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Fading embers hold clues to puzzle of gamma-ray bursts

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Fading embers hold clues
to puzzle of gamma-ray bursts

Afterglow points to long-sought shock-wave features

September 10, 1999: Sometimes the big fireworks aren't the whole show. Watching the embers fade away can help you understand what was hidden by a blinding flash of light - or gamma rays. In a new set of observations, astrophysicists have discovered that an afterglow can start in gamma rays during a gamma-ray burst, thus suggesting that more than one activity is causing what appears to be a chaotic explosion.

Right: Hubble Space Telescope visible light image of a gamma-ray burst taken a few days after it appeared on Jan. 23, 1999. Studying the gamma-ray signatures of fading fireballs in the few minutes immediately after a burst is yielding clues about what happens during the explosion. Credit: Space Telescope Science Institute

Gamma-ray bursts are one of the most mysterious events in modern astrophysics. They occur about once a day at completely random points in the heavens. Their cause remains unknown, although recent observations have allowed scientists to start narrowing the possibilities.

"Figuring out gamma-ray bursts is a lot like doing a jigsaw puzzle, except the pieces are all face-down," remarked Tim Giblin, a graduate student at the University of Alabama in Huntsville. "Every once in awhile, we see something in the data - like this afterglow - that allows us to turn over one or two pieces. It helps us to make measured progress towards a better understanding of these events. Nature doesn't give up all of her secrets so easily."

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Giblin has led a research team that has found one of those secrets, the first evidence for a prompt, high-energy afterglow component from a gamma-ray burst. Their results are presented in a paper accepted for publication in the Astrophysical Journal Letters. The gamma-ray afterglow observed in GRB980923 (observed on Sept. 23, 1998, hence the name) started 40 seconds after the burst began, and had a characteristically different signature than the burst emission itself, indicating that it was produced by a different process than the other gamma-ray emission in the burst. Thus the observation supports the idea that there can be multiple energy emission processes and mechanisms in action during the actual burst itself.

Left: Two graphs depict the dying afterglow of GRB 980923. The upper graph shows the burst profile from when it triggered the BATSE detectors to more than a minute after onset. The lower graph spans from almost a minute after the burst started to 5 minutes after burst onset. The vertical scale of the lower graph is exaggerated to show the faint glow against the background noise. Links to 800x850-pixel, 60K JPG. Credit: NASA/Marshall Space Flight Center and the University of Alabama in Huntsville.

In March 1997, scientists made the first observations of an afterglow to a gamma-ray burst. The faint x-ray glow observed in GRB970228 by the Italian-Dutch BeppoSax satellite led a team headed by Jan van Paradijs of the University of Alabama in Huntsville and the Universiy of Amsterdam to discover an optical counterpart.

The optical glow was observed by Hubble Space Telescope and many other telescopes, and persisted for months after the burst itself had come and gone in gamma-rays. This discovery allowed the first direct redshift/distance measurements for gamma-ray bursts, and sealed the answer to a decade-long question of "How far away are the bursts?" The answer is typically several billion light years, putting bursts near the edge of the observable universe.

Since March 1997, scientists have been able to catch several more optical and x-ray counterparts to gamma-ray bursts, including one caught "in the act" on Jan. 23, 1999. Observing these counterparts, and knowing the distance to bursts have allowed scientists to begin to explore the question "What could cause something like this?"

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But nagging questions still persist. How was the afterglow made? When does it start in the burst process? What energies are produced in the afterglow? What is its relation to the burst itself?

As is usual in science, the answer to one question - the distance scale - prompted many more detailed questions about the nature of the events. The afterglow from GRB980923 is providing some insight.

GRB 980923 was observed by the Burst and Transient Source Experiment (BATSE) aboard NASA's Compton Gamma Ray Observatory shortly after 3 p.m. EDT on Sept. 23, 1998. It lasted about 400 seconds before it faded below BATSE's ability to detect, and it was the 12th brightest of more than 2,000 bursts seen by BATSE since its mission started in April 1991. Aside from being very bright, the burst also displayed a very unusual brightness profile.

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Scientists catch another gamma-ray burster in visible light - May 18, 1999. Several telescopes observe optical counterpart
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GOTCHA! The Big One That Didn't Get Away - Jan. 27, 1999. For the first time, images of visible light from a gamma ray explosion is captured by a robotic telescope.
Gamma-ray Bursters cross the 'Line of Death' - Oct. 13, 1998. A study of gamma ray burst spectra shows one more thing that these mysterious, cosmological gamma ray bursts are not.
Blast from the past: the latest clue in solving the gamma-ray burst mystery (May 6, 1998).
Gamma-ray burst identification earns top prize (Jan. 12, 1998)
Twinkle, twinkle, massive fireball - reports from the 4th Huntsville Gamma-ray Burst Symposium (Sept. 17, 1997)
Discovery may be "smoking gun" in gamma-ray mystery (March 31, 1997).

The first 40 seconds of the burst emission are highly variable, with the burst source rapidly changing its intensity. Then the variability stops, and the intensity smoothly diminishes over time according to a simple mathematical curve.

"It's as if the variable part of the burst simply switched itself off, leaving the smoothly-decaying gamma-ray afterglow or tail," said Giblin. It's like watching an ember die after it falls out of the fire.

Furthermore, the spectrum of this smoothly-decaying emission - how the gamma-rays are distributed in energy - is markedly different than the spectrum in the variable phase. The emission from the tail looks similar to the emission expected from hot, charged particles cooling and slowing down as they spiral in the presence of a strong magnetic field, what scientists call a 'cooling synchrotron spectrum.' In this process, a particle's energy is converted to the gamma-rays that are detected by the spacecraft.

"Because the spectrum is consistent with this kind of cooling in the burst environment doesn't mean that this has to be the explanation," noted Giblin. "but it sure is suggestive. And if this is what's going on, we've begun to use the tail emission as a probe of the physical conditions in the burst environment."

 

 

Left: A color-color diagram, a technique that Giblin recently applied to gamma-ray bursts, shows the burst itself falling into a distinctive crescent pattern that is typical of gamma-ray bursts, while the tail has a uniquely different pattern. Right: Evolution of the spectral index with time shows the burst (bottom left) followed by the afterglow (upper) on a different course, a characteristic of colling synchrotron radiation. Links to 647x616-pixel, 87K JPG (left) and 659x613-pixel, 72KB JPG (right). Credit: NASA/Marshall Space Flight Center and the University of Alabama in Huntsville.

The analogy to fireworks is a limited one since what we see on the Fourth of July is chemical combustion. Much of what is emitted by a gamma-ray burst may be the result of shock waves as hot, dense materials slam into each other at high speed.

Theorists have hypothesized since as early as 1994 that the emission we observe from gamma-ray bursts might be produced by shocks within a rapidly expanding fireball and by shocks created by the interaction of the rapidly expanding medium with surrounding material that was relatively cold and undisturbed until it was hit. The prompt gamma-ray emission that BATSE observes as a burst might come from the internal shocks, while the decaying counterparts may be caused by the secondary interaction between the blast material and the environment surrounding the burst site.

Such "shock heating" and emission are commonly observed in astrophysics, for example as recently as in the first-light image from Chandra X-ray Telescope of the supernova remnant Cas-A.

Left: The "first light" image of Casseopeia A by the Chandra X-ray Observatory shows the fading shock wave from a 320-year-old supernova. Cas A was almost feeble compare to a gamma-ray burst, but produced similar shock-wave heating effects. Credit: NASA and Chandra Science Center.

One can envision this by thinking about a snowplow. The snow plow (an expanding blob of material) crashing into the snow (stationary material) excites the snow so it is energized and emits radiation such as visible light, x-rays, or gamma-rays. In the case of a gamma-ray burst, the blob of material is moving at speeds close to the speed of light, causing exceptionally energetic emissions.

What's more, the blob of material is immensely large and powerful. In order to appear so bright from such a great distance, a gamma-ray burst must release as much energy in a few seconds as our Sun will produce in its entire 10-billion-year life.

With bits like the prompt afterglow now turned face up, Giblin and other astrophysicists hope they can piece together more of the gamma-ray burst puzzle.

Giblin's work, along with the work of nearly every other scientist in the field of gamma-ray bursts will take center-stage at the 5th Huntsville Gamma Ray Burst Symposium, to be held Oct.19-22 in Huntsville, Ala.

Held every two years, the Huntsville Symposium is among the premier international scientific meetings focused on the phenomena of gamma-ray bursts. At the last meeting in 1997, the first-ever Hubble images of a burst optical counterpart were revealed, which helped fix the billion-light-year distances to gamma-ray bursts.

Abstract

Evidence for an Early High-Energy Afterglow Observed with BATSE from GRB980923. T. W. Giblin (UAH/MSFC), J. van Paradijs (UAH), C. Kouveliotou (USRA/MSFC), V. Connaughton (NASA/MSFC), R. A. M. J. Wijers (SUNY), M. S. Briggs (UAH/MSFC), R. D. Preece (UAH/MSFC), G. J. Fishman (NASA/MSFC). Accepted for publication in Astrophysical Journal Letters (Oct. 10, 1999, V. 524).

In this Letter, we present the first evidence in the BATSE data for a prompt high-energy (25-300 keV) afterglow component from a gamma-ray burst (GRB), GRB980923. The event consists of rapid variabilty lasting ~40 s followed by a smooth power law emission tail lasting ~400 s. An abrupt change in spectral shape is found when the tail becomes noticeable. Our analysis reveals that the spectral evolution in the tail of the burst mimics that of a cooling synchrotron spectrum, similar to the spectral evolution of the low-energy afterglows for GRBs. This evidence for a separate emission component is consistent with the internal-external shock scenario in the relativistic fireball picture. In particular, it illustrates that the external shocks can be generated during the gamma-ray emission phase, as in the case of GRB990123.

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