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

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 He II λ4686 absorption features, blueshifted by ∼600 km s^-1. Remarkably these features are also variable on the ∼500 s dipping period, indicating their likely association with structure in the inner accretion disc. We interpret this as arising in a dense, hot (≳30 000 K) 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 ≳ 4 × 10³⁶ erg s^-1. 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 ≳ 10²⁴ 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 λ4686 absorption, the first (and only) variable dip period and is possibly the first black hole `accretion disc corona' candidate.