Multiwavelength spectral evolution during the 2011 outburst of the very faint X-ray transient Swift J1357.2-0933

TL;DR

This study analyzes the multiwavelength spectral evolution of the 2011 outburst of the very faint X-ray transient Swift J1357.2-0933, revealing its spectral states, correlations, and potential black hole nature.

Contribution

First detailed multiwavelength analysis of Swift J1357.2-0933's outburst, highlighting its spectral evolution and accretion characteristics at very faint luminosities.

Findings

X-ray peak luminosity ~1E35 erg/s classifies it as very faint transient.

Spectral softening observed as luminosity decreases.

Correlations suggest a non-irradiated viscous disc accretion system.

Abstract

We report our multiwavelength study of the 2011 outburst evolution of the newly discovered black hole candidate X-ray binary Swift J1357.2-0933. We analysed the Swift X-ray telescope and Ultraviolet/Optical telescope (UVOT) data taken during the ~7 months duration of the outburst. It displayed a 2-10 keV X-ray peak luminosity of ~1E35(D/1.5 kpc)^2 erg s-1 which classifies the source as a very faint X-ray transient. We found that the X-ray spectrum at the peak was consistent with the source being in the hard state, but it softened with decreasing luminosity, a common behaviour of black holes at low luminosities or returning to quiescence from the hard state. The correlations between the simultaneous X-ray and ultraviolet/optical data suggest a system with a black hole accreting from a viscous disc that is not irradiated. The UVOT filters provide the opportunity to study these correlations up to ultraviolet wavelengths a regime so far unexplored. If the black hole nature is confirmed, Swift J1357.2-0933 would be one of the very few established black hole very-faint X-ray transients.