2011A&A...528A.150D

The neutron star low-mass X-ray binary EXO0748-676 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 ∼3keV 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 5.6x10³³(d/7.1kpc)²erg/s and a temperature of the neutron star crust of 109eV. 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 λ6kpc. For this value of the distance, the derived mass and radius contours are consistent with a number of EoSs with nucleons and hyperons.