X-ray Burst

X-ray bursts occur in low-mass X-ray binary systems where 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 hydrogen is drawn into an accretion disk around the neutron star. This hydrogen is eventually deposited on the surface of the neutron star and immediately is converted into helium due to the extreme temperatures and pressures that exist there. A thin surface layer of helium is built up, and once a critical mass of helium is reached, it ignites explosively, heating the entire surface of the neutron star to several tens of millions of degrees releasing a sudden burst of X-rays. Once the outburst is over, the binary system temporarily returns to its quiescent state while the neutron star begins to re-accumulate the helium surface layer. The process repeats resulting in recurrent X-ray bursts.

There are many similarities between this process and that which results in recurrent novae. For an X-ray burst, the compact object is a neutron star which accretes a surface layer of helium that undergoes explosive burning to produce the outburst. For recurrent novae, the compact object is a white dwarf which accretes a surface layer of hydrogen that undergoes explosive burning to produce the outburst.

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

All of the above refers to Type I X-ray bursts which make up the vast majority of this class of object. In fact, there are only two Type II X-ray bursts known, one of which also shows Type I outbursts. A Type II burst is thought to be due to an increase in the rate of accretion from the companion star and is distinguished from a Type I burst through its burst profile. While Type I bursts have a rapid rise followed by a slow decline, Type II bursts start and stop abruptly with no gradual decay from peak. They can also show rapid successions of bursts separated by a few minutes.

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