Gamma-Ray Burst: A black hole is born

Gamma-ray bursts, the most violent cosmic explosions, are the birth announcements for black holes. Several new reports support this increasingly popular notion, but they disagree on how and when the black holes are produced.

SUPERJETS. In the collapsar model, jets from a black hole generate a gamma-ray burst and a supernova. Spectrum Astro

According to one theory, known as the collapsar model, gravity rapidly crushes the core of a star at least 20 times as massive as the sun down to a black hole–a superlatively dense cinder whose gravity is so strong not even light can escape its grasp. Jets of subatomic particles generated within a swirling disk of material around the black hole race outward at nearly the speed of light. This generates high-energy radiation–gamma-ray bursts that last from seconds to several minutes. At about the same time, the jets also blow apart the collapsed star’s outer layers, creating a cataclysmic, yet less energetic, supernova explosion (SN: 7/10/99, p. 28).

This model predicts that observers will first detect a gamma-ray burst. Only when its afterglow has dimmed can they see a temporary upswing in brightness as the supernova’s light suddenly dominates the radiation.

In a competing theory, known as the supranova model, a supernova comes first and is followed, days to months later, by a gamma-ray burst. In this scenario, a massive star rotates so rapidly that it can’t immediately collapse into a black hole. Instead, it temporarily forms into a less dense cinder, an ultraheavy neutron star. In the process, a shock wave jettisons the star’s outer layers in a supernova explosion. Later, the spin of this ultraheavy neutron star is slowed by its own magnetic field, and the body can no longer resist its fate. Gravity squeezes it down to a black hole, and a gamma-ray burst is finally generated.

Two papers scheduled for an upcoming Astrophysical Journal Letters appear to support the collapsar model. Joshua S. Bloom of the California Institute of Technology in Pasadena and his collaborators studied the afterglow of the closest known gamma-ray burst, GRB 011121, which was recorded Nov. 21, 2001. Examining the visible-light afterglow with the Hubble Space Telescope in the next few months, the team found evidence for a “red bump” in the fading light. This bump has the exact color expected from a supernova exploding at the same location as the burst.

Similar signs of a supernova have been spotted in the afterglow of other bursts, but never so clearly, the team notes. The researchers have yet to rule out models that could create the red bump without invoking supernovas.

Nonetheless, Bloom and his collaborators say their results exclude the supranova model, since the proposed supernova would have to have exploded at about the same time as the burst rather than several days to months before.

In an article in the April 4 Nature, however, James R. Reeves of the University of Leicester in England and his colleagues arrive at a different conclusion. Their argument is based on analysis of the X-ray afterglow of a gamma-ray burst recorded on Dec. 11, 2001, and dubbed GRB 011211.

Studying the spectrum of the burst’s afterglow with the European Space Agency’s X-Ray Multi-Mirror-Newton Telescope, Reeves and his collaborators found traces of magnesium, sulfur, silicon, argon, and calcium. These heavy elements are produced by nuclear fusion within a star’s core and are hurled into space by supernova explosions. Reeve’s team estimates that the hot, supernova-driven shell of material carrying these elements is expanding into space at one-tenth the speed of light.

Given this rate of expansion and the shell’s estimated size, the researchers find that the supernova exploded about 4 days earlier than the gamma-ray burst occurred. This appears to support the supranova model.

Theorist Stan Woosley of the University of California, Santa Cruz, who helped develop the collapsar model, says that regardless of which model ultimately wins out, “the preponderance of evidence now shows that gamma-ray bursts accompany and are a direct consequence of the deaths of massive stars” and thus the birth of black holes.

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