A relativistically broadened O VIII Lyalpha line in the ultra-compact X-ray binary 4U 0614+091

TL;DR

This paper reports the detection of a broad O VIII Lyalpha emission line in the X-ray spectrum of the ultra-compact binary 4U 0614+091, attributed to reflection near the neutron star, revealing details of the accretion environment.

Contribution

First detection of a relativistically broadened O VIII Lyalpha line in an ultra-compact X-ray binary, indicating reflection close to the neutron star.

Findings

Detection of a broad emission feature at ~0.7 keV.

Interpretation as O VIII Lyalpha emission from reflection.

Reflection occurs in the strong gravitational field near the neutron star.

Abstract

Ultra-compact X-ray binaries consist of a neutron star or black hole that accretes material from a white dwarf-donor star. The ultra-compact nature is expressed in very short orbital periods of less than 1 hour. 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 analyzed high-resolution RGS and broad band 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 Lyalpha emission caused by reflection of X-rays off highly ionized oxygen, in the strong gravitational field close to the neutron star.