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.4686 absorption features, blueshifted by similar to 600 km s(-1). Remarkably these features are also variable on the similar to 500 s dipping period, indicating their likely association with structure in the inner accretion disc. We interpret this as arising in a dense, hot (greater than or similar to 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 greater than or similar to 6 kpc, implying an X-ray luminosity L-X greater than or similar to 4 x 10(36) 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 greater than or similar to 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 lambda 4686 absorption, the first (and only) variable dip period and is possibly the first black hole 'accretion disc corona' candidate.