X-ray burster

X-ray bursters are a class of binary stars which have periodic outbursts luminous in X-rays. They contain a neutron star and an accreting companion.

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 the neutron star. Alternatively, the partner star may undergo mass loss by exceeding its Eddington luminosity, 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 as a dense layer of electron degenerate gas, another result of the extremely high gravitational field. Degenerate matter does not follow the ideal gas law, and so changes in temperature do not lead to notable changes in pressure. After enough of this material accumulates on the surface of the neutron star, thermal instabilities set off exothermic nuclear fusion reactions, which causes an increase in temperature (greater than 1 x 10⁹ kelvins), eventually giving rise to a runaway thermonuclear explosion. This explosive stellar nucleosynthesis begins with the hot CNO cycle which quickly yields to the rp-process. 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 burst separated by minutes. However, only from two sources have Type II X-ray bursts been observed, and so most X-ray bursts are assumed to be 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.