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 H alpha and H beta 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 H beta emission lines in the spectrum of Swift 1357.2-0933 and the H alpha 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)x10(3 )Rg and (1.94 - 20)x10(4) Rg, respectively. We estimate an inclination angle of 81 +/- 5 degrees for Swift J1357.2-0933 and an angle of 73 +/- 4 degrees for MAXI J1305-704. This is entirely consistent with the values reported in the literature. In agreement with the recent literature, our analysis of the outburst spectrum of MAXI J1305-704 revealed that the radius of the emission region deviates from expected values. It is larger than the orbital separation of the system. This outcome implies several potential scenarios, including line profile contamination, an alternative disc configuration that deviates from the Keplerian model, or even the possibility of a circumbinary disc. We caution that these results were derived from a simplistic model that may not fully describe the complicated physics of accretion discs. Despite these limitations, our results for the inclination angles are remarkably consistent with recent complementary studies, and the proposed description of the emitting region remains entirely plausible.