Thursday, September 07, 2006

Bursts and Flashes

If we would have new knowledge, we must get a whole world of new questions. ~Susanne K. Langer

Years ago, when I was starting out in college I tried majoring in physics, which didn't work out so well. After a considerable struggle, I ended up majoring in Management Science, a rather strange amalgam of economics, finance, behavioral science, and computer science (which left me with an impressive-sounding major that no one has ever understood, but it looked good on a resume). Physics took its toll on a lot of my fellow classmates, and for some reason, many of them gave astronomy a try, as if that was something easy. Now I had taken a course in radio astronomy, so I knew that thinking astronomy was easier than physics was like thinking that a volcano was less dangerous than an earthquake. Either one can get you, if you don't watch out.

One thing that makes astronomy difficult is that you are trying to understand very complex systems that are very far away. Yet astronomers make up convincing theories to explain what they observe, along with astrophysicists and physicists as well. Unfortunately, since the universe is large and our means of observing it get more sophisticated all the time, astronomers keep finding things that upset the applecart, causing major reappraisals in thinking and intense (and sometimes acrimonious) debate. In science, this is called “fun.”

One of the current areas of “fun” involves Gamma Ray Bursters (GRBs). GRBs are titanic explosions that propel huge amounts of gamma rays in our direction. Originally, they were thought to be events occurring relatively near by, in astronomical terms, at least. For that to be the case, the GRBs should have fallen in the plane of the Milky Way, since everything near to us falls into that area. As more data was gathered, though, it was found that GRBs were all over the sky, which meant that they were coming from a long way off.

Ultimately, thanks to timely observations, the distance to one of these things was determined, which turned out to be very, very far away. This information was very disconcerting because to generate the strength of gamma radiation that was detected over the huge distances, the energy being released had to be so enormous as to violate Einstein's famous equation, E=mc2.

Now you can mess with a lot of things in science, but the mass-energy equivalency is not something to be tinkered with lightly. Yet, if the GRBs were viewed as traditional explosions, radiating uniformly in all directions, then some, if not each one, would have to be converting an amount of mass equal to all the visible mass in the universe into energy.

Somehow, that seemed unlikely.

But, there was an observed phenomenon that would explain the intense bursts. If, instead of a uniform blast, the gamma rays were ejected as “beacons” or “jets”, say from each magnetic pole, then the mass-energy conversion works just fine. Just such jets of radiation have been observed for years, first by radio astronomers, later by visual observations from Hubble. If the Earth lies in the path of the jet, we see a brilliant outpouring of gamma rays. So one problem was solved. But the question still remained: What are GRBs anyway?

There has been no shortage of theories, including colliding neutron stars, matter falling into a massive black hole, even colliding black holes. Now, a new observation has astronomers thinking in a more traditional direction: Supernovas.

Supernovas would make excellent candidates for GRBs in a number of ways. First, supernovas are known to explode in a symmetrical way but not in a uniformly radiating manner. The best way to describe supernova explosions is to think of them as expelling matter and radiation in the shape of an hourglass. At the equator of the star, a disk of matter is ejected, but the main burst is from the poles (Eta Carinae is an excellent example).

Secondly, since there are a lot of stars, there are a lot of supernovas, which would explain why we see so many. Thirdly, early stars were bigger, shorter-lived stars, so we'd expect to see more GRBs at great distances, which we do. Things come together nicely.

However, not everything is so rosy. The supernova that was observed in conjunction with the GRB event lasted longer than normal. It also was less energetic than most GRBs, so, of course, scientists have created a sub-class of GRBs called “X-ray flashes”. I don't know if the IAU had anything to do with this (sneaky reference to Pluto; sorry).

Also, not all supernovas create GRBs. While this could be due to the directional nature of GRBs (that is, we see the light but the gamma ray burst is aimed in another direction), it's probably more a sign that not all GRBs can be explained by supernovas. Since GRBs are associated with black holes and not all supernovas result in black holes, it makes sense that there is yet another source or sources for the mysterious outbursts.

What makes this observation so interesting is that the star doesn't appear to be massive enough to collapse into a black hole. In addition, the GRB actually preceded the stellar explosion. So X-ray flashes may be a precursor to certain types of supernova events, but not all GRBs are necessarily associated with such events.

So, we have a new piece of information that, as is often the case, raises more questions than it answers. We are still a ways from knowing what GRBs are and how they fit into the great cosmic story, but we've come a tiny step closer. A lot more analysis will need to be done of the current data, and many more observations will need to be made. And, in the meantime, there will be much theorizing, debating, and arguing about what has been found.

As I said, this is what scientists call “fun.”

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