X-ray burster

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 (M_☉)) or low mass (less than 1 M_☉) X-ray binary, abbreviated as HMXB and LMXB, respectively.^{[further explanation needed]}

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,^[2] compared to the steady luminosity which is of the order 10³⁰ W for steady accretion onto a neutron star.^[3] 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.^[2] 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.^[4] The abbreviation XRB can refer either to the object (X-ray burster) or to the associated emission (X-ray burst).