Media Contact:
Mike Robertson, Director of Media Relations
College of Charleston, Charleston, S.C.
E-mail: robertsonm@cofc.edu
Phone: 843-953-5667

For release at 10 AM CDT (11 AM EDT) May 28, 2003

Nashville, Tenn. -- Scientists at the College of Charleston have found a rare subset of gamma-ray bursts that, once turned off, can turn back on full blast. By bringing into sharp focus a sometimes-overlooked but perplexing problem associated with gamma ray bursts (GRBs), the findings pose a challenge to the leading theory about the origins of these explosions, the most energetic in the Universe.

The findings are presented today at the 202nd meeting of the American Astronomical Society in Nashville, Tenn.

Shining with an energy exceeding a million trillion Suns, gamma-ray bursts are exceedingly distant, fleeting flashes of gamma ray light occurring nearly daily from random directions in the sky that are often followed by an "afterglow" of fading, less energetic light lasting hours, days, or even weeks.

Focusing on the little-known phenomena of "post-quiescent" emissions, the scientists say underscores just how much we have yet to learn, most notably about the "missing link" between the burst itself and the afterglow.

"The missing link refers to the distinction between the gamma-ray burst and the afterglow emission component," said Tucker Freismuth, who studied one of the largest GRB data sets recorded during a nine year period by the now-defunct Burst and Transient Source Experiment (BATSE). The study was Freismuth's senior thesis toward his just-completed Bachelors of Science degree in physics with astronomy concentration.

Freismuth explained that the only way to figure out this missing link is to have enough observations to produce a set of statistics on the distribution of time delays between the gamma-ray burst and the afterglow. This, in turn, would let astronomers figure out whether post-quiescent emissions represent a continuation of the burst or a rare especially energetic type of afterglow.

"We aren't saying that every gamma-ray burst with a delay between the burst and afterglow therefore has a quiescent episode," Freismuth said. "But it is also true we are not certain that the peak frequency of the afterglow is always less than the burst itself."

Working with his advisor Dr. Tim Giblin and Dr. Jon Hakkila, both faculty members of the college's department of physics and astronomy, Freismuth examined 2,704 gamma-ray bursts recorded by BATSE, the all-sky detector that flew aboard the Compton Gamma-Ray Observatory from 1991 - 2000. He was looking for bursts that, having shut off or gone "quiescent," suddenly flared again at gamma-ray energies.

They were looking for bursts whose quiescent episodes were as long or longer than the initial burst. The resurgence of gamma-ray emission they dubbed the "post-quiescent emission" since it comes after the quiescent component.

Analyzing the BATSE database, they found 23 GRBs showing a post-quiescent emission strong enough to analyze, although many more showed at least some evidence of such emissions.

Freismuth's advisor, Dr. Giblin, said it is important to understand the nature of these post-quiescent emissions in order to figure out if they are part of the burst or part of the afterglow phenomena. Such information would help refine the theoretical models involving GRBs, the chief one of which is the so-called "collapsar/fireball" model. This model attributes the burst phenomenon to the cataclysmic explosion of a highly energetic supernova (dubbed a "hypernova") that marks the deaths of especially massive stars and the afterglow phenomena to the collision of exploding stellar material with gas and dust in the surrounding interstellar medium.

Establishing that the post-quiescent emission is actually part of the afterglow component would mean, at least in some cases, its peak energy could occasionally be at gamma-ray levels rather than, as long predicted by theory, at the somewhat lower X-ray levels. This, in turn, would require at least some fine-tuning of the collapsar/fireball model.

Under this model, which has met with a good measure of success in describing both the characteristics of the burst itself and the afterglow, the burst is initiated as the dying star's core collapses into a black hole. This core collapse triggers a pressure wave that blasts out of the star in a particular direction as a fireball of heated stellar material.

Blobs of stellar material moving at different relativistic speeds inside the elongated fireball collide with each other, setting up internal shock waves that result in the release of the most energetic type of energy, gamma rays. The afterglow occurs as the blobs collide with gas and dust existing in the region around the star (the interstellar medium), creating external shock waves. These collisions are not as energetic, releasing X rays, and thereafter, as the fireball dissipates, the energy released drops down to visible light and even microwaves before disappearing entirely.

"The post-quiescent emissions may be originating in the shock wave as a result of variations in speed and density of plasma blobs being ejected by the star," Giblin said. "Or it may be we are seeing after some bursts the onset of the afterglow at gamma-ray energies. This would happen if the plasma blobs slammed into the interstellar medium at very high speeds."

Based on the spectral properties of the post-quiescent emission light curves, the team has found evidence that, in at least some cases, the post-quiescent emissions look suspiciously like those produced during the afterglow phase.

"We are fortunate to have seen a few cases in the BATSE data where the afterglow appears to overlap the GRB in time," Freismuth said. "Therefore, it is possible that in some cases we may see the afterglow begin some tens or hundreds of seconds after the burst, thus producing the observed quiescent period."

Freismuth, who will be attending graduate school at the University of Iowa in the fall physics, said many more observations are needed in order to build up a set of statistics.

"We need simultaneous GRB detection at as many wavelengths as possible, and we need high time-resolution data at as many wavelengths as possible in order to be able to answer these questions."

Science Contact:
Dr. Tim Giblin
Physics & Astronomy Department
College of Charleston, Charleston, S.C.
E-mail: giblint@cofc.edu
Phone: 843-953-5061