期刊
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
卷 412, 期 3, 页码 1409-1418出版社
OXFORD UNIV PRESS
DOI: 10.1111/j.1365-2966.2010.17562.x
关键词
accretion,accretion discs; binaries: eclipsing; stars: individual: EXO 0748-676; stars: neutron; X-rays: binaries
资金
- Netherlands Organisation for Scientific Research (NWO)
- NASA
- Chandra grant [GO8-9045X]
- Science and Technology Facilities Council [ST/G002339/1] Funding Source: researchfish
- STFC [ST/G002339/1] Funding Source: UKRI
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 similar to 1 x 1034 to 6 x 1033 (D/7.4 kpc)2 erg s -1, whereas the inferred neutron star effective temperature decreases from similar to 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.
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