The X-ray binary Swift J1749.4-2807 contains a neutron star that rotates around its own axis at a dazzling rate of 518 times per second. To date, only 14 of such fast spinning accreting X-ray pulsars are known. Amongst these, Swift J1749.4-2807 is the only one that shows eclipses: a temporary dramatic drop in the X-ray emission that lasts for approximately 36 minutes and repeats every 8.8 hours. These are caused by the companion star that periodically moves into our line of sight, thereby blocking the X-ray bright central part of the binary.
The unique combination of X-ray pulsations and eclipses makes Swift J1749.4-2807 a particularly promising target to precisely constrain the mass of the neutron star. This is one of the key objectives of modern astrophysics. We used the European satellite XMM-Newton to study the source in quiescence, when the accretion is thought to have switched off and the surface of the neutron star may become directly visible. Quiescent X-ray observations are an important aspect of the challenge to accurately constrain the mass of the neutron star.
Contrary to that seen for the majority of neutron stars, we found that the quiescent X-ray spectrum of Swift J1749.4-2807 consists primarily of high-energy (> 2 keV) photons and shows no evidence for heat radiation that comes from the surface of the neutron star. Its unusual properties can possibly be explained if matter continues to fall onto the neutron star in quiescence. This severely complicates the determination of its mass. It is of utmost importance to understand whether quiescent accretion is common amongst neutron star X-ray binaries.
Degenaar, Patruno, Wijnands 2012, ApJ 756, 148: The Quiescent X-Ray Properties of the Accreting Millisecond X-Ray Pulsar and Eclipsing binary Swift J1749.4-2807
Paper link: ADS
Discovery of eclipses in Swift J1749.4-2807 (2010): NASA press release