Tag Archives: neutron star

Quiescent but not quite?

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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

Schematic representation of the eclipsing binary Swift J1749.4-2807. Credit: NASA/GSFC.

Schematic representation of the eclipsing binary Swift J1749.4-2807.
Credit: NASA/GSFC.

Chasing the faint ASCA X-ray sources

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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.

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

Now you see me, now you don’t

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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.

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

Peeking into the crust of a neutron star

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The outer layer of a neutron star, its crust, covers about one-tenth of the total stellar radius and consists of ions, electrons and neutrons (see image). Studying the structure and composition of a neutron star crust is of interest because it plays an important role in the emission of gravitational waves and the evolution of the neutron star’s magnetic field.

When neutron stars reside in X-ray binaries, their crusts can be temporarily heated due to the accretion of matter. Once the accretion stops, the crust will cool again as it transports the gained heat towards the stellar core (where it is radiated away in the form of neutrino’s) and towards the surface (where the heat is lost in the form of thermal X-ray emission). Studying the heating and subsequent cooling of the crust of a neutron star carries unique information about its structure and gives insight into a variety of nuclear reaction processes.

We used the Chandra satellite to study the neutron star X-ray binary IGR 17480-2446, which is located in the globular cluster Terzan 5 and discovered in 2010 October. We observed the neutron star at different epochs after it had ended a 10-week episode of accretion. Our first observation revealed that the neutron star was hotter just after the accretion outburst than it had been before. In our subsequent observation the temperature had markedly decreased, although it was still higher than before the accretion activity. This suggests that the crust of the neutron star was heated during the accretion phase and is currently cooling down.

It is the first time that cooling of an accretion-heated neutron star crust has been observed for a neutron star with a “normal” accretion phase of a few weeks. Previous results concerned neutron stars that were heated for several years before the accretion stopped and the crust started to cool. By comparing the observed change in temperature with theoretical calculations, we found evidence for the presence of (strong) sources of heat in the outer layers of the crust. It remains a puzzle what should produce heat at such shallow layers. Further Chandra observations are planned to further investigate the temperature evolution of this neutron star.

Degenaar, Brown & Wijnands 2011, MNRAS Letters 418, L152: Evidence for crust cooling in the transiently accreting 11-Hz X-ray pulsar in the globular cluster Terzan 5

Paper link: ADS

Dutch Press release: Astronomie.nl

A schematic overview of the interior of a neutron star.

A schematic overview of the interior of a neutron star.

Some neutron stars go BOOOOOOOM

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Matter that accumulates onto the surface of an accreting neutron star undergoes thermonuclear burning. This process can be unstable and result in a sudden, bright flash of X-ray emission that is referred to as a thermonuclear X-ray burst (or type-I X-ray burst).

Thousands of X-ray bursts have been observed from about 100 neutron star X-ray binaries. Most of these events last about 10-100 seconds, have an energy output of ~10^39 erg (which is far more energetic than an atomic bomb!) and repeat on a timescale of minutes to hours. On rare occasions, however, X-ray bursts have been observed that are both longer (tens of minutes to hours) and 10-100 times more energetic. A few tens of such intermediately long X-ray bursts have been observed to date.

On 2010 August 13, the Burst Alert Telescope (BAT) onboard the Swift satellite triggered on an event coming from the direction of the neutron star X-ray binary XMMU J174716.1-281048. We analyzed the Swift data and found that the BAT had caught an intermediately long X-ray burst from this X-ray binary, which had a duration of nearly 3 hours. This was only the second X-ray burst ever recorded from this source.

The X-ray emission of XMMU J174716.1-281048 is unusually faint for an X-ray binary. This suggests that matter is transferred to the neutron star at a very slow rate. This might be the reason why the neutron star does not display regular X-ray bursts, but rather these rare energetic ones.

Degenaar, Wijnands & Kaur 2011, MNRAS Letters 414, L104: Swift detection of an intermediately long X-ray burst from the very faint X-ray binary XMMU J174716.1-281048

Paper link: ADS

An artist impression of an interacting binary. Image credits: David. A. Hardy / STFC

An artist impression of an interacting binary.
Image credits: David. A. Hardy / STFC