Titanic explosions that dwarf even the brightest supernovas, one scientist says, may account for mysterious gamma-ray bursts that flash once a day or so from random directions in the sky. But this provocative theory, submitted to the Astrophysical Journal, is having a hard time winning over skeptics.
Since gamma-ray bursts were discovered in 1973, astronomers have been trying to figure out what causes them. Earlier this year, observers determined that the bursts emanate from the far reaches of the universe, implying that the cataclysms that generate the flashes are more powerful than anything known to astronomy. They also noticed an "afterglow" of lower energy radiation following the burst of gamma-rays. "The discovery of the afterglow was a dramatic breakthrough," says Bohdan Paczynski, an astrophysicist at Princeton University. "If afterglows are for real, they offer a tremendously powerful tool to study gamma-ray bursts."
Paczynski didn't hesitate to put that tool to work. He has conceived a scenario in which one star in a binary system might "spin up" by sucking in gas—and angular momentum—from its companion. Like a figure skater drawing in her arms, the star would spin faster and faster, eventually making each revolution in just milliseconds, like the fastest pulsars. Paczynski argues that if the spinning star later exhausted its fuel and collapsed—the event that normally triggers a supernova—its spin could vastly amplify the cataclysm.
If the star were to collapse into a black hole, its magnetic field would become extremely intense. This field would transfer a huge amount of energy from the spinning core to the star's outer gas envelope, blowing it out in what Paczynski calls a "hypernova"—an explosion as great as 1000 supernovas combined. Most of the gas envelope would be thrown off at a third of the speed of light, and the outer edge would race outward at more than 99.999% of light speed. As the shock wave slammed into the interstellar medium, it would emit gamma-rays: the burst itself. The slower gas behind would result in the less energetic afterglow.
"It's an interesting idea," says astrophysicist Shrinivas Kulkarni of the California Institute of Technology. "It's one of those papers where you ask, "Why didn't you think of it?' " But he's not completely sold on the idea. Kulkarni argues that the scenario does not explain most gamma-ray bursts, in part because the bursts don't seem to come with the usual hallmarks of supernovas: very bright emissions of visible light and radio waves. "There really is no evidence for spectacular supernovae," he says. "But there's a new realization that gamma-ray bursts are more energetic than supernovae—this is a good first attempt to explain them."