transients | Nathalie Degenaar

In 1993, the Japanese Advanced Satellite for Cosmology and Astrophysics (ASCA) was successfully launched. This satellite was operated for 7 years (until 2000) and was the first mission that provided X-ray imaging capabilities in a relatively broad energy band (0.3-10 keV). During its lifetime, ASCA carried out two dedicated surveys of the Galactic Center and Plane, where it discovered around 200 distinct X-ray sources.

Up to date, about 1/3 of the ASCA-discovered X-ray sources could not be classified. They have relatively faint X-ray intensities that can trace a variety of Astronomical objects such as strongly magnetized neutron stars (called ‘magnetars’), bright accreting white dwarfs (‘polars’ and ‘intermediate polars’), sub-luminous accreting neutron stars and black holes, X-ray emitting massive stars, as well as foreground stars and background active galaxies (‘active galactic nuclei’).

In 2006, we launched a program to observe 35 of the unclassified ASCA-sources with the Swift satellite. The goal of this program was to study the X-ray spectrum of these objects, to find possible indications of temporal variations in the X-ray intensity and to obtain more accurate X-ray positions that would aid in conducting follow-up observations at other wavelengths (optical, infra-red, radio). With this approach we aim to gain more insight into the nature of the faint unclassified ASCA sources.

With our Swift observations we were able to tentatively identify three accreting compact objects: one likely magnetized white dwarf, one neutron star and one object that is likely a neutron star or a black hole. In addition, we found that three objects are possibly nearby X-ray emitting stars. Finally, we found evidence that two of the ASCA-detected sources likely undergo strong variations in their X-ray intensity, since these were not detected during our Swift observations.

Degenaar, Starling, Evans et al. 2012, A&A 540, 22: Swift follow-up observations of unclassified ASCA sources

Paper link: ADS

X-ray image from the ASCA survey of the Galactic Centre.
Credit: Sugizaki et al. 2001.

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.

Nathalie Degenaar

Tag Archives: transients

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