Journal Article

Further X-ray observations of EXO 0748−676 in quiescence: evidence for a cooling neutron star crust

N. Degenaar, M. T. Wolff, P. S. Ray, K. S. Wood, J. Homan, W. H. G. Lewin, P. G. Jonker, E. M. Cackett, J. M. Miller, E. F. Brown and R. Wijnands

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 412, issue 3, pages 1409-1418
Published in print April 2011 | ISSN: 0035-8711
Published online April 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17562.x
Further X-ray observations of EXO 0748−676 in quiescence: evidence for a cooling neutron star crust

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In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748−676 started a transition from outburst to quiescence, after it actively accreted for more than 24 yr. In a previous work, we discussed Chandra and Swift observations obtained during the first 5 months of this transition. Here, we report on further X-ray observations of EXO 0748−676, extending the quiescent monitoring to 1.6 yr. Chandra and XMM–Newton data reveal quiescent X-ray spectra composed of a soft, thermal component that is well fitted by a neutron star atmosphere model. An additional hard power-law tail is detected that changes non-monotonically over time, contributing between 4 and 20 per cent to the total unabsorbed 0.5–10 keV flux. The combined set of Chandra, XMM–Newton and Swift data reveals that the thermal bolometric luminosity fades from ∼ 1 × 1034 to 6 × 1033 (D/7.4 kpc)2 erg s −1, whereas the inferred neutron star effective temperature decreases from ∼124 to 109 eV. We interpret the observed decay as cooling of the neutron star crust and show that the fractional quiescent temperature change of EXO 0748−676 is markedly smaller than observed for three other neutron star X-ray binaries that underwent prolonged accretion outbursts.

Keywords: accretion, accretion discs; binaries: eclipsing; stars: individual: EXO 0748−676; stars: neutron; X-rays: binaries

Journal Article.  8561 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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