Douglas Isbell
Headquarters, Washington, DC April 1, 1997
(Phone: 202/358-1547)
Ray Villard
Space Telescope Science Institute, Baltimore, MD (Phone: 410/338-4514)
Dave Drachlis
Marshall Space Flight Center, Huntsville, AL (Phone: 205/544-0034)
RELEASE: 97-63
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 universe.
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 suspected
GRB.
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 universe.
"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:
http://wwwssl.msfc.nasa.gov/newhome/headlines/ast31mar97_1.htm