The incident was so extreme, it almost defies description. In the wee hours of the morning of 19 March, astronomers detected from more than halfway across the universe a burst of gamma rays brighter than a hundred-billion suns--and aimed squarely at Earth. Now, after 6 months of analysis, an international team of astronomers has discovered why the event was so extraordinary.
From brief glimpses throughout the past decade, astronomers have pieced together a standard theory of gamma ray bursts. When massive stars exhaust their nuclear fuel and can no longer resist the relentless, crushing force of gravity, they collapse violently. As a star much more massive than the sun contracts to the size of an asteroid--or even smaller if it becomes a black hole--it creates unimaginable densities, temperatures, and energy. A great deal of that energy rebounds outward in a jet of particles traveling at nearly the speed of light. When that jet meets surrounding gas or dust, it generates gamma rays.
New details are revising that picture. A team of 93 scientists report tomorrow in Nature that the "naked eye" burst from March--originating in a galaxy about 7.5 billion light-years away in the direction of the constellation Bootes--shows a feature that has never been seen before. After analyzing data from NASA's Swift spacecraft, which was designed to move very quickly in the direction of gamma radiation, and other data from both satellite and ground-based telescopes and detectors, the team says the super bright beam was surrounded by a less energetic, slower jet about 20 times wider. This could mean, the team suggests, that a bright, narrow beam accompanies all gamma ray bursts but can't be spotted unless it is aimed directly at Earth.
Events such as the naked-eye burst "are cracking away at the simple picture" theorists have assembled over the past decade, says astrophysicist Joshua Bloom of the University of California, Berkeley. Moreover, this incident could constitute the first of a remarkable series of glimpses into the most distant parts of the universe, he says. The burst was so bright, Bloom explains, that it could be observed with relatively small telescopes--and even with the naked eye. If astronomers get lucky again, they might spy similar bursts from even farther away, perhaps among the very earliest stars.