A relativistically broadened O<scp>viii</scp> Lyα line in the ultracompact X-ray binary 4U 0614+091

Abstract

Abstract Ultracompact X-ray binaries consist of a neutron star or black hole that accretes material from a white dwarf donor star. The ultracompact nature is expressed in very short orbital periods of less than 1 h. In the case of 4U 0614+091 oxygen-rich material from a CO or ONe white dwarf is flowing to the neutron star. This oxygen-rich disc can reflect X-rays emitted by the neutron star giving a characteristic emission spectrum. We have analysed high-resolution Reflection Grating Spectrometer and broad-band European Photon Imaging Camera (EPIC) spectra of 4U 0614+091 obtained by the XMM–Newton satellite. We detect a broad emission feature at ∼0.7 keV in both instruments, which cannot be explained by unusual abundances of oxygen and neon in the line of sight, as proposed before in the literature. We interpret this feature as O viii Lyα emission caused by reflection of X-rays off highly ionized oxygen, in the strong gravitational field close to the neutron star.