FDR? JFK?, No... GRB!

Here we are again with our Canada-France-Hawaii Telescope astronomy column. In this (almost) weekly chronicle, you already read about various kinds of astronomical objects, from rocky asteroids to dark matter in the universe. Today, I'd like to introduce you to a new ÒspeciesÓ. Sorry, no cute name for this one, nor in Hawaiian nor in any other language spoken on Earth... It's simply, in a very descriptive way, called for what it is: Gamma Ray Burst, better know with three letters in the astronomical community, like FDR or JFK in US History, as "GRB".

Don't stop reading at the view of 'gamma rays'. They are just like the photons making the visible light (the one you see coming from the Sun, or the stars on a clear night). They have, a much higher energy than visible photons though (even more than X-rays), far away on the blue side of the visible spectrum you see so often in the rainbows over the Kohala meadows. These gamma rays are produced on Earth by radioactive material (the detonation of a nuclear bomb would produce a short intense gamma-ray burst). They are widely used in medicine for cancer treatment, to diagnose a medical condition due a blockage in a particular organ, or to sterilize packaged instruments, catheters, needles and other medical instruments in order to eliminate the risk of infection. Gamma rays at low doses can almost perfectly sterilize biologically inert foods, such as meat and poultry, and highly storable foods, such as grains and dry beans. For that purpose, a radioactive source, like a certain form of Cobalt 60 or Cesium 137, is used, which had the advantage of not making anything around them radioactive. Well, this is getting close to a sensitive issue on the Big Island and I'll stop here my considerations on gamma rays on Earth...

Gamma rays themselves are very rarely coming to the ground. They interact with the atmosphere, and for them there is not much difference between oxygen, ozone or even carbon. It means that their absorption is not likely to be affected by the changes in our atmosphere induced by natural (big volcanic explosions) or human activities. How did we get to know about GRB's, bursts of gamma rays happening somewhere in the sky, if the atmosphere blocks them the gamma rays? Let us move back to the 1960s, the height of the cold war... The United States decided to launch a series of satellites in order to detect the gamma ray emission which nuclear bomb test explosion would produce, as a way to monitor these nuclear tests. Surprise: many detections of gamma ray bursts were made, but coming from space and not from the Earth. The information was kept secret from astronomers until it was finally declassified in 1973.

Since then, new satellites have been launched, dedicated to the observation of the universe in gamma rays. Many GRB's have been detected in all directions, without any idea on their exact distance from us. It has been quickly found that they were not coming from effects of the fluctuating solar activity and that they were likely to be generated outside our Solar System. As the subject was becoming more an more puzzling, refinements made in the spacecrafts as well as an intense activity in ground based astronomy led to the first observation of a GRB in visible light. Why is that so difficult to do? Well, a GRB is avery short event. It can be from a few milliseconds to a couple of minutes long. It seems to come from a cataclysmic explosion somewhere in the universe. It should be a burst of energy not only in gamma rays, but also in other radiations including the visible light. But the flash we could see in visible light is going to be bright enough to be easily detected for a very short time only! I'm sure you realize that it's very difficult to patrol the whole sky on a continuous basis to watch for any unidentified light flash. Thanks to these new satellites, it is now possible to detect a GRB in gamma rays, to locate it with a reasonable precision, and to send the information back to the Earth in a very short time. Thanks to a network of telescopes spread all around the globe, there is a slight chance to detect what we call the optical counterpart of a GRB. And yes! a 'star' has been found where a GRB had been seen. A source of light quickly fading to become in a matter of days or weeks invisible even with the largest telescopes. The most famous one happened on January 23, 1999. It has been observed in many places (including a couple of telescopes on Mauna Kea). Analyzing its light embedded in the light of a surrounding galaxy, it has been found to be 10 billion light years from Earth.

Why is it so important to know the distance of a GRB? Mainly to get an idea about the energy dissipated in what is clearly a very violent explosion. The results are really gigantic: a GRB could be millions of times as bright as our galaxy, but only for a short time though. Don't forget that there are probably 10 to 100 billion galaxies in the Universe, so the GRB was ÒonlyÓ a 10,000th as bright as the entire Universe. What could produce such an intense and brief release of energy? Possible explanation, among various others, is the collapse of star in a black hole, or the collapse of two stars on each other, which at the end is likely to produce a black hole. To be honest, the astronomers have a hard time explaining a GRB at the moment. Only a few GRB's have been observed in enough details to be able to get an accurate view of their decay. Thanks to the efforts of many observatories (including those on Mauna Kea), we should have one day a better understanding of this strange astronomical species.

On these two pictures, you can see the optical transient source associated to a Gamma Ray Burst which happened on March 1st, 2000.
The picture on the left has been taken with one of the CFHT instruments 2 days after  the burst. The picture on the right has been taken 9 days later with a much more sensitive  instrument. The Optical Transient (OT) is still visible, but now much fainter, as it can be seen comparing it to the close bright star on its right which kept the same brightness between the two pictures. This GRB has been found to be very far away, at more than 15 billion light years from Earth...

Christian Veillet
Canada-France-Hawaii Telescope