Tuesday, March 25, 2008

What's a gamma ray burst?

Artists concept of collapsar, one scenario for gamma ray bursts
Image Credit: Tony Piro

In a couple of posts over the last few weeks (one on the "Death Star Pinwheel" and one on a very bright explosion), I've mentioned gamma ray bursts. But what is a gamma ray burst? Today, a little history, and tomorrow, the current hypothesis.

In the 1963, the US and Soviet Union entered into a treaty that prohibited the testing of nuclear weapons in the atmosphere, in outer space, or under water. The US decided that one way to verify that the Soviets were not testing weapons in space was to launch a satellite to look for gamma rays. Gamma rays are the most energetic form of light, and are released in copious quantities by nuclear reactions. The atmosphere blocks gamma rays, so the only way to look for them is in space.

So, the US launched a series of Vela satellites to look for short flashes of gamma rays -- and they found them! After a few tense weeks of detecting almost a flash a day, it was realized that the amount of nuclear material the Soviets would have to be using to test so many space weapons would be depleting their entire stockpile, which would not be a smart move, and the Soviets were not dumb. A little more research proved that the explosions were coming from deep space, and so were astronomical.

Most people though that gamma ray bursts must be material falling onto neutron stars in our galaxy causing small nuclear explosions. But in the the 1990s, the Compton Gamma Ray Explorer (the gamma-ray equivalent of the Hubble Space Telescope) found that gamma ray bursts do not come from the parts of the sky where the Milky Way galaxy is found -- they come from every direction. That is a strong indication that these gamma ray bursts come from the distant universe, which looks about the same in every direction. But if the events that caused gamma ray bursts sent gamma rays in every direction, the energies required were at least 10 times bigger than supernovae, the most powerful explosions in the Universe.

To further muddy the waters, the Compton Gamma Ray Observatory started to reveal evidence that there are two types of gamma ray bursts. One type, the short burst, has higher-energy gamma rays but only lasts a fraction of a second. The second type, or long burst, had lower-energy gamma rays but last ten to 30 seconds. I remember seeing some talks about this data when it was first emerging. Lots of people had lots of ideas as to what gamma rays were, why there might be two different types, and whether or not they actually believed any other theory. But we needed some more information, and that information came from an unexpected source: an Italian X-ray satellite called "Beppo-Sax."

To be continued...

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