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image002.jpgWhen we look up with gamma-ray eyes (or use satellite’s like Swift above the atmosphere to watch the sky with gamma-ray sensitive detectors), many different things draw our attention. There is gamma-ray emission from pulsars, from quasars, from accretion disks around black holes. There are flickers from anti-matter – matter self-annihilation. There are many cool and wonderful things. Most cool and wonderful of all are gamma-ray bursts. Lasting anywhere from a few thousandths of a second to a couple of minutes, these short-lived high-energy events give off ~70 times the light the Sun will give off in its entire lifetime. Randomly appearing all over the sky, we observe roughly 1 gamma-ray burst a day using orbiting observatories.In general, if you make a plot of gamma-ray brightness versus time for all gamma-ray bursts, a pattern emerges, where the plots naturally segregate themselves into short-bursts and long bursts, with the short bursts only lasting 2 seconds or less. Strangely, it looks like this single population of short bursts may have two different groups. (image credit: NASA)

The long bursts are known (because we can seem them in the optical sometimes) to originate in the death of giant stars going not just supernova, but hypernova! The shorter bursts have been much more enigmatic, with only a handful (1 that I can find record of, but I leave room for more), being observed in wavelengths other than gamma-ray and X-ray. We had some ideas of causes: merging neutron stars, neutron stars with re-arranging magnetic fields (generally called soft-gamma ray repeaters, like SGR 1806-20), something not as powerful as exploding stars, but still pretty exotic. There is now reason to believe both the possibilities could be true.

In a pre-print posted on arXiv by Robert Chapman and collaborators, a statistical perspective is taken, and the question is asked, can all the short gamma-ray bursts observed be accounted for by just one of these populations, or does a combined model make the most sense?

Soft gamma-ray burst repeaters like SGR1806-20 are visible as far as 50 million parsecs away (for comparison, Andromeda Galaxy is 775 thousand parsecs away). In general, short gamma ray bursts can be correlated on the sky with galaxies as far away as 155 million parsecs.

In trying to sort out the cause of short gamma-ray bursts, this team combined models called luminosity functions describing the potential distributions of neutron star – neutron star binaries and and soft gamma-ray repeaters. They were able to reproduce the observed number of short gamma-ray bursts using a model that had the number of bursts coming from stars with rearranging magnetic fields becoming flat at a distance of about 200 million parsecs, and the number of bursts from neutron star – neutron star binaries increasing beyond 200 million parsecs.

Their attempt to fit observations using either just the soft gamma-ray bursts or neutron star – neutron star populations failed. No good fit could be found.

What is kind of neat about this is short gamma ray bursts always involve neutron stars, they just don’t always involve the same physical mechanism. Everything is tightly wound up in these little stars, and energy is just waiting to escape and light up our sky with gamma-rays.