UV and X-ray observations of the neutron star LMXB EXO 0748–676 in its quiescent state
Abstract
The accretion behaviour in low-mass X-ray binaries (LMXBs) at low luminosities, especially at <1034 erg s−1, 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, C iv. The line is relatively broad (≳3500 km s−1), 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.
Citation
Parikh , A S , Degenaar , N , Hernández Santisteban , J V , Wijnands , R , Psaradaki , I , Costantini , E , Modiano , D & Miller , J M 2021 , ' UV and X-ray observations of the neutron star LMXB EXO 0748–676 in its quiescent state ' , Monthly Notices of the Royal Astronomical Society , vol. 501 , no. 1 , pp. 1453-1462 . https://doi.org/10.1093/mnras/staa3734
Publication
Monthly Notices of the Royal Astronomical Society
Status
Peer reviewed
ISSN
0035-8711Type
Journal article
Description
Funding: AP, ND, and JHS are supported by a Vidi grant awarded to ND by the Netherlands Organization for Scientific Research (NWO). IP and EC are supported by the Vidi grant 639.042.525. Further support for HST program GO-13108 was provided by NASA through a grant from the STScI.Collections
Items in the St Andrews Research Repository are protected by copyright, with all rights reserved, unless otherwise indicated.