Divya Rawat

We probe the spectropolarimetric properties of the black hole binary source 4U 1630−47 in the steep power-law state. We detect a significant polarization fraction of ∼7 per cent at a polarization angle of ∼21°. The 2–12 keV NICER spectrum can be fitted with a combination of a thermal and a Comptonization component, the latter characterized by a spectral index, Γ ∼ 2.1, along with a reflection feature at ∼7.0 keV. In the 2–8 keV band, the degree of polarization of 4U 1630−47 in the steep power-law state is 4.4σ different from the value previously measured in the high-soft state. In the steep power-law state, the polarization fraction increases as a function of energy but exhibits an overall drop in each energy band compared to that of the high-soft state. We propose that the decrease in the polarization fraction in the steep power-law state could be attributed to the presence of a corona. The observed polarization properties in both states can be explained by the self-irradiation of the disc around a Kerr black hole, likely influenced by the frame dragging effect.

MAXI J1535−571 is a black-hole X-ray binary that in 2017 exhibited a very bright outburst which reached a peak flux of up to 5 Crab in the 2–20 keV band. Given the high flux, several X-ray space observatories obtained unprecedented high signal-to-noise data of key parts of the outburst. In our previous paper, we studied the corona of MAXI J1535−571 in the hard-intermediate state (HIMS) with Insight-HXMT. In this paper, we focus on the study of the corona in the soft-intermediate state (SIMS) through the spectral-timing analysis of 26 NICER detections of the type-B quasi-periodic oscillations (QPOs). From simultaneous fits of the energy, rms and lag spectra of these QPOs with our time-dependent Comptonization model, we find that in the SIMS the corona size is ∼6500 km and vertically extended. We detect a narrow iron line in the energy spectra, which we interpret to be due to the illumination of the outer part of the accretion disc by this large corona. We follow the evolution of the corona and the radio jet during the HIMS–SIMS transition, and find that the jet flux peaks after the time when the corona extends to its maximum vertical size. The jet flux starts to decay after the corona contracts vertically towards the black hole. This behaviour points to a connection between the X-ray corona and the radio jet similar to that seen in other sources.