Swift J1357.2-0933: a massive black hole in the Galactic thick disc

TL;DR

This study characterizes Swift J1357.2-0933 as a massive black hole system located in the Galactic thick disc, using spectroscopy to constrain its properties and orbital parameters.

Contribution

It provides new constraints on the distance, mass, and location of the black hole system, and confirms a high orbital inclination through spectroscopic analysis.

Findings

Black hole mass > 9.3 solar masses

System located > 2.29 kpc from Earth in the thick disc

Orbital inclination > 80 degrees

Abstract

Swift J1357.2-0933 is one of the shortest orbital period black hole X-ray transients (BHTs). It exhibited deep optical dips together with an extremely broad H$\alpha$ line during outburst. We present 10.4-m GTC time-resolved spectroscopy during quiescence searching for donor star absorption features. The large contribution of the accretion flow to the total luminosity prevents the direct detection of the companion. Nevertheless, we constrain the non-stellar contribution to be larger than $\sim 80\%$ of the total optical light, which sets new 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-$K_2$ correlation, results in a massive black hole ($M_1>9.3 \, \rm{M_\odot}$) and a $\sim$ M2V companion star ($M_2\sim 0.4\, \rm{M_\odot}$). We also present further evidence supporting a very high orbital inclination ($i\gtrsim 80^\circ$).