Scientists uncover chronology of gamma-ray burst
May 22 2008
Stellar explosion sets new records
For 40 seconds on March 13, a pinpoint of light
suddenly appeared in the constellation Bootes, shining more
brightly then 10 million galaxies. The radiation accompanied a
powerful stellar explosion halfway across the universe and set
two records: It was the most distant naked-eye object ever
recorded from Earth and the most luminous.
Astronomers have studied in detail hundreds of these stellar
explosions, known as gamma-ray bursts, and most lie even farther
from Earth. But researchers had particular good fortune in
observing the March 13 burst, dubbed GRB 080319B, because they
were already tracking another burst that had erupted less than
30 minutes earlier in a nearby region of the sky. So they were
able to view the new outburst in record time after NASA’s Swift
satellite observed its gamma rays.
telescope captured this infrared image of the afterglow of GRB
080319B just four minutes after the eruption was observed on
Perley, J. Bloom et al., PAIRITEL
“The combination of these unique optical data
with simultaneous gamma-ray observations provides powerful
diagnostics of the detailed physics of this explosion within
seconds of its formation,” Judith L. Racusin of Pennsylvania
State University in University Park and her colleagues note in
an article posted online
Three different components contribute to the
visible-light emission, from the first seconds after the burst
to hours and days afterward, says coauthor Enrico Ramirez-Ruiz
of the University of California, Santa
Cruz. Researchers have never before seen
all three components in a single burst, he says.
“It confirms a lot of the standard model, but
more importantly, it also shows the complexity of the
processes,” Ramirez-Ruiz adds.
GAMMA BURSTThe Hubble
Space Telescope captured the fading visible-light afterglow
(arrow) of the gamma-ray burst dubbed GRB 080319B on April 7.
Although the burst originated halfway across the universe, its
visible-light emission was for the first 40 seconds bright
enough to be seen with the naked eye, providing a cornucopia of
data for telescopes that quickly pointed to the burst. N.
Tanvir, A. Fruchter, A. Levan, E. Rol, NASA, ESA
According to that model, a burst occurs when a
jet of material breeches the surface of a massive star and zooms
far beyond it. Collisions between blobs of material within the
jet produce the gamma rays. The data collected on GRB 080319B
strongly suggest that the earliest visible-light photons from
the burst, produced during the first 50 seconds, are also
generated within the jet but by a different process.
The second component, which endures from 50
seconds post-explosion to 800 seconds, shows characteristics of
what astronomers call a reverse shock. This occurs when the
high-speed jet encounters surrounding interstellar material;
that material sends a shock into the slowed jet. The final
component, after 800 seconds has elapsed, is the familiar
visible-light afterglow of a gamma-ray burst, generated by the
forward shock created when the jet slams into gas and dust in
the surrounding space.
The initial avalanche of visible-light radiation,
known as prompt optical emission, is most likely produced by
fast electrons gyrating around strong magnetic fields within the
jet, the researchers suggest. In contrast, the gamma rays, which
have much higher energies, may be generated by collisions
between some of the visible-light photons and the fast
electrons, which kick these photons up to gamma-ray energies.
In order for astronomers to see the initial
visible-light emission, the jet from the exploding star must
have been traveling at 0.999998 the speed of light, Ramirez-Ruiz
calculates. Such a jet quickly leaves the exploding star far
behind and enters a low-density region where visible-light
photons can easily escape the jet rather than undergo repeated
collisions with the jet’s electrons. A high-speed jet is also
narrowly focused, and the researches suggest that this burst is
a rare example of a highly concentrated outburst that by luck
happened to be aimed directly toward Earth.
“We deduce that we happened to view this monster
down the barrel of this very narrow and energetic jet,” the
researchers note in their article.
The brilliance of the visible-light emission
gives researchers new hope that even more distant bursts could
be used to probe some of the earliest epochs of the universe, a
time before the first stars were born, says Josh Bloom of the
University of California, Berkeley. He and his colleagues posted
their observations of the burst online.
The brightness of the burst "really crowns these
as kings for the next few decades" for probing the early
universe, Bloom adds.
These new studies, comments theorist Andrew
MacFadyen of New
“are telling us what the jet structure is, and that’s very
useful for constraining models of the explosion.”
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