Spectropolarimetric study of 4U 1630−47 in steep power-law state with IXPE and NICER

ABSTRACT

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.