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

TL;DR

This paper analyzes X-ray observations of the black hole system Swift J1357.2-0933 during its 2017 outburst, revealing a truncated accretion disk and spectral components at very low luminosity.

Contribution

It provides the first detailed spectral analysis of this system during an early low hard state at extremely low luminosity, incorporating relativistic effects and high inclination assumptions.

Findings

Detection of a disk blackbody component with T_in ~ 0.06 keV

Evidence for a truncated accretion disk within a few RISCO

Spectral modeling consistent with a high inclination of 70°

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.