GRBs
Published on Dec 21, 2005 at 7:47 pm.
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Filed under astronomy.
Now, for another astrophysical mystery. OK, I know that I haven’t finished with the dark matter thing, but here is one that we are perhaps further along in figuring out.
In about three weeks, the American Astronomical Society (the professional astronomer organization for North America) will have a meeting. One of the sessions will include papers about Gamma Ray Bursters (GRBs). So, I thought that I’d mention something about them.
In the early 1960’s, treaties were signed to end atmospheric nuclear testing. This did not put an end to testing nuclear devices, but rather just confined such tests to underground explosions. Weapons engineers did not like this, since the dynamics of an underground explosion are quite different from those of a surface or above surface blast. Thus, these tests did not give a true indication of what a weapon would actually do if it were used for real. So, there was real concern that nations might cheat on the provisions of the treaty. To monitor against such cheating, satellites were deployed with powerful gamma ray detectors. The only known sudden burst of gamma rays was from detonating nuclear weapons, so this seemed to be a good way of finding any cheaters. Much to the surprise of the intelligence agencies monitoring the satellites, a great number of gamma ray bursts were recorded from random directions in the sky! Naturally, this information was classified for a long time.
Why weren’t these gamma ray bursts discovered from ground based measurements? Well, it turns out that gamma rays and X-rays don’t really pass through all that much air before they are absorbed. Most of the radiation is absorbed in only feet of air. Only really massive bursts, like those from nuclear explosions, could travel the miles needed to be detected through the atmosphere. Cosmic gamma ray bursts were simply too weak for their radiation to penetrate the atmosphere (thankfully).
By the 1990’s, the existence of the gamma ray bursts from the early spy satellites had been declassified. Since gamma rays don’t penetrate the atmosphere well, special astronomical gamm ray telescopes were placed in orbit around. Hundreds, and then thousands, of gamma ray bursts (GRBs) were discovered. However, no one knew what these things were.
Two types of GRBs were found: long duration and short duration bursters. Long duration is a matter of perspective, since this meant from a couple seconds up to a few minutes. The short duration bursts were tens of milliseconds up to about a second or so. All kinds of wild ideas were proposed. But nobody even knew how far away these events were happening! After all, it makes a difference if they are nearby and fairly weak, or far away and very powerful. Both would appear the same brightness to us on Earth. The problem was that the events lasted such a short time that by the time astronomers monitoring the gamma ray detectors had contacted other astronomers to point their instruments towards the GRB to see what was there, it had already faded! Eventually, in the late 1990’s, though, astronomers managed to capture the afterglow of a GRB. This turned out to be in the outer parts of a galaxy nearly 7,000,000,000 lightyears distant! This was an amazing discovery, since this was WAY farther than anyone had ever suspected. This meant that the GRBs had to have many times more energy than the entire energy output of a galaxy!!!! Now, we had a major problem. How do we account for such energy production.
Even wilder explanations came about to try to explain how such energy could be liberated in such a short time in such a confince space. Ideas were floated about such as exploding black holes, merging neutron stars, matter-antimatter annihilation, flareups of active galaxy nuclei (still not understood at that time, though we now know more about these events, too), collisions of cosmic strings, etc. Part of the difficulty was that GRBs were always detected far away, so study was difficult. It became apparent, though, that we really didn’t want a GRB nearby. In 2003, a GRB “only” 2.6 billion lightyears away was powerful enough to partially ionize the Earth’s upper atmosphere. If a GRB were to occur anywhere within tens of thousands of lightyears, then it would be energetic enough to fry Earth, and perhaps to sterilize the surface of the planet of life (including us). It would be pretty, of course, since the air would glow a pretty blue color all over the side of the planet facing the GRB for a few seconds. Then, we’d die of radiation poisoning. Darn! Probably, we’d die long before we even figured out what caused the GRB. Well, at least it would be pretty. 
A few other GRBs were identified quickly enough for their afterglow to be seen and studied. Then, a mystery surfaced, because some parts of the spectrum of the afterglow seemed to be similar to those of a supernova. A supernova occurs when the core of a very massive star collapses into a neutron star or a black hole. The sudden release of energy of this collapse is sufficient to blow the rest of the star apart in a massive explosion that temporarily makes the exploding star as bright as the entire light ouput of the galaxy in which it resides. Yet, GRBs were brighter still. It was suggested that perhaps an even bigger form of supernova might occur from the collapse of stars of over 100 solar masses, a sort of super-supernova. The term for coined for this event was hypernova. A hint came when some supernovae were found that appeared to be slightly under luminous. This led to the suggestion that perhaps supernovae are not symmetric. Up to that point, all the equations and theoretical work had been with spherically symmetric explosions. Spherical symmetry makes solving the equations involved much easier. MUCH, MUCH, easier. Extremely easier! But, if the explosion isn’t symmetric, then excess energy would be channeled in certain directions. A suggestion was made that perhaps a very massive star collapsing into a black hole might develop a fast rotating accretion disk around the black hole for a moment (or longer). Such an accretion disk would have a powerful magnetic field, and this field would channel the bulk of the ionized particles from the explosion into two jets heading out the magnetic north and south poles. This is much the idea of how shaped explosive charges work — most of the explosion is directed in one direction. So, this would make the explosion of the supernova more powerful in the direction of the poles. So, rather than a hypernova, the GRB would be what you would see if one of these very large assymetric supernovae were to occur with the jet pointed towards Earth.
The assymetric supernova model seems to work fairly well to explain long duration GRBs, but it fails to fully explain short duration GRBs, which still remain a mystery (and still have lots of wild theories running around about them). More observations are needed, though, to fully support this model. At any rate, we are finally getting an idea of what is going on.
So, I hope that this wasn’t too much or too involved for y’all. I know that there is a wide range of readership out there.
-Astroprof
