Date: Tue, 1 Apr 1997 15:05:57 -0500 (EST)
Subject: Hubble Tracks the Fading Optical Counterpart of
a Gamma-Ray Burst
Headquarters, Washington, DC April 1, 1997
Space Telescope Science Institute, Baltimore, MD
Marshall Space Flight Center, Huntsville, AL
HUBBLE TRACKS THE FADING OPTICAL COUNTERPART OF A GAMMA-RAY BURST
NASA's refurbished Hubble Space Telescope has made an
important contribution toward solving one of astronomy's
greatest enigmas by allowing astronomers to continue watching
the fading visible-light counterpart of a gamma-ray burst
(GRB), one of the most energetic and mysterious events in the
The so-called optical counterpart is presumably a cooling
fireball from the catastrophic event that triggered the massive
burst of invisible gamma rays -- the highest-energy radiation
in the universe. This event may have unleashed as much energy
in a few seconds as the Sun does in ten billion years.
The burst was detected by several space-based, high-energy
astrophysics observatories on Feb. 28. The visible GRB
counterpart, the first ever detected, was then discovered in a
pair of ground-based telescopic images of the region where the
burst occurred. Taken a week apart, the later picture showed
that an object that could be seen in the first image had
disappeared in the field, suggesting it was the decaying
fireball from the event. A week after that discovery,
astronomers at the New Technology Telescope and the Keck
telescope identified an extended source at the location of the
Hubble's high resolution and sensitivity were brought in to
hunt down the rapidly dimming fireball -- plunging from 21st to
below 23rd magnitude in eight days -- after it had grown so
faint that it could not be resolved by ground-based telescopes
by March 13. On March 26, Hubble allowed astronomers to
reacquire the lost remnant and continue following the behavior
of the fading source. The Hubble observation clearly shows that
the visible GRB source has two components: a point-like object
and an extended feature.
This observation demonstrates Hubble's unique capability
for monitoring the aftermath of gamma-ray bursts, long after
they have faded from the view of Earth-based telescopes. And
there will be no shortage of targets: once a day, a gamma-ray
burst occurs somewhere in the universe.
"Now we know that, at least in some cases, we can follow
the aftermath of GRBs for several weeks, using a coordinated
effort between ground-based telescopes, Hubble and other
spacecraft," said Kailash Sahu, leader of a team of scientists
at The Space Telescope Science Institute, Baltimore, MD, who
used Hubble to resolve the fading GRB remnant. "The fact that
we were able to resolve the extended feature and measure its
brightness separately provides us with an unprecedented
opportunity to solve the mystery of these enigmatic objects,"
added team member Mario Livio. A scientific paper on the
team's findings has been submitted to the journal Nature.
A much anticipated second observation with Hubble,
scheduled for April 7, should help clarify the nature of the
extended feature and place meaningful constraints on theories
about the mechanism behind these extraordinary detonations.
Hubble also may provide an answer to the question of whether
GRBs originate in our Milky Way galaxy, or come from far more
energetic events scattered at cosmological distances across the
far reaches of the universe.
If Hubble's follow-up observations show the extended object
adjoining the GRB has not faded, it is probably related to a
host galaxy. This would confirm the notion that GRBs are
cosmological in origin, far removed from Earth in space and
time. Any measurable fading would present the startling
alternative that the extended object is a cloud of gas
illuminated by a GRB source within our own Milky Way.
"This opens up a whole new era in gamma-ray burst research.
We now know that it is possible to see the fading optical
emission by rapid follow-up observations with powerful
telescopes. With several more of these, we should be able to
narrow the models of what could be causing these gigantic
outbursts," said Gerald Fishman of the Marshall Space Flight
Center, Huntsville, AL, a principal investigator on NASA's
Compton Gamma Ray Observatory.
Hubble's contribution to solving the GRB mystery is the
latest in a series of extraordinary ground- and spacecraft-
based observations, across the electromagnetic spectrum, that
has carried astronomers on a fast-paced detective hunt for the
mechanism powering the most energetic and elusive events in the
"Hubble's unmatched ability to see the faintest traces of
the universe is helping solve one of astronomy's most
perplexing unsolved problems," said Robert Williams, director
of The Space Telescope Science Institute, who provided some of
his discretionary time for the observation. "This has been a
textbook example of the importance of coordinated telescope
observations," he said.
Although more than 2,000 separate GRBs have been catalogued
as they randomly occur across the sky, the outbursts have
perplexed astronomers for more than two decades. This is
because the source of a GRB had never been seen until a team of
astronomers led by Jan van Paradijs of the University of
Alabama in Huntsville, and the University of Amsterdam, found a
diffuse object at the location of a gamma ray burst using a
4.2-meter telescope at La Palma Observatory in the Canary Islands.
The burst had been detected by the Gamma-Ray Burst Monitor
aboard the Italian-Dutch BeppoSAX satellite.
Within eight hours after the burst was detected, the BeppoSAX
spacecraft was maneuvered to point its more precise X-ray imaging
instruments at the location. Hubble observing time was then set aside
to allow astronomers to take images with Hubble's Wide Field Planetary
Camera 2 which clearly show a point-like source, at 25.7 magnitude,
and the extended object.
The raw data from the Hubble image has been posted to the
Internet at the following URL:
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