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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 517; no. 3; pp. 4489 - 4496
• Main Authors: Mudambi, S P, Gudennavar, S B, Misra, R, Bubbly, S G
• Format: Journal Article
• Language: English
• Published: 02.11.2022
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stac2952

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 and 2019. Modelling with a disc emission, thermal Comptonization, and blurred reflection components revealed that the source was in the high-soft state with the disc flux ∼87 per cent of the total and high-energy photon index ∼2.6. There is an evidence that either the inner disc radius varied by ∼25 per cent or the colour hardening factor changed by ∼12 per cent. The values of the inner disc radius imply that for a non-spinning black hole, the black hole mass is  < 7 M ⊙ 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 ⊙ and a source distance of ∼10 kpc. Fixing the distance to 10 kpc and using a relativistic accretion disc model, constrained the black hole mass to 6 M⊙ and inclination angle to 72°. A positive correlation is detected between the accretion rate and inner radii or equivalently between the accretion rate and colour factor.