As if gamma-ray bursts weren't nasty enough, astronomers announced today that the brightest GRB ever seen shot a double-barrel blast toward Earth at near-light speed. One jet was incredibly narrow, and was nested inside a wider jet. It's like shooting a gun with a deadly laser beam embedded within a lethal spray of buckshot.
The evidence for this double-barrel blast comes from a gamma-ray burst detected by NASA's Swift satellite on March 19, 2008. GRBs are intense explosions that appear about once per day from random directions in space. Most GRBs occur when the core of a massive star collapses to form a black hole. Infalling stellar gas forms a disk around the black hole, and some of that material is shot away from the black hole in two jets traveling in opposite directions at near the speed of light. These jets can be likened to cosmic blowtorches whose energy boggles the imagination. For several seconds to several minutes, the gamma rays from these jets can greatly outshine an entire galaxy's worth of stars.
Normally, GRBs are detected in gamma rays and X rays. Eventually, the jets slam into surrounding gas clouds and dissipate their energy, generating an afterglow that astronomers can detect in visible light or in X rays. This optical emission is extremely faint and comes many minutes or hours after the burst itself. But in this March 19th burst, optical telescopes recorded a flaring source that peaked at about magnitude 5.3, visible to the naked eye from a dark site. If this doesn't sound particularly bright, then consider the distance — the GRB took place 7.5 billion years ago, effectively halfway across the visible universe. In other words, we are looking so far back in space and time that the star exploded several billion years before the solar system had even formed! For about 40 seconds, the GRB's optical flash was by far and away the most distant object that could be seen with the naked eye.
The immediate and obvious question was, "Why was this burst so bright?" In a paper published in the September 11th issue of Nature, Judith Racusin (Penn State University) and 92 coauthors provide an answer. The Swift satellite usually detects evidence of one jet slamming into nearby gas clouds and breaking up. But in this case, Swift found two "jet breaks," one from a very narrow jet and one from a much wider jet. The narrow jet has an opening angle of just 0.4 degrees, whereas the the wider jet had an angle of about 8 degrees. The GRB was so bright because the narrow jet was aimed exactly toward Earth. Swift's observations indicate that the particles in the narrow jet traveled at an astounding 99.99995% the speed of light.
As Racusin explains, "If the narrow jet was not pointed at us, we would not have seen its signature." A jet this narrow is so pencil thin that a satellite like Swift might see just one per decade. Since very few GRBs would be so perfectly aligned to aim their narrow jets toward Earth, this implies that most or all GRBs produce a narrow jet within a wider jet. (And since GRBs probably produce jets that shoot away in opposite directions, they are actually producing two narrow jets inside wider jets.)
GRBs were already known to contain staggering amounts of energy. But Racusin adds, "If all GRBs do have these narrow jets, it could potentially double (or more) the amount of energy that would be emitted by these explosions." Such a jet aimed at Earth from a collapsing star in our Milky Way Galaxy would seriously damage our atmosphere, perhaps triggering a mass extinction. Fortunately, only a tiny percentage of stars generate GRBs, and it's extraordinarily unlikely that one would occur in our galaxy with the perfect geometry to aim the barrel of the gun directly at Earth. GRBs are indeed nasty critters, but despite what you might have seen in sensationalist TV shows, they pose such a negligible threat to humanity that you should not lose any sleep over them. Still, we can marvel at Mother Nature's creativity and fury.