geometry of the sources in different spectral states and the possible origin of the polarized emissions. Finally, we discuss the prospects of India’s first X-ray polarimetric mission, XPoSat in the context of IXPE results.