Gamma ray bursts (GRBs) are probably the most violent events that happen today in universe. In a few seconds or less, a gamma-ray burst releases an enormous quantity of energy (1044 - 1046 J), which is comparable to burning up the entire mass-energy of the sun in a few tens of seconds, or comparable to the energy emitted by our entire Milky Way does in a hundred years. GRBs are scary - if just one GRB would occur near to us (in our Milky Way for example), it would simply wipe out the ozone layer on Earth, causing life extinction. Hopefully, we will not witness anything like this soon :-). However, it is estimated that a few hundred GRBs occur every day in the observable universe, and not only that - there were GRBs detected at an extreme distance - more 13 billion light-years.
Over the last decades, scientists struggled to understand where they come from, and how far they are. There seems to be two main classes of bursts which seem to behave pretty different: short bursts (which last less than two seconds) and the long ones.
Recently, Don Lamb and his team at University of Chicago found out a solid theory for the generation of short bursts. Their conclusion will be published in the Oct 6. issue of the journal Nature. According to this theory, short-gamma ray bursts are generated by the collision of two (closely-orbiting) neutron stars. This theory is largely confirmed by a significant GRB that was detected earlier this year on July 9, officially known as GRB 050709. There are a few interesting things that came as a consequence of this event:
- First observation of the optical afterglow of a short burst. Afterglows are produced when the jets emitted by the newly formed black hole slam into the interstellar gas that surrounds it. Scientists need these afterglows to track the bursts to their source.
- First identification of the galaxy in which a short burst had occurred. "The observation of that optical afterglow led to the identification of the host galaxy," Lamb said.
- First secure measurement of distance to a short burst. Astronomically speaking, the host galaxy of the July 9 burst has a redshift of 0.16. This translates to a distance of approximately 1 billion light years from Earth, "so it's about 10 times closer than is typical for long GRBs," he said. "That makes short bursts a thousand times less luminous and a thousand times less energetic than long GRBs."
- First determination of where in the host galaxy the burst occurred. It took place in the outskirts of its host galaxy, meaning that it is a very old object. "This alone is very strong evidence that this burst was due to merging neutron stars," Lamb said.