UV and X-ray observations of the neutron star LMXB EXO 0748–676 in its quiescent state
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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.
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
Monthly Notices of the Royal Astronomical Society
Copyright © 2020 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This work has been made available online in accordance with publisher policies or with permission. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the final published version of the work, which was originally published at https://doi.org/10.1093/mnras/staa3734.
DescriptionFunding: 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.
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