Seconds after NASA’s Swift satellite recorded a gamma-ray burst on Sept. 4, astronomers’ cell phones buzzed, beepers went off, and an e-mail alert flashed on more than 1,000 computer screens. It was the 68th time that Swift had notified astronomers about a cosmic explosion, and this one didn’t initially appear to be exceptional.
But this week, a team of astronomers announced that the burst is the most distant one ever detected, hailing from the long-ago era when galaxies and stars first lit up the heavens. The finding suggests that researchers may soon detect even more-distant bursts that will illuminate the epoch before there were massive light-emitting objects such as quasars and large galaxies.
“This opens the door for gamma-ray bursts as unique and powerful probes of the early universe,” theorist Don Lamb of the University of Chicago said during a Sept. 12 telephone briefing. Five years ago, he and Daniel Reichart of the University of North Carolina at Chapel Hill, a codiscoverer of the Sept. 4 burst, predicted that remote bursts might be relatively common.
According to the leading theory of gamma-ray bursts, most of these events occur when a massive, rotating star collapses to become a black hole, hurling jets of particles into space. During its lifetime—ranging from less than a second to a few minutes—a burst can be 10 billion to 100 billion times as bright as an entire galaxy.
In principle, then, bursts can be seen at much greater distances than can galaxies or quasars, the especially bright beacons at the centers of some galaxies. Moreover, because the bursts mark the demise of individual stars and not collections of stars, they may have preceded the formation of galaxies and quasars in cosmic history, notes Lamb.
The newfound burst, GRB 050904, lies about 12.8 billion light-years from Earth. That’s 500 million light-years farther away than the most-distant previously known burst. The newly observed burst hails from when the universe was only 900 million years old. The youngest known galaxies hark back to an only slightly earlier epoch.
A few hours after Swift notified astronomers of the 200-second-long burst, Reichart’s team observed the burst’s infrared afterglow with the Southern Observatory for Astrophysical Research (SOAR) telescope atop Cerro Pachon in Chile. Excitement grew when Reichart’s team compared the infrared images with visible-light observations from detectors a mountaintop away, on Cerro Tololo. This array, known as Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes, had found nothing.
Reichart’s team proposed that either the burst had exploded in a relatively nearby galaxy that was shrouded in dust or the burst was so distant that much of the visible light from its afterglow was absorbed by intervening clouds of hydrogen gas before it could reach Earth.
Follow-up observations with SOAR indicated that the dusty scenario was unlikely. Then, on Sept. 6, a team led by Nobuyaki Kawai of the Tokyo Institute of Technology in Japan used the near-infrared Subaru Telescope on Hawaii’s Mauna Kea to measure the record-breaking distance to GRB 050904.
Discovering such a distant burst less than a year after Swift’s launch suggests that astronomers may discover “scores and even hundreds” of distant bursts, says Lamb.
Simulations suggest that the cosmos’ first-generation stars were more massive than those of later generations and therefore more likely to have produced powerful gamma-ray bursts when they died, notes Stan Woosley of the University of California, Santa Cruz.
