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The Question

(Submitted December 13, 1996)

What is the cause of X-ray bursts?

The Answer

Thanks for your question on X-ray bursts. X-ray bursts were discovered by X-ray satellites, and are characterized by a rapid, and very dramatic increase in X-ray flux from an X-ray source, lasting about a second or so. X-ray bursts are seen coming from Globular Clusters, which are in the halo of our galaxy, and from sources along the plane of our galaxy. This distribution suggests they come from a population of stars that are common in the disk: X-ray binaries.

An X-ray binary is a pair of stars where one member is a compact object (such as a black hole or neutron star) and the other star is a normal star. The two stars orbit each other at a separation where the gravitational pull of the compact object distorts the normal star and material streams off of it, and onto the compact object. (For more information on X-ray binaries, take a look in the Basic or Advanced High Energy Astrophysics sections of Imagine the Universe!)

In the case of X-ray bursts, we have a very clear hypothesis for what is going on. Researchers have studied many bursts in detail, and from the spectra and light curves we know that a tremendous amount of energy (in about 10 seconds, 10^39 ergs---what the Sun's X-ray corona emits in about 3000 years!) is being released from an area of about 15 km. This is the size that neutron stars are expected to be. Hydrogen accreted from the normal star onto the neutron star is continuously fused into helium. A layer of helium is them formed near the surface of the neutron star. When there is enough helium present, an unstable reaction occurs: virtually all of the helium is fused to carbon at once, which we see as the explosive burst. After this, the accretion continues to take place between the two stars. After a while there is again enough material to explosively burn the helium, and another burst is seen.

Studying X-ray bursts give us insight into the accretion process between the two stars, and also the conditions on the surface of the neutron star. For a particular source, X-ray astronomers catalog the bursts, the duration between bursts, and the energy released in bursts to get an idea of what is happening in the helium and hydrogen layers just above the neutron star surface.

X-ray bursts shouldn't be confused with gamma-ray bursts. Gamma-ray bursts are more energetics, and are distributed isotropically on the sky. No optical counterparts have been conclusively identified with any gamma-ray burst. Neutron stars may also be involved in the production of gamma-ray bursts, but no one knows for sure what causes these.

Padi Boyd and Jim Lochner

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