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

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.