Poster abstract details

Swift J1357.2-0933 is one of the shortest period black hole X-ray transients (BHTs), and exhibits deep optical dips together with an extremely broad H$\alpha$ line during outburst. We present GTC time-resolved spectroscopy of this outstanding BHT during its quiescent state aiming to detect the companion star absorption features. The high accretion flow contribution to the total luminosity prevents the direct detection of the companion star. Nevertheless, we constrain the veiling factor to $X>0.81$, which sets more constraining lower limits to the distance ($d > 2.29\, \rm{kpc}$) and the height over the Galactic plane ($z>1.75\, \rm{kpc}$). This places the system in the galactic thick disc. We measure a modulation in the centroid of the H$\alpha$ line with a period of $P=0.11\pm0.04\, \rm{d}$, which combined with the recently presented FWHM-Radial velocity correlation results in a $\sim$M2V companion star ($M_2\sim 0.40\, \rm{M_\odot}$) as well as one of the most massive black holes in the Galaxy ($M_1>9.72 \, \rm{M_\odot}$).