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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- October 2023
- DOI:
- 10.1093/mnras/stad2327
- arXiv:
- arXiv:2307.15347
- Bibcode:
- 2023MNRAS.525..661R
- Keywords:
-
- accretion: accretion discs;
- polarization;
- techniques: polarimetric;
- software: data analysis;
- X-rays: binaries;
- X-rays: individual: 4U 1630-47;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 6 pages, 7 figures, 3 tables, accepted for publication in MNRAS