Expert: Courtney Seligman - 3/26/2011

Question
Hello,

I was curious, what exactally is a Gamma Ray Burst? I saw them on a television documentary once, but I didn't get to watch the whole thing.

Thanks!

Answer
A gamma-ray burst is a short-lived emission (hence a "burst") of extremely high energy photons (or gamma rays). They last from a only a second or so to as much as a few minutes. The longer-lived bursts are generally accompanied by smaller amounts of radiation at longer wavelengths, over a longer period of time, which sometimes allows an examination of the "afterglow". The shorter bursts are not, so it is harder to study them in any detail.

Gamma-ray bursts are observed to occur all over the sky, completely at random, which implies that they are associated with something going on in the distant Universe, rather than in our own galaxy (in which case they would be concentrated along the plane of the Milky Way). This also means that they are probably extremely bright, since distant objects have to be very bright to be seen at all.

There are two theories of how gamma-ray bursts form. The shorter lived ones are probably due to the collisions of neutron stars. This is an extremely rare event, but must occur once in a while, particularly in the dense cores of massive galaxies, or in the even denser cores of massive globular clusters. The reason they must occur is that the abundance of gold is much too high to explain by standard theories of how the elements are created inside stars (although considered rare, gold is actually fairly common compared to most other heavy elements; it's just relatively inactive chemically, which makes it hard to separate from its ores; if it were easier to get at, it would be almost as cheap as dirt). The overly high abundance of gold can be most easily explained by the collision of neutron stars, and since that also offers a good explanation of short-lived gamma ray bursts, that is the preferred theory for their origin.

The longer-lived bursts are believed to be due to the iron-core collapse of extremely massive stars, resulting in a supernova and (following that) the formation of a black hole. During the collapse, the rotation of the star should create a relatively dense flattened disc surrounding the forming black hole, so a large fraction of the most energetic radiation from the supernova (which would be gamma rays) is forced to radiate along the poles of the disc, where there is less material blocking its escape. There are a number of other types of accretion-disk emitters in which jets of energy and matter are forced to radiate along the polar axis of the disk, so this seems a reasonable explanation of the phenomenon.

One result of the polar radiation is that unless you happen to be almost exactly in the direction of the emission, you won't even notice it. So there must be far more objects giving off such bursts than the small number that happen to be pointing in our direction. Despite that, we see them all over the sky, in surprisingly large numbers. Given the relative rarity of the supernovae (and neutron star collisions) that are thought to create the bursts, to see so many of them, we must be observing them from practically everywhere in the observable Universe -- that is, over distances of many billions of light years. Even given how bright their sources must be, to see gamma-ray bursts at such distances is rather amazing; but the fact that their light is concentrated into a beam means it is much brighter than it would be if it went in all directions; so the thing that makes it less likely for us to see any given burst also allows us to see them over a much larger volume of space; and as a result, we still get to see a lot of them.

I should note that we have known about gamma-ray bursts for decades, but it is only in recent years that generally accepted theories have been developed. So it is possible that other theories may come along, which add to or replace the current ones.