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

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 0.7-78keV spectrum with a number of models, however the preferred model is one of a multitemperature disc with an inner disc temperature kTin=0.252 ± 0.003keV scattered into a steep power-law with photon index Γ=6.39+0.08-0.02 and an additional hard power-law tail (γ=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 (3kpc) and black hole mass (5M⊙), we find the unabsorbed luminosity (0.1-100keV) to be 0.60 percent 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+0.7-0.4, Rg or ~ 12 Rg 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°. © 2016 The Authors.