H$\beta$ spectroscopy of the high-inclination black hole transient Swift J1357.2-0933 during quiescence

Abstract

Swift J1357.2-0933 is a transient low-mass X-ray binary hosting a stellar-mass black hole. The source exhibits optical dips and very broad emission lines during both outburst and quiescence, which are thought to be the result of a high orbital inclination. We present phase-resolved spectroscopy obtained with the 10.4m Gran Telescopio Canarias (GTC). The spectra focus on the $\mathrm{H}\beta$ spectral region during X-ray quiescence. The emission line is exceptionally broad (full width at half maximum, FWHM > 4000 Å), in agreement with previous studies focused on $\mathrm{H}\alpha$. A two-Gaussian fit to the prominent double-peaked profile reveals a periodic variability in the centroid position of the line. We also produced a diagnostic diagram aimed at constraining additional orbital parameters. Together, they allow us to independently confirm the orbital period of the system using a new dataset obtained five years after the previous outburst. However, our estimates for both the systemic velocity and the radial velocity semi-amplitude of the black hole reveal larger values than those found in previous studies. We argue that this could be explained by the precession of the disc and the presence of a hotspot. We found evidence of a narrow inner core in the double-peaked H$\beta$ emission profile. We studied its evolution across the orbit, finding that it is likely to result from the occultation of inner material by the outer rim bulge, further supporting the high orbital inclination hypothesis.

6 pages, 4 figures

References in corpus (8)

• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• Optical precursors to X-ray binary outbursts
• Swift J1357.2-0933: the faintest black hole?
• Dynamical constraints on the neutron star mass in EXO 0748-676
• Optical spectroscopy of the dwarf nova U Geminorum
• A correlation between Ha trough depth and inclination in quiescent X-ray transients: evidence for a low-mass black hole in GRO J0422+32
• Spectral analysis of the low-mass X-ray pulsar 4U 1822-371: Reflection component in a high-inclination system
• Puzzling Blue Dips in the Black Hole Candidate Swift J1357.2-0933, from ULTRACAM, SALT, ATCA, Swift and NuSTAR

Cited by in corpus (1)

• 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