They were first spotted in the late 1960s by satellites designed to ferret out secret thermonuclear tests by the Chinese and Soviets, but some of the bursts of high-energy gamma rays picked up by military spy agencies half a century ago confounded the intelligence community. The blasts weren’t coming from anything as puny as an H-bomb being detonated on the earth below. Instead, scientists eventually realized, they resulted from titanic explosions more than halfway across the universe.
By now, astrophysicists generally understand these gamma-ray bursts, or GRBs, which pop off once a day, on average, and are most likely triggered by supernovae, or exploding stars. But the details are still sketchy, which is why a new burst seen in the constellation Leo by two different NASA satellites, the Swift mission and the Fermi Gamma-ray Space Telescope, is so potentially important: it’s by far the most vivid event ever seen — “eye-wateringly bright,” according Fermi project scientist Julie McEnery. That means follow-up observations will be unusually easy to do — and those kinds of aftershock studies are where the real work gets done.
In the past, follow-up analyses were what helped astronomers figure out what causes GRBs in the first place. The only clues they had back in the 1990s, thanks to the important but limited insight provided by NASA’s now defunct Compton Gamma Ray Observatory, was that the bursts seemed to come randomly from all directions in the sky.
That meant they couldn’t be originating mostly in the Milky Way, which spreads outward in a disk shape, not in all directions like a sphere. The only two things that completely surround earth that way are the outer reaches of the solar system — where the Oort cloud of comets lives — and the cosmos itself. GRBs are hardly likely to come from comets — dirty snowballs of ice and rock — which leaves the cosmos as the only plausible source. Eventually, observers managed to aim telescopes in the direction of GRBs very shortly after they popped off, and spotted an afterglow of X-ray and optical light in the very same spot. That is the fingerprint of a supernova, so one part of the mystery was solved; but the exact mechanism behind the phenomenon was still uncertain.
At present, the best explanation for that piece is that a giant, aging star collapses abruptly to form a black hole. Some of the remaining material is shot outward to form a rapidly expanding shell of gas. Some gas also falls into the black hole, where it’s compressed and heated, and sometimes shoots a jet of matter back outward. When the jet runs into the expanding shell of gas, it triggers a cataclysmic explosion — and the result is a GRB. It probably happens when any supermassive star collapses to form a black hole, but, says McEnery, “we’re not going to see most of them; we can only detect them when the jets point right at us.”
The value of the newest, biggest burst is that it is also the best opportunity to refine and improve the existing theory. Technically, this one isn’t actually the brightest GRB detected in absolute terms: some have been more dazzling, but so far away that they’ve seemed relatively dim. This one, known as GRB 130427A, was a “mere” 3.6 billion light-years away — still far, far beyond the edges of the Milky Way, but close enough, says McEnery, that “it outshone everything else in the night sky for several seconds.”
That brightness is allowing the scientists to dissect the burst in unprecedented detail, which could help them understand some of the still murky details of how supernovas unfold. In fact, says McEnery, “we can study this event in ways we could never dreamed of. We’re likely to be seeing the afterglow for several months.”
That’s the formal scientific explanation for her obvious excitement, and that of GRB watchers and supernova theorists around the world. Then there’s the informal explanation: “Stuff that blows up,” she says. “What’s not to like?”