Re: what are some of the theories behind the cause of gamma ray bursts?

Theories proposed or theories currently accepted? It turns out that there's a huge difference.

Soon after gamma ray bursts (GRBs) were first identified, in the late 1960s and early 1970s, astronomers were able to establish that they were not coming from within Pluto's orbit. Unfortunately, this wasn't much of a constraint. GRBs could still have been as close as the Oort cloud---the hypothesized cloud of comets around the Sun---which would put them as close as 1/10 of a light year or they might have been coming from the distant reaches of the Universe, billions of light years away. Literally hundreds of explanations were put forward. Explanations ranged from exploding comets to colliding stars to exotic matter to cosmic strings (essentially "defects" in the Universe) to ....

The reason for this vast uncertainty was the poor resolving power of gamma-ray telescopes. One way of thinking about the resolving power of a telescope (or pair of binoculars or the human eye) is to ask how well the telescope can identify the location on the sky of an object. The human eye has an angular resolution of roughly 1/100 of a degree; for reference, the full Moon covers 1/2 of a degree. (The fact that the Moon appears to cover a huge amount of the sky while stars are mere points indicates that the Moon is much larger than the angular resolution of the eye while stars are much less.) Modern optical and radio telescopes have resolutions much better than that of the human eye. By contrast, a typical gamma-ray telescope might have an angular resolution of several degrees. Thus, when a GRB did occur, generally astronomers could say only that it occurred in a patch of sky much larger than the full Moon. In such a large region of sky, there are thousands of stars, even thousands of galaxies.

One way of identifying what a GRB is would be to detect something besides its gamma-ray light emission. For instance, an optical or radio telescope would give a much more accurate position, which might allow a GRB to be identified with a particular kind of star or with a galaxy. GRBs, as their name suggests, though, don't last very long (between 1/1000 of a second to 1000 seconds). Thus, the GRB is often finished before astronomers at other telescopes can even be notified.

Recently, the BeppoSAX satellite changed this situation. BeppoSAX carries an X-ray telescope. X-ray telescopes generally do not have angular resolutions as good as optical telescopes, but X-ray telescope resolutions tend to be much better than those of gamma-ray telescopes. Thus, if a gamma-ray telescope on BeppoSAX detects a GRB, the X-ray telescope on BeppoSAX can be pointed in that direction.

What the BeppoSAX researchers found was that sometimes after the GRB finished, they could see a source of X-ray light, a fading source. A fading source is just what one would expect after a huge explosion. Moreover, since the BeppoSAX X-ray telescope can provide a much more accurate position, other telescopes could be pointed in that direction. There's now been a good number (more than a dozen) X-ray sources identified with GRBs, and a handful of these have also been detected in visible light.

For a couple of the GRBs for which a visible-light "afterglow" has been detected, it's been possible to estimate a distance. Essentially, the GRB afterglow is seen shining through a great cloud of hydrogen gas, and so the GRB must be more distant that the cloud of hydrogen gas. The distance estimates for these GRBs are in the millions to billions of light years, so GRBs occur in the distant reaches of the Universe.

As far as I'm aware, there are two currently popular models to explain GRBs, both associated with the end of a star's life. Stars about 10--25 times more massive than the Sun form a neutron star when they run out of fuel. (Stars are an exquisite balancing act. Their mass produces gravity, which causes them to contract and heat up, which causes fusion reactions, which causes pressure, which balances the force of gravity. If the star runs out of fusion fuel, i.e., hydrogen, gravity can take over and sqash the star.) If two neutron stars were to collide, the result would be a powerful explosion, potentially powerful enough to produce a GRB. Another possibility is that a star 100 times more massive than the Sun might end its life in a "hypernova." A star that forms a neutron star undergoes an explosion called a supernova during the formation of the neutron star. In a supernova, the star blows its outer layers apart while the inner parts collapse to form a neutron star. A hypernova would be a super-supernova, possibly resulting from the outer layers being blown apart while the inner part collapses to form a black hole.

I should note that my explanation somewhat implicitly assumes that there's only one explanation for GRBs. That need not be true, of course. Some GRBs might be colliding neutron stars, some might be hypernovae, and some (all?) might be none of the above.

For more info, you might want to check the Web sites of BeppoSAX and the Compton Gamma Ray Observatory.