Spectro-polarimetry of 4U 1957+115 with IXPE: Effects of spin and returning radiation on polarised emission of black hole in thermal state

TL;DR

This study uses IXPE and NICER observations to analyze the polarization and spectral properties of the black hole binary 4U 1957+115 in a thermal state, revealing energy-dependent polarization, high black hole spin, and effects of returning radiation.

Contribution

First comprehensive spectro-polarimetric analysis of 4U 1957+115 in thermal state, estimating black hole spin and exploring polarization effects due to system inclination and returning radiation.

Findings

Polarization degree increases with energy.

Black hole spin estimated as 0.988±0.001.

Polarization angle shows energy-dependent switching.

Abstract

We present a comprehensive spectro-polarimetric study of persistent Black hole X-ray binary 4U $1957\!+\!115$ with {\it IXPE} and {\it NICER} observations. The source is observed in disk dominated thermal state with disk temperature, kT$_{in}\approx1.4$ keV. The emission during thermal state from the source is found to be moderately polarized and \textit{IXPE} measures a degree of polarization (PD) $= 1.95\pm0.37\%$ ($>4.79\sigma$) along with a polarization angle (PA) $= -42.78^{\circ}\pm5.41^{\circ}$ in the energy range of $2-8$ keV. PD is found to be an increasing function of energy, whereas PA indicates switching within the energy range which could be due to high inclination and the returning radiation within the system. Simultaneous energy spectra ($0.6-10$ keV) from {\it NICER} are modelled to study the spectral properties. Furthermore, the spin parameter of the black hole is estimated with spectro-polarimetric data as a$_{\ast}=0.988\pm0.001\,(1\sigma)$, which is corroborated by {\it NICER} observations. Finally, we discuss the implications of our findings.