Thermonuclear X-ray bursts
When matter is accreted onto a neutron star, it piles up on its surface where it is being compressed. Due to the high densities and temperatures, the accreted matter undergoes thermonuclear reactions. Under some circumstances this burning process is unstable and causes an entire surface layer to ignite, resulting in an intense flash of X-ray emission that temporarily outshines the X-ray binary's accretion luminosity. Such thermonuclear X-ray bursts have a typical duration of a few seconds to a minute, although on some occasions X-ray bursts with a duration of tens of minutes, hours or even up a day have been seen. The short X-ray bursts are quite common, thousands have been observed to date, but the longer and more energetic ones are much more rare.
Thermonuclear X-ray bursts are considered to be a distinctive feature of neutron stars: black holes have no solid surface and can thus not display thermonuclear X-ray bursts. Therefore, whenever a thermonuclear X-ray burst is observed from a previously unclassified X-ray source, astronomers can immediately identify the accreting compact object as a neutron star. In some cases the X-ray burst emission shows regular intensity variations, called X-ray burst oscillations, which are thought to be due to hotspots on the surface of the neutron star, which form during the bursts and which come periodically into our line of sight because of the rotation of the neutron star. Hence these oscillations trace the frequency at which the neutron star rotates around its own axis. Researchers at API are involved in observations of thermonuclear X-ray bursts (using X-ray satellites), as well as the theoretical interpretation of these phenomena.
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