Spectral characteristics of the black hole binary 4U 1957+115: A multi-mission perspective

TL;DR

This study analyzes the spectral properties of the black hole binary 4U 1957+115 using multi-mission data, revealing its high soft state and constraining the black hole's mass, spin, and distance through detailed spectral modeling.

Contribution

It provides the first comprehensive multi-mission spectral analysis of 4U 1957+115, constraining black hole parameters and accretion properties with advanced modeling.

Findings

Source was in high soft state with 87% disk flux.

Black hole mass estimated to be less than 10 solar masses.

Detected correlation between accretion rate and inner disk radius.

Abstract

We report spectral analysis of the persistent black hole X-ray binary, 4U 1957+115, using AstroSat, Swift and NuSTAR observations carried out between 2016-2019. Modelling with a disk emission, thermal comptonization and blurred reflection components revealed that the source was in the high soft state with the disk flux $\sim 87$ % of the total and high energy photon index $\sim2.6$. There is an evidence that either the inner disk radius varied by $\sim25$ % or the colour hardening factor changed by $\sim12$ %. The values of the inner disk radius imply that for a non-spinning black hole, the black hole mass is $<7$ M$_\odot$ and the source is located $>30 $ kpc away. On the other hand, a rapidly spinning black hole would be consistent with the more plausible black hole mass of $< 10$ M$_\odot$ and a source distance of $\sim10$ kpc. Fixing the distance to $10$ kpc and using a relativistic accretion disk model, constrained the black hole mass to 6 M$_\odot$ and inclination angle to 72$^{\circ}$. A positive correlation is detected between the accretion rate and inner radii or equivalently between the accretion rate and colour factor.