Postmortems in the sky

October 24, 1999

October 25, 1999: While it's not quite Halloween, a radio astronomer struck that chord when he described astrophysicists' fascination with gamma-ray bursts.

"We're interested in dead and dying things," said Dr. Dale Frail of the National Radio Astronomy Observatory in Socorro, N.M. "Our highest ambition is to know who that dying thing is."

Frail spoke during the third day of the week-long Fifth biennial Huntsville Gamma Ray Burst Symposium. Gamma-ray bursts are mysterious flashes of high-energy radiation that come from the edge of the observable universe. Since their discovery by gamma-ray detectors designed to watch for nuclear weapons tests in space, scientists have tried to find counterparts in other parts of the spectrum so they might figure out what causes the bursts.

Astrophysicists engage in "forensic science," Frail said, when they study gamma-ray bursts because they look at the remains without having ever seen the victim alive. Frail works in radio astronomy, the end of the spectrum that comes into play when the "corpse is cold and still," volunteered one member of the audience. By comparison, scientists using instruments like the Burst and Transient Source Experiment (BATSE) are "interested in hearing the death rattle," Frail joked.

The death rattles as detected in various parts of the electromagnetic spectrum are all anyone can examine so far in the bid to uncover what causes such horrible deaths out at the edges of the known universe.

The death rattle and last gasp are mostly what scientists have to go on since it's impossible to bring major observatories to bear on a burst as it occurs, and far too much to hope that a telescope will be observing something when a burst happens to go off nearby. While BATSE and a few other instruments can record the gamma-ray flash of a burst, the rest of the astronomy community have to work with the afterglow - if a counterpart is found in other parts of the spectrum - which can last hours or a year.

Frail believes that radio emissions from the afterglow provide unique information on the burst environment and the burst progenitor itself. Using the radio telescopes in Socorro and other institutions, he has looked for radio counterparts in bursts where optical or X-ray counterparts were found. In some cases, they were seen in radio and X-rays, but not visible light.

"I believe that we are seeing a class of events that are optically dark (because) they are obscured by dust in the areas where the gamma-ray bursts exploded," he said. The observations have provided a powerful test of models of fireballs that expand outward from bursts.

Afterglows are also seen in visible and near-visible wavelengths and continue to be among the most valued because they help scientists in trying to locate the hosts of bursts. A total of 14 have been observed with the Hubble Space Telescope and point very strongly to a home for bursts.

"In every case, the gamma-ray burst is right on top of the stellar field," said Dr. Andrew Fruchter of the Space Telescope Science Institute in Baltimore. "It is not in the open where you find gaps" between galaxies near the edge of the observable universe.

Further, many of the bursts are associated with blue galaxies, which are observed to have high rates of star birth.

"All of these things are consistent with star formation," Fruchter said.

In many cases, the optical afterglow components are barely visible to Hubble, despite its incredible light gathering power. GRB 970228 (the numbers are the date of the burst) is the landmark sighting because it was the first optical component to be captured in visible light. When Hubble was able to look at it some weeks later, scientists saw "a small smudge in the sky," a dwarf galaxy with the fading embers of the burst.

In several cases, Hubble's resolution and its ability to distinguish colors have allowed scientists to pick out burst afterglows and barely observable host galaxies. Fruchter described one irregular galaxy as "having the morphology of a train wreck."

Back in the radio end of the spectrum, Dr. Shri Kulkarni of the California Institute of Technology outlined evidence from radio astronomy and other fields that support two leading theories of what causes bursts. One is that two neutron stars coalesce to form a black hole, the other is that a massive supernova or "collapsar" explodes and also forms a black hole.

"It's like when people discovered supernovae," Kulkarni said. "People didn't know what they were, but eventually they figured it out." Supernovae now are known to be massive stars that blow themselves out with a great fury after a short but brilliant life.

Kulkarni said that there is "strong indirect evidence connecting [bursts] to massive stars in dusty hosts or with dust along the line of sight." Whatever the cause, it is followed by an intense blast wave as the materials move outward from the source through interstellar space. But even then scientists are still puzzling over exactly what they are seeing. Dr. Titus Galama, who recently joined CalTech after a long stay at the University of Amsterdam, described the different spectra recorded for the famous burst of Jan. 23, 1999 (GRB 990123). The Robotic Optical Transient Search Experiment (ROTSE) caught this burst in optical wavelengths within seconds of its detection by BATSE.

The burst was brilliant enough that had it been in the nearby M31 galaxy in Andromeda, it would have appeared as bright as the full moon. But despite having ROTSE and BATSE data that overlap in time, "there is no simple relationship between ROTSE's observations and BATSE." Galama said. He showed a graph with data from both instruments. The ends of data lines from BATSE and ROTSE did not point towards each other to indicate that one was a continuation of the other.

"The prompt optical emission is not a simple extrapolation of the BATSE data to lower wavelengths," Galama said, "so we conclude that ROTSE and BATSE are seeing different components."

He suggested that BATSE sees gamma rays produced by internal shock waves as the exploding gas interacts with itself. The optical or visible part is caused by the external shock wave blazing forward through space and ramming into whatever dust and gas are there. Since that material can vary: some regions like the Coal Sack Nebula are so dense that the absorb light from stars and galaxies behind them, others are empty "superbubbles" swept clean by previous star explosions.

Finally, a late afterglow appears in gamma rays as the external shock wave causes reverse shocks within the expanding explosion. This appears in gamma rays as a smooth tail whose high spot early in the blast is masked by the blast itself.

But again in the spirit of Halloween, gamma-ray bursts can wear different masks, and not every one follows this basic model.

"To a first order," Galama concluded, "we have excellent agreement with the fireball model. But we have some oddities to investigate."

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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