Phase-resolved spectroscopy of the low-mass X-ray binary V801 Ara

Abstract

We present phase-resolved optical spectra of the low-mass X-ray binary system V801 Ara. The spectra, obtained in 2014 with Inamori Magellan Areal Camera and Spectrograph on the Magellan/Baade telescope at Las Campanas Observatory, cover the full binary orbit of 3.8 h. They contain strong emission features allowing us to map the emission of H α, H β, He II λ4686, and the Bowen blend at λ4640. The radial velocity curves of the Bowen blend shows significantly stronger modulation at the orbital period than H α as expected for the former originating on the secondary with the latter consistent with emission dominated by the disc. Our tomograms of H a and H β are the most detailed studies of these lines for V801 to date and they clearly detect the accretion disc. The H β emission extends to higher velocities than H α, suggesting emission from closer to the neutron star and differentiating temperature variance in the accretion disc for the first time. The centre of the accretion disc appears offset from the centre-of-mass of the neutron star as has been seen in several other X-ray binaries. This is often interpreted to imply disc eccentricity. Our tomograms do not show strong evidence for a hotspot at the point where the accretion stream hits the disc. This could imply a reduced accretion rate or could be due to the spot being drowned out by bright accretion flow around it. There is enhanced emission further along the disc, however, which implies gas stream interaction downstream of the hotspot.