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 discovered on February 28 by the Gamma-Ray Burst Monitor aboard the Italian-Dutch BeppoSAX satellite. The burst was also within the field of view of one of the SAX Wide Field Cameras. Followup observations were conducted by several other space-based astronomical observatories. 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, 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 may also 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 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."
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 lead 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.
NASA Headquarters, Washington, DC
Space Telescope Science Institute, Baltimore, MD
Marshall Space Flight Center, Huntsville, AL