X-ray Bursts

Bursts of X-ray energy that occur in low-mass X-ray binary systems in which a neutron star and low-mass main sequence star are in orbit around one another. Due to their close proximity and the extreme gravity of the neutron star, the companion star overflows its Roche lobe and H is drawn into an accretion disk around the neutron star. H is eventually accreted to the surface of the neutron star, where it is immediately converted into He by the extreme temperatures and pressures that exist there. A thin surface layer of He builds up, and once a critical mass is reached, the He ignites explosively, heating the entire surface of the neutron star and releasing a sudden burst of X-rays. After the burst, the system returns to a quiescent state and the neutron star begins to re-accumulate the He surface layer. The process repeats resulting in recurrent X-ray bursts. The mechanisms that produce X-ray bursts and recurrent novae are similar. Recurrent novae form when a white dwarf accretes a surface layer of H that undergoes explosive burning.

X-ray bursts generally occur at regular intervals separated by several hours or days. They last from a few seconds to a few minutes, with the burst profile showing a rapid rise (0.3-10 s) followed by a slower decline (5-100 seconds). The rapid rise reflects the sudden increase in temperature brought about by explosive He ignition, while the longer decline reflects the slower cooling of the surface of the star.