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) peaked in the X-ray regime of the electromagnetic spectrum. These astrophysical systems are composed of an accreting compact object, typically a neutron star or occasionally a black hole, and a companion 'donor' star; the mass of the donor star is used to categorize the system as either a high mass (above 10 solar masses) or low mass (less than 1 solar mass) 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 1039–40 ergs, compared to the steady luminosity which is of the order 1037 ergs for accretion onto a neutron star. As such, the ratio of the burst flux to the persistent flux, denoted in the literature as α, 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 to this class of object (X-ray burster) or the astronomical observation of the associated emission (X-ray burst).
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 the neutron star. The partner 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 rich in hydrogen and helium. Because compact stars have high gravitational fields, the material falls with a high velocity towards the neutron star, usually colliding with other accreted material en route, forming an accretion disk. In an X-ray burster, this material accretes onto the surface of the neutron star, where it forms a dense layer as a result of the extremely high gravitational field. After mere hours of accumulation and gravitational compression, nuclear fusion starts. Often the increase in temperature (greater than 1 x 109 kelvins) gives rise to a thermonuclear runaway. This explosive stellar nucleosynthesis begins with the hot CNO cycle which quickly yields to the rp-process. Within seconds most of the accreted material is burned, powering a bright X-ray flash that is observable with X-ray telescopes. Theory suggests that in at least some cases the hydrogen in the accreting material burns continuously, and that it is the accumulation of helium that causes the bursts.
Observation of bursts
Because an enormous amount of energy is released in a short period of time, much of the energy is released as high energy photons in accordance with the theory of black body radiation, in this case X-rays. This release of energy may be observed as in increase in the star's luminosity with a space telescope, and is called an X-ray burst. These bursts cannot be observed on Earth's surface because our atmosphere is opaque to X-rays. Most X-ray bursting stars exhibit recurrent bursts because the bursts are not powerful enough to disrupt the stability or orbit of either star, and the whole process may begin again. Most X-ray bursters have irregular periods, which can be on the order of a few hours to many months, depending on factors such as the masses of the stars, the distance between the two stars, the rate of accretion, and the exact composition of the accreted material. Observationally, X-ray bursts are put into two distinct categories, labeled Type I and Type II. A Type I X-ray burst has a sharp rise followed by a slow and gradual decline of the luminosity profile. A Type II X-ray burst exhibits a quick pulse shape and may have many fast bursts separated by minutes. However, only from two sources have Type II X-ray bursts been observed, and most X-ray bursts are of Type I.
Applications to astronomy
Luminous X-ray bursts can be considered standard candles, since the mass of neutron star determines the luminosity of the burst. Therefore, comparing the observed X-ray flux to the predicted value yields relatively accurate distances. Observations of X-ray bursts allow also the determination of the radius of the neutron star.
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