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Swift J1749.4-2807 is a transient neutron star low-mass X-ray binary thatcontains an accreting millisecond X-ray pulsar spinning at 518 Hz. It is thefirst of its kind that displays X-ray eclipses, which holds significant promiseto precisely constrain the mass of the neutron star. We report on a ~105 kslong XMM-Newton observation performed when Swift J1749.4-2807 was inquiescence. We detect the source at a 0.5-10 keV luminosity of ~1E33(D/6.7kpc)^2 erg/s. The X-ray light curve displays three eclipses that are consistentin orbital phase and duration with the ephemeris derived during outburst.Unlike most quiescent neutron stars, the X-ray spectrum is best described witha simple power law, while a pure-hydrogen atmosphere model does not fit thedata. We place an upper limit on the 0.01-100 keV thermal luminosity of thecooling neutron star of <2E33 erg/s and constrain its temperature to be <0.1keV (for an observer at infinity). Timing analysis does not reveal evidence forX-ray pulsations near the known spin frequency of the neutron star or its firstovertone with a fractional rms of <34% and <28%, respectively. We discuss theimplications of our findings for dynamical mass measurements, the thermal stateof the neutron star and the origin of the quiescent X-ray emission.

The quiescent X-ray properties of the accreting millisecond X-ray pulsar and eclipsing binary Swift J1749.4-2807