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

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 $\sim 2.6$. There is an evidence that either the inner disk radius varied by $\sim 25$ % or the colour hardening factor changed by $\sim 12$ %. 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 $\sim 10$ 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.

arXiv comment8 Pages, 4 figures and 3 Tables. Accepted for publication in Monthly Notices of Royal Astronomical Society (MNRAS)

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Cited by in corpus (2)

• NICER/NuSTAR Characterization of 4U 1957+11: A Near Maximally Spinning Black Hole Potentially in the Mass Gap
• Probing the soft state evolution of 4U 1543-47 during its 2021 outburst using AstroSat