Neutron stars in X-ray binaries often accrete matter only for a few weeks, after which the accretion stops and the binary remains quiescent for several years. As the naming suggests, it is generally assumed that accretion has completely stopped in quiescence. Yet, the binary still emits X-ray emission (albeit orders of magnitude lower than during the active phase), which is thought to result from the radiation of heat from the neutron star.

The neutron star X-ray binary EXO 1745-248 is located in the globular cluster Terzan 5 (see images) and has been studied in quiescence using Chandra observations obtained in 2003. Unlike the majority of neutron stars, surprisingly, its quiescent emission did not resemble thermal emission. This poses a puzzle for the origin of the quiescent X-ray emission of this X-ray binary.

We used three additional Chandra observations taken in 2009 and 2011 to further study the quiescent X-ray emission of EXO 1745-248. While in 2009 the neutron star was detected at a similar brightness as previously seen, the source had disappeared in 2011! The implied large variation in the quiescent X-ray intensity can possibly be explained if the accretion did not fully stop and the neutron star continued to slowly accumulate matter. Alternatively the 2011 disappearance might be caused by a temporarily obscuration of the X-ray source, for example by the outer edge of the accretion disk.

Degenaar & Wijnands, 2011, MNRAS 422, 581: Strong X-ray variability in the quiescent state of the neutron star low-mass X-ray binary EXO 1745-248

Paper link: ADS

Three-color images of the globular cluster Terzan 5, obtained with the Chandra X-ray satellite.