X-ray Burster - Burst Astrophysics

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. 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 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.