What is a Gamma Ray Burst?

The biggest releases of energy in the universe are detected at about one per day from an even spread all over the sky. These massive bursts of energy, that only last a few seconds, can release 300 times more electromagnetic radiation than our Sun gives off over it’s entire 10 billion year lifespan. What is going on to produce such massive releases of energy? Scientists are not exactly sure what causes gamma ray bursts (GRB) but think it has something to do with the core collapse of massive stars that can cause what is known as a hypernovae. A hypernova is just like a supernova only bigger, much bigger. They might also be caused by the collisions of neutron stars and/or blackholes. What we do know about them is that they are hugely powerful and happen evenly all over the sky and have been happening for over 13 billion years.

What do GRBs look like? Well this is the challenging bit, you can’t see them directly as GRBs have a much shorter wave length than visible light though they are accompanied by an after glow which is often visible and there has even been a couple of GRBs with visible afterglows that could have been seen by the naked eye. GRB bursts are so powerful that they outshine even active galaxy nuclei (AGN) that contain massively powerful quasars. There are two types of GRBs that have been observed delineated by the length of the burst. The long duration GRBs are longer than about two seconds and the short duration are shorter than 2 seconds. The short duration bursts are thought to have something to do with neutron star or black hole collisions and the longer duration bursts are those associated with hypernova explosions. These long duration bursts have been associated with the active star forming regions of distant galaxies.

Even though GRBs have been broken into two groups they are still highly variable with some lasting only a few milliseconds and other than can last up to 1000 seconds. When large numbers of GRBs are plotted on a graph they tend to have two main groupings that are on either side of two seconds duration.

The first GRBs were detected in the late 1967 when the United States was looking for evidence of nuclear tests in space by the Soviet Union. They instead picked up unusual bursts of gamma ray energy that couldn’t be explained at the time. It wasn’t until 1973 that the first GRB was identified. Now there are many observatories around the world that are focused on understanding GRBs including the space telescope Fermi which was launched in 2008. It was designed to examine the source of gamma rays and other high energy particles in the universe. Both Fermi and SWIFT, the other satellite looking at GRBs, has produced a wealth of data that has greatly increased our understanding of GRBs.

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Fermi getting packed for launch in 2008 (Credit: NASA)

The ESO Very Large Telescope recoded a GRB that was so old it is estimated that it occurred about 600 million years after the Big Bang. One of the closest GRBs recorded was last year in the galaxy NGC4993, only 130 million light years away, it was also the same event where a gravitational wave was observed. Generally though, GRBs are usually found more than a few billion light years away so the opportunity to study one a bit closer was welcomed by scientists. Its thought that GRBs are generated in intense beams of radiation streaming out from the opposites poles of either a collapsing massive stars or the result of a stellar remnant collision. These beams are very narrow, only a few degrees in width so to detect them on Earth we would have to have one of them pointing right at us. If this was the case then there are clearly many more GRBs than we observe as the beams would be pointing in other directions as well.

There’s a lot we don’t know about these super powerful bursts of radiation but the more that get observed the more we end up knowing.