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 multi-temperature disk with an inner disk temperature $\mathrm{k}{T}_{\mathrm{in}}=0.252\pm 0.003$ keV scattered into a steep power-law with photon index $\Gamma =6.39_{-0.02}^{+0.08}$ and an additional hard power law tail ($\Gamma =1.79\pm 0.02$). We report on the emergence of a strong disk-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 year prolonged outburst. Using reasonable estimates for the distance to the source ($3$ kpc) and black hole mass ($5M_{\odot }$), we find the unabsorbed luminosity (0.1-100 keV) to be $\approx 0.60$% of the Eddington luminosity, making this one of the lowest luminosity soft states recorded in X-ray binaries. We also find that the accretion disk extended towards the compact object during its transition from hard to soft, with the inner radius estimated to be $R_{\mathrm{in}}=28.0_{-0.4}^{+0.7}{R}_g$ 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 $i=55^{\circ }$.

10 pages, 5 figures, accepted for publication in MNRAS