Tag Archives: thermonuclear burst

A mathematical tool to study X-ray bursts

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Neutron stars can rip off gas from a nearby companion star and pull this material towards them; a process called accretion. The material that is accreted on to the neutron star undergoes nuclear reactions that can cause detonations generating more energy than an atomic bomb. Such thermonuclear bursts are observed as brief, bright flashes of X-ray emission that last for seconds to hours and repeat on a timescale of hours to months.

Not surprisingly, these violent explosions can be destructive to the surroundings of a neutron star. Indeed, evidence has been accumulating that X-ray bursts have a profound effect on the  accretion flow that transports material toward the neutron star. For example, the explosions can cool, blow away, or accelerate the in-falling flow of material. One way to investigate this is by studying how the X-ray energy spectrum of the accretion flow changes during an X-ray burst. This is not an easy task, however, because the X-ray burst emission typically outshines that of the accretion flow. Small changes in the weak accretion emission are therefore swamped by the bright burst emission.

Mathematical techniques that involve decomposing complex data into matrices (e.g., “non-negative matrix factorization”) have been previously applied to reveal subtle changes in the X-ray emission of super-massive and stellar-mass black holes. Motivated by those successes, we investigated the applicability of such techniques to study changes in the accretion emission induced by X-ray bursts.

For this purpose we used high-quality data obtained with the NuSTAR satellite of a well-known neutron star X-ray binary and thermonuclear burster 4U 1608-52. Our case study revealed that these mathematical techniques can be a very powerful tool to reveal changes in the accretion emission that remain hidden in conventional spectral analysis. Applying these techniques to NuSTAR data is particularly promising, as this instrument can provide valuable information on the  accretion geometry, that helps interpret the results from the X-ray burst analysis.

Degenaar, Koljonen, Chakrabarty, Kara, Altamirano, Miller, Fabian 2016, MNRAS 456, 4256: Probing the effects of a thermonuclear X-ray burst on the neutron star accretion flow with NuSTAR

Paper link: ADS

nustar_nasa

Artist impression of NASA’s NuSTAR mission (launched in 2012). Image credits: NASA.

The effects of a violent thermonuclear burst

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Thermonuclear X-ray bursts manifest themselves as intense flashes of X-ray emission that have a duration of seconds to hours, during which a total energy of approximately 1039 to 1042 erg is radiated. These events are caused by unstable thermonuclear burning that transforms hydrogen and/or helium that has falling onto the surface of a neutron star into heavier chemical elements. X-ray bursts are a unique signature of neutron stars in low-mass X-ray binaries. In such interacting binary star systems, a Roche-lobe overflowing late-type companion star feeds matter to the compact object via an accretion disk.

There is a delicate connection between the properties of X-ray bursts and that of the accretion flow. On the one hand, the rate at which mass is accreted onto the neutron star determines the duration, recurrence time, and radiated energy of the X-ray bursts, whereas the accretion geometry can strongly influence the observable properties. On the other hand, it has been proposed that particularly powerful X-ray bursts may be able to influence the accretion flow.

IGR J17062-6143 is an X-ray source that was discovered in 2006, but remained unclassified until the Burst Alert Telescope onboard the Swift satellite detected an X-ray burst in 2012. This unambiguously identified IGR J17062-6143 as a neutron star low-mass X-ray binary. But not just any ordinary neutron star. The X-ray burst was highly energetic (classifying as a so-called intermediately long X-ray burst), and displayed three very unique features that indicate that the explosion was violent enough to disrupt the accretion disk surrounding the neutron star.

Firstly, the 18-min long X-ray burst displayed dramatic, irregular intensity variations that were clearly visible in the X-ray light curve. Similar fluctuations have only been seen on a handful of occasions. They are likely caused by swept-up clouds of gas or puffed-up structures in the accretion disk. The time-scale of the fluctuations suggest this gas is located at a distance of approximately 103 km from the neutron star. Secondly, the X-ray spectrum of the X-ray burst showed a highly significant emission line around an energy of 1 keV (most likely a Fe-L shell line). This emission feature can be explained as irradiation of relatively cold gas. The width of the line suggests that this material is located at a distance of 103 km from the neutron star. Thirdly, significant absorption features near 8 keV were present in the X-ray spectrum (in the Fe-K band). These likely result from hot, ionized gas along the line of sight. Fitting these features with photo-ionization models points towards a similar radial distance as inferred from the emission line and the light curve fluctuations. Spectral emission and absorption features have never been (unambiguously) detected during an X-ray burst, making this a highly exciting discovery.

In conclusion, three independent observational features suggest that the energetic X-ray burst from IGR J17062-6143 swept up gas along our line of sight, out to a distance of roughly 1000 km from the neutron star (approximately 50 gravitational radii). This provides strong evidence  that powerful X-ray bursts can indeed disrupt the (inner) accretion disk.

Degenaar, Miller, Wijnands, Altamirano, Fabian 2013, ApJ 767, L37: X-Ray Emission and Absorption Features during an Energetic Thermonuclear X-Ray Burst from IGRJ17062-6143

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

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