NuSTAR and Swift observations of Swift J1357.2-0933 during an early phase of its 2017 outburst

Abstract

We present a detailed spectral analysis of Swift and NuSTAR observations of the very faint X-ray transient and black hole system Swift J1357.2-0933 during an early low hard state of its 2017 outburst. Swift J1357.2-0933 was observed at $\sim$0.02% of the Eddington luminosity (for a distance of 2.3 kpc and a mass of 4 M$_{\odot}$). Despite the low luminosity, the broadband X-ray spectrum between 0.3 and 78 keV requires the presence of a disk blackbody component with an inner disk temperature of T$_{\mathrm{in}}$ $\sim$ 0.06 keV in addition to a thermal Comptonization component with a photon index of {\Gamma} $\sim$ 1.70. Using a more physical model, which takes strong relativistic effects into account, and assuming a high inclination of 70$^\circ$, which is motivated by the presence of dips in optical light curves, we find that the accretion disk is truncated within a few RISCO from the black hole, independent of the spin.