X-ray burster

X-ray bursters are one class of X-ray binary stars exhibiting periodic and rapid increases in luminosity (typically a factor of 10 or greater) that peak in the X-ray regime of the electromagnetic spectrum. These astrophysical systems are composed of an accreting compact object, and a main sequence companion 'donor' star. A compact object in an X-ray binary system consists of either a neutron star or a black hole; however, with the emission of an X-ray burst, the companion star 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. The star's mass is drawn on to the surface of the neutron star where the hydrogen fuses to helium which accumulates until it fuses in a burst, producing X-rays.

The mass of the donor star is used to categorize the system as 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. X-ray bursters differ observationally from other X-ray transient sources (such as X-ray pulsars and soft X-ray transients), showing 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 10^39–40ergs, compared to the steady luminosity which is of the order 10³⁷ ergs for steady accretion onto a neutron star. As such the ratio α, of the burst flux to the persistent flux, ranges from 10 to 10³ 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 the object (X-ray burster) or the associated emission (X-ray burst).

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