Tuesday, January 02, 2007

Baffling Bursts

The important thing in science is not so much to obtain new facts as to discover new ways of thinking about them. ~William Lawrence Bragg

If I were starting out today in a career in astronomy, I would make Gamma Ray Bursters (GRB's) my specialty. These energetic events are causing no end of consternation in astrophysical circles, and when that happens, people start jumping at possible explanations. That's PhD material.

GRB's have been a mystery for some time now. Basically, a GRB is an object that very briefly emits a huge outburst of gamma rays. Because the bursts are brief, it was a long time before astronomers gained sufficient data to know that they were frequent events. Initially, it was thought that GRB's were reasonably local happenings, occurring within our own galaxy. But with satellites doing the observing, it was found that these things were all over the sky, not just in the plane of the galaxy. That left GRB-watchers with two discomfiting facts:
  • Most GRB's were very far away, which meant that

  • Their energy output was almost incomprehensible.
In fact, if one considered a GRB as if it were an ordinary explosion, the amount of energy being released would have required a galaxy-worth of mass. Yet these things were happening all over the sky, and no galaxies were disappearing, which meant that one would be forced to conclude that E=mc2 was being violated. It's one thing to mess with Mother Nature, it's another to screw around with one of the most fundamental equations of physics.

Fortunately, there was a way out if one got rid of the assumption of a traditional radiating-in-all-directions explosion. When astronomers looked at supernova remnants, they saw all sorts of evidence of very directional outbursts. They also saw the jets of energy from super massive black holes. So GRB's must radiate their energy in the same way, so the GRB's we observe happen to be pointed in our direction. It also made a fair case for GRB's to be the result of supernova explosions, although there were still some questions to be answered (as I discussed here).

I should point out that supernovas have been primarily associated with long-duration GRB's. Brief ones are thought to be a consequence of collisions between neutron stars or black holes (or a neutron star and a black hole).

But Nature loves to tease scientists. In May and June of 2006, scientists saw two GRB's that were most unsettling. They were long-duration events; the June burst lasted 102 seconds, which is forever in GRB-speak (the May burst went for 4 seconds, which is still considered “long duration”). Since the launch of the Swift satellite, designed to find GRB's, astronomers have set up an alarm system to try to find what might be seen where the burst happened. When these two were spotted by the SWIFT satellite, telescopes around the planet (and, in space, Hubble) were turned to the coordinates, expecting to find supernovas, as they have often done in the recent past.

This time, they found nothing, which was a fine howdy-do. Well, technically, they did find something, but the observed behavior was definitely not like that of a supernova. The June burst, for example, was studied for a long period, with astronomers looking for a rebrightening object that would indicate a supernova had occurred, but despite looking for almost two months, nothing was seen after the first four days. An object was found at the site of the May burst, but it dimmed within a day. These would be vanishingly dim supernovas.

So, to add to the conundrum I discussed in my earlier article that not all supernovas create GRB's, we now had GRB's with no supernovas. It took a couple of months, but a new theory has emerged, one that has a new take on how some stars die. It seems that some massive stars, for reasons which are not clear, don't end in a massive supernova explosion; they simply collapse into a black hole, without so much as a whimper.

Well, that's not exactly correct, because the star's material falling into the black hole does let out a mighty shout, which is detected as the gamma ray outburst. The problem is that we don't understand what mechanism is at work here. All of the current theories describe the final fate of stars based on their mass. Stars like our Sun slough off outer layers, then collapse inward to become white dwarf stars. Massive stars go out in a blaze of glory and gamma rays. Except that maybe some just go out in a gamma ray burst.

It's an interesting idea, but no one understands what mechanism could be at work to cause the star to simply collapse on itself yet be massive enough to create a black hole. One possibility is that the star is collapsing into a companion black hole, but there would be difficulties with that scenario as well. There is considerable work to do based on these recent observations.

Fortunately for the astronomers, given the frequency of GRB's, they'll have no shortage of data with which to work.

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