# Hot, dense He II outflows during the 2017 outburst of the X-ray transient Swift J1357.2-0933

## TL;DR

This study reveals dense, hot outflows in the 2017 outburst of Swift J1357.2-0933, characterized by variable He II absorption features, high luminosity, and unique accretion disc wind properties, challenging previous distance estimates.

## Contribution

It provides the first evidence of He II absorption in this source, suggests a greater distance and luminosity than previously thought, and introduces a new candidate for black hole accretion disc corona phenomena.

## Key findings

- Detection of variable He II absorption features.
- Revised distance estimate >6 kpc, implying higher luminosity.
- Identification of a dense, hot outflowing wind at high inclination.

## Abstract

Time-resolved SALT spectra of the short-period, dipping X-ray transient, Swift J1357.2-0933, during its 2017 outburst has revealed broad Balmer and HeII4686 absorption features, blue-shifted by ~600 km/s. Remarkably these features are also variable on the ~500s dipping period, indicating their likely association with structure in the inner accretion disc. We interpret this as arising in a dense, hot (>~30,000K) outflowing wind seen at very high inclination, and draw comparisons with other accretion disc corona sources. We argue against previous distance estimates of 1.5 kpc and favour a value >~6 kpc, implying an X-ray luminosity L_X>~4x10^{36} erg/s$. Hence it is not a very faint X-ray transient. Our preliminary 1D Monte-Carlo radiative transfer and photoionization calculations support this interpretation, as they imply a high intrinsic L_X, a column density N_H>~10^{24} cm^{-2} and a low covering factor for the wind. Our study shows that Swift J1357.2-0933 is truly remarkable amongst the cohort of luminous, galactic X-ray binaries, showing the first example of He II absorption, the first (and only) variable dip period and is possibly the first black hole 'accretion disc corona' candidate.