X-ray view of emission lines in optical spectra: Spectral analysis of the two low-mass X-ray binary systems Swift J1357.2–0933 and MAXI J1305–704

We propose a novel approach for determining the orbital inclination of low-mass X-ray binary systems by modelling the Hα and Hβ line profiles emitted by the accretion disc, with a Newtonian (i.e. non-relativistic) version of DISKLINE. We applied the model to two sample sources, Swift J1357.2-0933 and MAXI J1305-704, which are both transient black hole systems, and analyse two observations that were collected during a quiescent state and one observation of an outburst. The line profile is well described by the DISKLINE model, although we had to add a Gaussian line to describe the deep inner core of the double-peaked profile, which the DISKLINE model was unable to reproduce. The Hβ emission lines in the spectrum of Swift 1357.2-0933 and the Hα emission lines in that of MAXI J1305-704 during the quiescent state are consistent with a scenario in which these lines originate from a disc ring between (9.6-57)× 103 Rg and (1.94-20)× 104 Rg, respectively. We estimate an inclination angle of 81a...