The cooling, mass and radius of the neutron star in EXO 0748-676 in quiescence with XMM-Newton

Cheng, Z., Méndez, M., Díaz-Trigo, M. & Costantini, E., 1-Nov-2017, In : Monthly Notices of the Royal Astronomical Society. 471, 3, p. 2605-2615 11 p.

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We analyse four XMM-Newton observations of the neutron-star low-mass X-ray binary EXO 0748-676 in quiescence. We fit the spectra with an absorbed neutron-star atmosphere model, without the need for a high-energy (power-law) component; with a 95 per cent confidence the power law contributes less than 1 per cent to the total flux of the source in 0.5-10.0 keV. The fits show significant residuals at around 0.5 keV which can be explained by either a hot gas component around the neutron star or a moderately broad emission line from a residual accretion disc. The temperature of the neutron star has decreased significantly compared to the previous observation, from 124 to 105 eV, with the cooling curve being consistent with either an exponential decay plus a constant or a (broken) power law. The best-fitting neutron-star mass and radius can be better constrained if we extend the fits down to the lowest possible energy available. For an assumed distance of 7.1 kpc, the best-fitting neutron-star mass and radius are 2.00(0.24)(+0.07) M-circle dot and 11.3(1.0)(+1.3) km if we fit the spectrum over the 0.3-10 keV range, but 1.50(1.0)(+0.4) M-circle dot and 12.2(3.6)(+0.8) km if we restrict the fits to the 0.5-10 keV range. We finally discuss the effect of the assumed distance to the source upon the best-fitting neutron-star mass and radius. As systematic uncertainties in the deduced mass and radius depending on the distance are much larger than the statistical errors, it would be disingenuous to take these results at face value.

Original languageEnglish
Pages (from-to)2605-2615
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - 1-Nov-2017


  • dense matter, equation of state, stars: individual: EXO 0748-676, stars: neutron, X-rays: binaries, X-RAY TRANSIENTS, PHOTOIONIZATION CROSS-SECTIONS, EMISSION-LINE, LIGHT CURVES, ANGULAR-DISTRIBUTION, BINARY EXO-0748-67, MAXI J0556-332, BURST SPECTRA, ABSORPTION, DISCOVERY

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