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 kT_(in) = 0.252 ± 0.003 keV 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 (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 R_(in)=28.0^(+0.7)_(−0.4)R_g or ∼12R_g, 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°.

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