Volume 528, April 2011
|Number of page(s)||11|
|Section||Stellar structure and evolution|
|Published online||18 March 2011|
XMM-Newton observations of the low-mass X-ray binary EXO 0748−676 in quiescence
ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
2 Observatoire Astronomique de Strasbourg, 11 rue de l’Université, 67000 Strasbourg, France
3 SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
4 Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
5 European Space Astronomy Centre (ESAC), Science Operations Department, Villanueva de la Cañada (Madrid), Spain
Received: 25 November 2010
Accepted: 5 February 2011
The neutron star low-mass X-ray binary EXO 0748−676 started a transition from outburst to quiescence in August 2008, after more than 24 years of continuous accretion. The return of the source to quiescence has been monitored extensively by several X-ray observatories. Here, we report on four XMM-Newton observations elapsing a period of more than 19 months that started in November 2008. The X-ray spectra contain a soft thermal component that we fit with a neutron-star atmosphere model. In only the first observation do we find a significant second component above ~3 keV accounting for ~7% of the total flux, which might be indicative of residual accretion. The thermal bolometric flux and the temperature of the neutron star crust decrease steadily by 40% and 10%, respectively, between the first and the fourth observations. At the time of the last observation in June 2010, we obtain a thermal bolometric luminosity of 5.6 × 1033 (d/7.1 kpc)2 erg s-1 and a temperature of the neutron star crust of 109 eV. The cooling curve is consistent with a relatively hot medium-mass neutron star cooling by standard mechanisms. From the spectral fits to a neutron-star atmosphere model, we infer limits to the mass and the radius of the neutron star. We find that to achieve self-consistency between the neutron-star masses derived using the different methods, the value of the distance is constrained to be ≲6 kpc. For this value of the distance, the derived mass and radius contours are consistent with a number of EoSs with nucleons and hyperons.
Key words: X-rays: binaries / accretion, accretion disks / X-rays: individuals: EXO 0748 − 676 / stars: neutron
© ESO, 2011
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