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Time-resolved SALT spectra of the short-period, dipping X-ray transient,Swift J1357.2-0933, during its 2017 outburst has revealed broad Balmer andHeII4686 absorption features, blue-shifted by ~600 km/s. Remarkably thesefeatures are also variable on the ~500s dipping period, indicating their likelyassociation with structure in the inner accretion disc. We interpret this asarising in a dense, hot (>~30,000K) outflowing wind seen at very highinclination, and draw comparisons with other accretion disc corona sources. Weargue against previous distance estimates of 1.5 kpc and favour a value >~6kpc, implying an X-ray luminosity L_X>~4x10^{36} erg/s$. Hence it is not a veryfaint X-ray transient. Our preliminary 1D Monte-Carlo radiative transfer andphotoionization calculations support this interpretation, as they imply a highintrinsic L_X, a column density N_H>~10^{24} cm^{-2} and a low covering factorfor the wind. Our study shows that Swift J1357.2-0933 is truly remarkableamongst the cohort of luminous, galactic X-ray binaries, showing the firstexample of He II absorption, the first (and only) variable dip period and ispossibly the first black hole 'accretion disc corona' candidate.

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