What is a Gamma Ray Burst? Its location…

continue from last week: What is a Gamma Ray Burst? Its History…

We now know that gamma ray bursts (GRBs) came from space. But exactly where? From our Solar System? within our Milky Way Galaxy? or galaxies far far away?

In 1990s, the Compton Gamma-Ray Observatory (CGRO) was launched. Within a few years, this satellite had detected thousands of bursts. If we plot these bursts on a “sky map”, it turned out that their distribution was uniform across the sky. This means that the bursts should come from outside our galaxy. If they were related to our Milky Way, then they should be more or less confined to the plane of our galaxy, though we weren’t 100% certain because it could be something that was associated with the halo.

BATSE map of GRBsBATSE map of its 2704 detected GRBs. Note that the GRBs are distributed all over the sky.

While many astronomers were convinced GRBs came from far far away, some still think that they should be local because of the extreme energy release. These bursts seemed impossibly powerful: to appear so bright from so far away, they must vastly outshine entire galaxies containing hundreds of billions of stars.

To know the distance of a GRB, we must be able to pinpoint its location and transmit the coordinates fast enough back to Earth so that we can zoom in with our ground-based telescopes to find out what it was, how far it was located and what kind of galaxies it was in (if the bursts occurred in galaxies).

The answer came in 1997. BeppoSAX, an Italian-Dutch satellite, manage to identify the location of a GRB (GRB 970228) fast enough that astronomers on the ground were in time to photograph its optical afterglow. After the burst had faded away, they found a faint galaxy there, took its spectra and figure out that it’s a galaxy far far away.

GRB 970228. Click to enlarge.
BeppoSAX observations of GRB970228. On the left is the original observation of the burst on 28 February 1997. The right image shows the burst object has dimmed 3 days later.

Afterglow is the radiation emitted after a GRB, and can be seen in other parts of the electromagnetic spectrum – X-rays, optical and radio wavelengths. Afterglows can last for days to several years, and they fade away over time in a well-understood manner. Most of what we know about GRBs comes from afterglow observations. That’s why it is important to pinpoint the exact location of GRBs fast enough to observe their afterglows.

Afterglows helped astronomers determine the distance to GRBs. The discovery of afterglow made redshift measurements possible, which can be translated into distance. And we found out that all of the GRBs are really very very far away, at a distance of few billion light-years. The current record-holder is located 12.8 billion light-years away, almost to the edge of the observable Universe.

So, gamma ray bursts are far far away, releasing extremely HUGE amount of energy (GRB only needs a few seconds to release the energy our Sun produce in its entire 10 billion years lifetime). They are by far the brightest and most energetic phenomena in the known universe, second only to the Big Bang itself.

What could possibly cause them?

…to be continued…


~ by thChieh on November 4, 2008.

One Response to “What is a Gamma Ray Burst? Its location…”

  1. […] What is a Gamma Ray Burst? Its types and causes…  …continue from Part 1 – its history and Part 2 – its location… […]

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