SALT time-resolved optical observations reveal a hot dense wind in the black hole candidate LMXB Swift J1357.2-0933

Time-resolved SALT photometry and spectroscopy of the X-ray transient black hole candidate, Swift J1357.2-0933, during its 2017 and 2019 outbursts, has revealed both continuum dips and transient broad Balmer and HeII 4886 absorption lines. These both occur on a characteristic timescale of several 100 s, which evolves during the outburst to longer periods, over timescales of weeks - months. The transient absorption lines are blue-shifted by up to $\sim$600 km s$ ^{-1}$, but over the course of an individual absorption event converge to rest velocities. We interpret these observations as evidence of structures in the accretion disc which occult the inner regions on a timescale associated with the Keplerian period. The spectral line behaviour is consistent with the presence of a dense, hot ($\sim$30,000K) out-flowing wind, seen at high inclination, and we draw comparisons with other accretion disc corona sources. We argue that the distance is $\sim$6 kpc, much larger than than the previously estimated 1.5 kpc, which implies an X-ray luminosity of L$ _{X}$ $\ge$ 4 x 10$ ^{36}$ erg s$ ^{-1}$. Our initial 1D Monte-Carlo radiative transfer and photoionization calculations support this interpretation, as they imply a high intrinsic L$ _{X}$, a column density of N$ _{H}$ $\ge$ 10$ ^{24}$ cm$^{-2}$ and a low covering factor for the wind. We conclude that Swift J1357.2-0933 is quite unique in comparison to other luminous galactic X-ray binaries, showing variable absorption dips which evolve in period and exhibits the first example of HeII 4686 absorption from an accretion disc corona.