# The black hole X-ray transient Swift J1357.2-0933 as seen with Swift and NuSTAR during its 2017 outburst

## TL;DR

This study used Swift and NuSTAR observations to analyze the 2017 outburst of black hole transient Swift J1357.2-0933, finding no evidence of the previously hypothesized high-inclination toroidal structure and suggesting reprocessed flux mainly emerges in the ultraviolet.

## Contribution

The paper provides new X-ray and multi-wavelength observational data that challenge the previously proposed high-inclination torus model for Swift J1357.2-0933.

## Key findings

- No intense optical dips observed during 2017 outburst.
- X-ray spectra fit an absorbed power-law without reflection signatures.
- Strong correlation between X-ray and ultraviolet/optical data.

## Abstract

We report on observations of black hole Swift J1357.2-0933, a member of the modest population of very faint X-ray transients. This source has previously shown intense dips in the optical lightcurve, a phenomena that has been linked to the existence of a "unique toroidal structure" in the inner region of the disc, seen at a high inclination. Our observations, carried out by the Neil Gehrels Swift and NuSTAR X-ray observatories, do not show the presence of intense dips in the optical light curves. We find that the X-ray light curves do not show any features that would straightforwardly support an edge-on configuration or high inclination configuration of the orbit. This is similar to what was seen in the X-ray observations of the source during its 2011 outburst. Moreover, the broadband spectra were well described with an absorbed power-law model without any signatures of the cut-off at energies above 10 keV, or any reflection from the disc or the putative torus. Thus, the X-ray data do not support the unique obscuring torus scenario proposed for J1357. We also performed a multi-wavelength study using the data of X-ray telescope and Ultraviolet/Optical Telescope aboard Swift, taken during the 4.5 months duration of the 2017 outburst. This is consistent with what was previously inferred for this source. We found a correlation between the simultaneous X-ray and ultraviolet/optical data and our study suggests that most of the reprocessed flux must be coming out in the ultraviolet.