The Line of Death, Ignoring Theories, and Other Stops in the Hunt for Bursts
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and Other Stops in the Hunt for Bursts
September 18, 1997
The Line of Death
With all the data that have been collected about gamma ray bursts, understanding of the bursts themselves remains elusive. Bursts are just that, sudden flashes of energy that carry little apparent information about the source. Still, scientists know that something is in there. Wednesday, several scientists here at the 4th Huntsville Gamma Burst Symposium described their efforts to extract clues from flashes - including signs that some have crossed "the line of death."
"It's a limit that's predicted by the synchrotron
shock model that, in principle, cannot be violated," said Dr. Rob Preece
of the University of Alabama in Huntsville.
Synchrotron radiation is emitted by electrons as they spiral along magnetic field lines. The most common form on Earth comes from synchrotron particle accelerators where accelerated electrons emit radiation at a tangent to their circular paths. This effect is also observed in neutron stars and supernovae.
In the shock wave of material blasting outward from whatever causes gamma ray bursts, electrons will gyrate around the magnetic field lines. To anyone moving with that cloud of material, their behavior and emissions will look quite normal. Because the matter is moving almost at the speed of light, observers on the outside (like satellites orbiting the Earth) will see the radio waves are blue-shifted into gamma rays that must be emitted straight ahead rather than in all directions.
Preece said that the energies carried by the electrons can be plotted on something like a bell curve. Most are in the middle, while some are lower and some are higher in energy.
The "line of death" - which Preece admitted is a fanciful title - is the slope of the curve. In standard models, the slope cannot exceed a certain value. In other words the curve cannot rise and fall too sharply, which would mean that the energy of the burst is clumped in a very narrow band.
Yet about a third of the bursts that Preece has studied do just that.
"I am seeing steeper slopes than you would expect from this model," Preece said. "We should think of another model for those emissions." Preece said it is possible that the second model could describe just the odd third that falls outside the "line of death," or that the second model could explain bursts that obey and exceed it.
While much excitement has been generated by the discoveries of optical and X-ray counterparts to gamma ray bursts this year, no one knows which bursts will be the best candidates for counterparts in other wavelengths.
Dr. David Palmer of the Universities Space Research Association described how he and other scientists studied data on the February 28 (GRB 970228) burst as seen by the Transient Gamma Ray Spectrometer (TGRS) aboard the Wind geoscience satellite. The TGRS can detect greater detail in burst spectra than the Burst and Transient Source Experiment can do, for example.
Looking at the spectrum, Palmer said the first peak in the data did not follow the fireball model, but later peaks in the data might. "They can change their behavior," he said, "so the model is not invalidated."
"The gamma ray spectrum does not tell if [a burst] is worthwhile to look for a counterpart," Palmer said. "So just ignore theory and go out looking."
Other Stops in the Search
Although previous experiments have reported seeing features in the spectrum of gamma ray bursts that look like absorption lines, a search through nearly 2,000 bursts from BATSE has yet to reveal such concrete examples of line-features in BATSE spectra. Still, scientists hope to find some spectral lines that might form a simple link between gamma ray bursts and known physical phenomena.
Dr. Michael S. Briggs of the University of Alabama in Huntsville used a computer to analyze hundreds of bursts in hopes of finding distinctive lines that would match known phenomenon, just as most stars are known by spectral lines, narrowly defined sets of colors unique to atoms at specific energy levels.
Because the data have some noise and uncertainties, scientists have to be meticulous in their effort to find features in the spectra, and to come to an explanation of the feature that other scientists will accept as reasonable. Briggs said that so far that work has not produced any spectral lines.
"The data appear favorable so far," said Michael S. Briggs of the University of Alabama in Huntsville, "but we may not be able to distinguish spectral lines from other features like sharp low-energy breaks."
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