Science News

All I Want for Christmas is a Gamma Ray Burst

Sun, 18th Dec 2011

Dominic Ford

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This is something of a Christmas firework leftover from last year.  A gamma-ray burst was observed at about dinner time in the UK on Christmas day last year.  A gamma-ray burst is a burst of radiation similar to what you would find in a nuclear reactor, but thankfully very much weaker, coming out of the night sky.  These are normally very short events.  Many of them last less than a couple of seconds so they're very difficult to observe, but even longer lasting events are typically over within over a couple of minutes.  This one, however, was really quite a surprise because the gamma-ray particles carried on coming for about half an hour and some could even be seen after about 45 minutes!

It’s quite a challenge to explain what this could’ve been; an event so energetic it produced these high energy gamma-rays for such a long prolonged period.  We have various ideas about what causes most conventional gamma-ray bursts.  We think the short ones are probably caused by neutron stars colliding with one another, and the tremendous release of gravitational energy as those two stars combine and form a black hole.  That energy is released as gamma-rays over a period of about 2 seconds as neutron stars are so dense they can combine very quickly.  We think the longer bursts are caused by the supernovae at the end of the lives of very massive stars.  But even they will only last for a couple of minutes.  So how could they possibly prolong this process to last for half an hour?

Publishing in the journal Nature earlier this month, there were two papers presenting two very different theories as to what this could have been.  One of the theories is that this was a neutron star colliding with a massive star, just as it was about to go supernova, quite a realistic scenario because as a star is about to go supernova, it will expand;  If there is a neutron star in a close orbit around it, it will attract and engulf that neutron star.  This could lead to the two types of gamma-ray burst happening back to back, possibly leading to a prolonged emission of gamma-rays.

The other model is a complete contrast to that.  It suggests this was quite a small event, relatively nearby in our own galaxy.  Perhaps a rocky asteroid came to close to the neutron star it orbited, and became broken up by the tidal gravitational forces around the neutron star.  These pieces could then fall in, one by one, over the period of about half an hour, leading to lots of very weak gamma-ray bursts.  On Earth, we may see this as a continuous spread of gamma-rays.  

Both of these theories actually fit the observations pretty well, but this illustrates that there's probably not any one mechanism which is responsible for all gamma-ray bursts.  These interesting events are probably caused by a huge range of phenomena, and just happen to look quite similar when we observe them.

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