Cosmic Afterglow: Gamma-ray bursts may one-up themselves

Bursts of gamma rays that originate beyond our galaxy are already known to be the universe’s most energetic flashes. But new observations suggest that these cosmic outbursts may pack an even greater wallop than scientists had estimated.

Last Oct. 4, just 11 seconds after NASA’s High-Energy Transient Explorer II satellite recorded a gamma-ray burst, the craft alerted astronomers to the event.

For the first time, researchers closely monitored the visible-light afterglow of a burst almost as soon as it appeared. Since then, another gamma-ray burst has been observed from start to finish (SN: 2/1/03, p. 77: Available to subscribers at Gamma-ray burst leaves ephemeral afterglow).

The first half-hour of the observations is the most puzzling, report Derek W. Fox of the California Institute of Technology in Pasadena and his colleagues in the March 20 Nature. During that time, the afterglow decayed more slowly than the scientists had expected. The finding may be linked to the origin of gamma-ray bursts.

Theorists hold that gamma-ray bursts are generated when high-speed particle jets in the cosmos are dramatically slowed, probably by collisions between clumps of material within them. The afterglow is created when the jet slams into the surrounding interstellar medium.

The slow decay of the afterglow observed last October suggests that something is giving the jet an extra kick, Fox’s team concludes. That dovetails with a popular explanation for the origin of the jet: the collapse of a massive star into a black hole. As a black hole pulls in material from its surroundings, it generates jets of particles and then reenergizes them, suggests Stan E. Woosley of the University of California, Santa Cruz. If that model is correct, he adds, the total energy poured into a gamma-ray burst and its afterglow could be two to three times more than what some theorists had calculated from other models. The total power would be equivalent to that of more than a million trillion suns.

However, Tsvi Piran of the Hebrew University of Jerusalem says another mechanism accounts for the data even better, including a steep energy decline recorded after the first 30 minutes of the afterglow. The radiation from a jet moving near the speed of light beams outward in a cone-shaped zone, he notes. As the jet slows in the interstellar medium, the cone enlarges, and an observer sees more of more of the radiation. As the observer samples more of the radiation, the average energy may go up or down. In this model, the afterglow may appear brighter not because the jet is reenergized but because the larger area includes regions of high energy. Later, the overall area might include more regions of low energy, producing a steep decline in the observed afterglow.

A reenergized jet can’t account for the rapid falloff observed, notes Piran. But he acknowledges that both a revived jet and the enlargement of the radiation cone may play a role in the particular burst recorded last fall. “These observations are a new window on gamma-ray bursts, and when you open up a new window, there are often new surprises,” says Piran.

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