A low-luminosity soft state in the short-period black hole X-ray binary Swift J1753.5-0127

Abstract

We present results from the spectral fitting of the candidate black hole X-ray binary Swift J1753.5-0127 in an accretion state previously unseen in this source. We fit the 0.7–78 keV spectrum with a number of models, however the preferred model is one of a multitemperature disc with an inner disc temperature kTin = 0.252 ± 0.003 keV scattered into a steep power-law with photon index ?=6.39+0.08?0.02 and an additional hard power-law tail (? = 1.79 ± 0.02). We report on the emergence of a strong disc-dominated component in the X-ray spectrum and we conclude that the source has entered the soft state for the first time in its ?10 yr prolonged outburst. Using reasonable estimates for the distance to the source (3 kpc) and black hole mass (5?M?), we find the unabsorbed luminosity (0.1–100 keV) to be ?0.60?per?cent of the Eddington luminosity, making this one of the lowest luminosity soft states recorded in X-ray binaries. We also find that the accretion disc extended towards the compact object during its transition from hard to soft, with the inner radius estimated to be Rin=28.0+0.7?0.4Rg or ?12Rg, dependent on the boundary condition chosen, assuming the above distance and mass, a spectral hardening factor f = 1.7 and a binary inclination i = 55°.

More information

Identifiers

ORCID for P. Gandhi: orcid.org/0000-0003-3105-2615

ORCID for D. Altamirano: orcid.org/0000-0002-3422-0074

Catalogue record

Export record

Altmetrics