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 $\sim$7 % at a polarization angle of $\sim$21 ${}^{\circ }$. 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, $\Gamma \sim$2.1, along with a reflection feature at $\sim$7.0 keV. In the $2-8$ keV band, the degree of polarization of 4U 1630$-$47 in the steep power law state is 4.4 $\sigma$ 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 disk around a Kerr black hole, likely influenced by the frame-dragging effect.

6 pages, 7 figures, 3 tables, accepted for publication in MNRAS

References in corpus (10)

• X-ray Properties of Black-Hole Binaries
• Analyzing the Data from X-ray Polarimeters with Stokes Parameters
• ixpeobssim: a Simulation and Analysis Framework for the Imaging X-ray Polarimetry Explorer
• Design, Construction, and Test of the Gas Pixel Detectors for the IXPE Mission
• IXPE and NICER view of Black hole X-ray binary 4U 1630-47: First significant detection of polarized emission in thermal state
• Suzaku Observation of the Black Hole Binary 4U 1630--47 in the Very High State
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• Detection of X-ray polarized emission and accretion-disk winds with IXPE and NICER in the black-hole X-ray binary 4U 1630-47
• The First X-ray Polarization Observation of the Black Hole X-ray Binary 4U 1630-47 in the Steep Power Law State
• The first X-ray polarimetric observation of the black hole binary LMC X-1

Cited by in corpus (2)

• X-ray polarisation properties of thermal-radiative disc winds in binary systems
• The peculiar disk evolution of 4U 1630--472 observed by Insight-HXMT during the 2022 and 2023 outbursts