X-ray bursters are one class of X-ray binary stars exhibiting X-ray bursts, periodic and rapid increases in luminosity (typically a factor of 10 or greater) that peak in the X-ray region of the electromagnetic spectrum. These astrophysical systems are composed of an accreting neutron star and a main sequence companion 'donor' star. There are two types of X-ray bursts, designated I and II. Type I bursts are caused by thermonuclear runaway, while type II arise from the release of gravitational (potential) energy liberated through accretion. For type I (thermonuclear) bursts, the mass transferred from the donor star accumulates on the surface of the neutron star until it ignites and fuses in a burst, producing X-rays. The behaviour of X-ray bursters is similar to the behaviour of recurrent novae. In the latter case the compact object is a white dwarf that accretes hydrogen that finally undergoes explosive burning.
The compact object of the broader class of X-ray binaries is either a neutron star or a black hole; however, with the emission of an X-ray burst, the compact object can immediately be classified as a neutron star, since black holes do not have a surface and all of the accreting material disappears past the event horizon. X-ray binaries hosting a neutron star can be further subdivided based on the mass of the donor star; either a high mass (above 10 solar masses ()) or low mass (less than ) X-ray binary, abbreviated as HMXB and LMXB, respectively.
X-ray bursts typically exhibit a sharp rise time (1–10 seconds) followed by spectral softening (a property of cooling black bodies). Individual burst energetics are characterized by an integrated flux of 1032–1033 joules, compared to the steady luminosity which is of the order 1030 W for steady accretion onto a neutron star. As such the ratio α of the burst flux to the persistent flux ranges from 10 to 1000 but is typically on the order of 100. The X-ray bursts emitted from most of these systems recur on timescales ranging from hours to days, although more extended recurrence times are exhibited in some systems, and weak bursts with recurrence times between 5–20 minutes have yet to be explained but are observed in some less usual cases. The abbreviation XRB can refer either to the object (X-ray burster) or to the associated emission (X-ray burst).
Thermonuclear burst astrophysics
When a star in a binary fills its Roche lobe (either due to being very close to its companion or having a relatively large radius), it begins to lose matter, which streams towards its neutron star companion. The star may also undergo mass loss by exceeding its Eddington luminosity, or through strong stellar winds, and some of this material may become gravitationally attracted to the neutron star. In the circumstance of a short orbital period and a massive partner star, both of these processes may contribute to the transfer of material from the companion to the neutron star. In both cases, the falling material originates from the surface layers of the partner star and is thus rich in hydrogen and helium. The matter streams from the donor into the accretor at the intersection of the two Roche lobes, which is also the location of the first Lagrange point, L1. Because of the revolution of the two stars around a common centre of gravity, the material then forms a jet travelling towards the accretor. Because compact stars have high gravitational fields, the material falls with a high velocity and angular momentum towards the neutron star. The angular momentum prevents it from immediately joining the surface of the accreting star. It continues to orbit the accretor in the orbital plane, colliding with other accreting material en route, thereby losing energy, and in so doing forming an accretion disk, which also lies in the orbital plane.