UV and X-ray observations of the neutron star LMXB EXO 0748-676 in its quiescent state

TL;DR

This study investigates the quiescent state of the neutron star LMXB EXO 0748-676 through UV and X-ray observations, finding evidence that its X-ray emission likely originates from crust cooling rather than residual accretion.

Contribution

It provides new UV and X-ray observational data of EXO 0748-676 in quiescence, highlighting the possible dominance of crust cooling in X-ray emission and the presence of a broad emission line indicating outflows.

Findings

UV continuum not detected, but a broad Civ emission line observed.

X-ray and UV emissions appear unconnected, with high Lx/Luv ratio.

X-ray emission likely from crust cooling, not residual accretion.

Abstract

The accretion behaviour in low-mass X-ray binaries (LMXBs) at low luminosities, especially at <E34 erg/s, is not well known. This is an important regime to study to obtain a complete understanding of the accretion process in LMXBs, and to determine if systems that host neutron stars with accretion-heated crusts can be used probe the physics of dense matter (which requires their quiescent thermal emission to be uncontaminated by residual accretion). Here we examine ultraviolet (UV) and X-ray data obtained when EXO 0748-676, a crust-cooling source, was in quiescence. Our Hubble Space Telescope spectroscopy observations do not detect the far-UV continuum emission, but do reveal one strong emission line, Civ. The line is relatively broad (>3500 km/s), which could indicate that it results from an outflow such as a pulsar wind. By studying several epochs of X-ray and near-UV data obtained with XMM-Newton, we find no clear indication that the emission in the two wavebands is connected. Moreover, the luminosity ratio of Lx/Luv >100 is much higher than that observed from neutron star LMXBs that exhibit low-level accretion in quiescence. Taken together, this suggests that the UV and X-ray emission of EXO 0748-676 may have different origins, and that thermal emission from crust-cooling of the neutron star, rather than ongoing low-level accretion, may be dominating the observed quiescent X-ray flux evolution of this LMXB.