IXPE observation confirms a high spin in the accreting black hole 4U 1957+115

TL;DR

This study reports the first X-ray polarimetric observation of the black hole binary 4U 1957+115, revealing a high spin parameter consistent with strong gravitational effects near the black hole.

Contribution

It provides the first polarimetric data for 4U 1957+115 and constrains the black hole spin to be higher than 0.96, indicating a rapidly spinning black hole.

Findings

Polarisation degree of 1.9% in 2-8 keV range

Models with spin lower than 0.96 are disfavoured

Significant role of returning radiation in the observed polarisation

Abstract

We present the results of the first X-ray polarimetric observation of the low-mass X-ray binary 4U 1957+115, performed with the Imaging X-ray Polarimetry Explorer in May 2023. The binary system has been in a high-soft spectral state since its discovery and is thought to host a black hole. The $\sim$571 ks observation reveals a linear polarisation degree of $1.9\% \pm 0.6\%$ and a polarisation angle of $-41^\circ.8 \pm 7^\circ.9$ in the 2-8 keV energy range. Spectral modelling is consistent with the dominant contribution coming from the standard accretion disc, while polarimetric data suggest a significant role of returning radiation: photons that are bent by strong gravity effects and forced to return to the disc surface, where they can be reflected before eventually reaching the observer. In this setting, we find that models with a black hole spin lower than 0.96 and an inclination lower than $50^\circ$ are disfavoured.