Tag Archives: thermonuclear burst

Swift: A neutron star finding machine

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Many neutron stars reside in X-ray binaries, where they pull off and consume matter from a companion star. As the accreted material accumulates on the surface of the neutron star, the temperature and pressure rise, what can eventually lead to a gigantic explosion: a thermonuclear X-ray burst. These are very energetic, bright flashes of X-ray emission that can last from a few seconds to a few hours. These are a unique characteristic of neutron stars.

NASA’s Swift satellite carries instruments that can detect X-ray, ultra-violet and optical emission from astronomical objects. In addition, it is equipped with a Burst Alert Telescope (BAT). This instrument has a very wide field of view of about 2 steradians and monitors a large part of the sky with the aim to detect (rare) energetic events. The BAT has proven to be an very suitable instrument to detect thermonuclear X-ray bursts from accreting neutron stars.

Some neutron stars display X-ray bursts only very rarely (maybe only once every year). These events can be easily missed, so that the neutron star can remain hidden for a very long time. Indeed, Swift’s BAT has detected several thermonuclear X-ray bursts from previously unknown X-ray sources. In some cases, the BAT picked up an X-ray burst from an X-ray sources that had been discovered before, but was not known to harbor a neutron star. At present, approximately 100 X-ray bursting neutron stars are known (see this list of Galactic X-ray bursters and the MINBAR catalog for overviews). A significant fraction of these (about 10) have been discovered by the Swift satellite.

In 2011 and 2012, the BAT helped identify four new neutron stars via the detection of their thermonuclear X-ray bursts.

Degenaar, Altamirano, Wijnands 2012, Astronomer’s Telegram 4219: IGR J17062-6143 is likely a bursting neutron star low-mass X-ray binary

Paper link: ADS

Degenaar, Wijnands, Reynolds et al. 2014, ApJ 792, 109: The Peculiar Galactic Center Neutron Star X-Ray Binary XMM J174457-2850.3

Paper link: ADS

Degenaar, Linares, Altamirano, Wijnands 2012, ApJ 759, 8: Two New Bursting Neutron Star Low-mass X-Ray Binaries: Swift J185003.2-005627 and Swift J1922.7-1716

Paper link: ADS

 

Artist impression of the Swift satellite. Credit: NASA.

Artist impression of the Swift satellite.
Credit: NASA.

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