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Watching the Universe's Second Biggest Bang
25 March 1999 8:00 pm
On 23 January, astronomers detected a powerful pulse of gamma rays that originated billions of years ago in a far corner of the universe. Their observations, published in 6 studies in tomorrow's Science and next week's Nature, offer clues to the celestial catastrophes that generate these so-called gamma ray bursts.
First observed in the 1970's, gamma ray bursts (GRB's) remained a mystery for decades. But improved satellite detectors, together with an alert system for ground-based observers, allowed astronomers to trace GRB's to very remote galaxies. Theorists now think that GRB's result from the collision of two high-density neutron stars or from a "hypernova"--the total collapse of a very massive star. Both would form a central black hole and eject matter at close to the speed of light, creating shock waves that would generate both the initial flash and a lingering afterglow.
With GRB 990123, as the January bang was called, the alert system acted fast enough to enable scientists to record a complete portrait of the event, from the gamma ray burst and a simultaneous optical flash to radio waves crackling even days later. Combining the data, Titus Galama of the University of Amsterdam and his colleagues conclude in this week's Nature that they are seeing the effects of three kinds of shock waves within the fireball. "The initial gamma ray burst is believed to be caused by internal shocks in the ejecta," says Galama. "The optical flash recorded during the burst is probably due to the short-lived reverse shock, while the afterglow arises from the forward shock."
By analyzing the light, Michael Andersen of the Nordic Optical Telescope on La Palma in the Canary Islands and his colleagues show in tomorrow's Science that the source of GRB 990123 was several billion light-years away. That distance makes it the most luminous gamma ray burst seen so far. If the explosion radiated uniformly in all directions, its energy must have been a colossal 3.4 x 1054 ergs--what you would get if you converted the mass of two suns into energy. It also makes GRB 990123 the second biggest explosion--after the big bang--ever observed.
But if the blast emitted gamma rays in two opposite directions, and we happen to look down one of the two jets, less energy could account for the observed luminosity. A team led by Shrinivas Kulkarni of the California Institute of Technology in Pasadena claims that they see evidence for beaming in the afterglow of GRB 990123: About 2 days after the burst, the afterglow started to fade faster than before, which you would expect when a jet points more or less in your direction and, once it has cooled a certain amount, suddenly starts to expand sideways, increasing the cooling rate. Other scientists have found hints of beaming as well.
Some theorists are now coming up with explosion mechanisms that would naturally produce beams of radiation--emerging, for example, from the poles of a spinning black hole. But others are withholding judgment. "The theoretical evidence for beaming is quite compelling," says leading GRB theorist Martin Rees of Cambridge University, "but the observational evidence isn't very strong yet." Another titanic burst, and another haul of data, may change that.