GAMMA RAY BURSTER
September 17, 1997
After three months of observations with the NSF's Very Large Array (VLA) and Very Long Baseline Array (VLBA) radio telescopes, scientists have learned that the "fireball" of debris expands very closely to the speed of light. They estimate its current size to be about one-tenth of a light-year, or 170 times the distance between the sun and Pluto. The scientists reported their findings about the May 8 gamma ray burst in the September 18 issue of Nature.
"For 30 years, we've known almost nothing about these mysterious explosions in the sky. Our observations show that these events release truly incredible amounts of energy," said Dale Frail, of the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico.
"It was only a few months ago that our observations showed that such bursting objects are located far beyond our own galaxy. However, astronomers had little evidence for how this cosmological juggernaut actually works. The radio observations have revealed a size of the fireball, unobtainable by any other technique, thereby enabling astronomers to learn about inner workings of such objects," said Shri Kulkarni, professor of astronomy at the California Institute of Technology (Caltech).
In addition to Frail and Kulkarni, the astronomers are Greg Taylor of NRAO and Italians Luciano Nicastro and Marco Feroci of the BeppoSAX Gamma Ray Burst Team. BeppoSAX is an Italian-Dutch satellite, launched late last year, that detects gamma ray bursts and provides precise sky positions to allow ground-based telescopes to observe them.
Cosmic gamma ray bursts, occurring about once per day, have been observed for some 30 years. However, until this year, very little was known about them. Even their distances from Earth were the subject of great debate among astronomers.
This year, the rapid and improved positions provided by the BeppoSAX satellite have allowed astronomers to look for the bursts, or their optical/ radio counterparts, quickly with other instruments on the ground and in space. As a result, two bursters, including the one on May 8, have been detected optically. The VLA found radio emission from the May 8 burster on May 13, and the VLA and VLBA have been monitoring the object regularly since then.
"Shri (Kulkarni) and I searched for radio counterparts to gamma ray bursters for four years, but only when BeppoSAX provided us with accurate positions were we able to identify one," said Frail.
Gamma ray bursts, the causes of which still are unknown, are short-lived phenomena. The May 8 burst lasted only 15 seconds. However, X-ray, optical and radio emission continues in an "afterglow." It is that afterglow that is revealing details of the fireball created by the initial explosion. That initial explosion released, in 15 seconds, more energy than the sun will release in its entire, 10-billion-year lifetime.
Optical studies of the May 8 object using the 10-meter W.M. Keck Telescope in Hawaii indicated that it is very distant, at least seven billion light-years away.
While the burst's afterglow showed a steady decline in brightness at both optical and X-ray wavelengths, that was not the case at radio wavelengths. The radio emission rose and fell several times, and the relative intensity at different radio wavelengths also changed.
While puzzling over this behavior, Frail and Kulkarni learned that Jeremy Goodman of Princeton University already was predicting this effect. Irregularities in the extremely tenuous material between the stars could cause fluctuations in the radio intensity as seen from Earth. "As we collected more data, it became clear to us that indeed the radio object 'twinkled,'" Kulkarni said.
"Many amateur astronomers know that stars twinkle, but planets don't," Frail explained. "This is because the stars are so distant they appear as mere points, and irregularities in the atmosphere cause them to twinkle. Planets, on the other hand, are close enough that they are not mere points, and their larger apparent size squelches the twinkling. Over three months, we saw the radio twinkling of the gamma ray burster slowly cease. From this we could calculate both its apparent size and the rate of its expansion."
"It is interesting to note that the irregularities in the thin material between the stars enable us to obtain information unobtainable any other way. Without this effect, we would need a radio telescope array the size of the sun to measure this object," said Greg Taylor, who led the effort to observe the object with the VLBA.
Using the ultra-sharp "vision" of the continent-wide VLBA, the astronomers have pinpointed the burster's position in the sky with extremely high precision -- less than a thousandth of a second of arc. In the three months of their observations, they have noted that this position has not changed measurably, thus strengthening the case for the object's being at a great distance.
The VLA and VLBA are instruments of the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
NASA Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD
Space Telescope Science Institute, Baltimore, MD
September 16, 1997
The most recent finding from observations with Hubble's Space Telescope Imaging Spectrograph (STIS) made on Sept. 5 - nearly six months after the blast - is being reported today at the Fourth Huntsville Symposium on Gamma Ray Bursts, at the Hilton Hotel in Huntsville, Al.
"Hubble is the only telescope capable of continuing to watch the aftermath of this explosion, because it has faded to 1/500th its brightness when first discovered by ground based telescopes last March," says Andrew Fruchter of the Space Telescope Science Institute in Baltimore, Md. "These observations provide an unprecedented opportunity to better understand the catastrophe behind such incredible outbursts."
Hubble's key findings are:
1. The continued visibility of the burst, and the rate of its decline over time, support theories that the light comes from a gamma-ray burst in a "relativistic" fireball (expanding near the speed of light) located at extragalactic distances. A burst in our galaxy, at the observed brightness, would have been slowed by the interstellar medium within the first few weeks, and faded from sight by now.
2. The observations contradict earlier claims, by some astronomers that the gamma-ray burst is moving against the sky background (this offset is called proper motion). Had proper motion been detected, the gamma-ray burst would have had to be no further away than about 30,000 light-years, or about the distance to the center of the galaxy.
3. The fuzzy companion object the fireball is embedded in - as first confirmed by Hubble in March 26 observations -- has not noticeably faded. This means it is not a relatively nearby nebula produced by the explosion, but in all likelihood a host galaxy.
4. Since the burst did not occur at the center of the host galaxy, but near its edge, the gamma-ray burst phenomenon is not related to activity in the nucleus of a galaxy. The Hubble observations support the "fireball" model for a gamma-ray burst.
"These observations are consistent with colliding neutron stars creating the fireball, but do not require it. The cause of that fireball is still not determined. Though colliding neutron stars is one theoretical means of producing such a fireball it is not the only one," says Fruchter.
Hubble observations over the past six months show the fireball is fading at a constant rate, as predicted by theory. Eventually, gas plowed in front of the stellar tidal wave should build up enough resistance to bring the fireball to a halt like snow piling up in front of a plow - and it should blink out. But the fact that hasn't happened yet offers more clues to solving the gamma-ray burst mystery.
If the burst happened nearby, the resulting fireball should have had only enough energy to propel it into space for a month or so before "hitting the wall" of accumulated gas and dying out. The fact that this fireball has expanded to gargantuan size, sweeping out a bubble of space one light-year across, means the explosion was truly titanic and, to match the observed brightness, must have happened at the vast distances of galaxies.
When Hubble first acquired the fireball, on March 27 (several weeks after the initial discovery) it was at 26th magnitude. The magnitude scale is used to measure the brightness of objects in space. The lower the magnitude, the brighter the object. The unaided eye can detect objects of the 6th magnitude.
By the Sept. 5 observation, it had faded to 1/5th that brightness to 27.7 magnitude (approximately 1/500,000th) the brightness of the faintest star). The suspected host galaxy has remained at approximately 25th magnitude.
Only Hubble has the resolution and contrast capability to still distinguish the fading fireball from the now brighter host galaxy. The researchers hope for follow-up observations to continue keeping track of the burst's optical counterpart until it fades away.
The research team: Andrew Fruchter (STScI), Elena Pian (ITSR), Steve Thorsett (Princeton), Marco Tavani (Columbia), Mario Livio (STScI), Kailash Sahu (STScI), Filippo Frontera (ITSR), Larry Petro (STScI) and Duccio Macchetto (STScI).
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).
GIF and JPEG images, captions and press release text are available via the World Wide Web at
http://oposite.stsci.edu/pubinfo/PR/97/30.html
and via links in
http://oposite.stsci.edu/pubinfo/Latest.html or
http://oposite.stsci.edu/pubinfo/Pictures.html.
PHOTO CAPTION:
Though the visible component has faded to 1/500th its brightness (27.7 magnitude) from the time it was first discovered by ground- based telescopes last March (the actual gamma-ray burst took place on February 28), Hubble continues to clearly see the fireball and discriminated a surrounding nebulosity (at 25th magnitude) which is considered a host galaxy.
The continued visibility of the burst, and the rate of its fading, support theories that the light from a gamma-ray burst is an expanding relativistic (moving near the speed of light) fireball, possibly produced by the collision of two dense objects, such as an orbiting pair of neutron stars.
If the burst happened nearby, within our own galaxy, the resulting fireball should have had only enough energy to propel it into space for a month. The fact that this fireball is still visible after six months means the explosion was truly titanic and, to match the observed brightness, must have happened at the vast distances of galaxies. The energy released in a burst, which can last from a fraction of a second to a few hundred seconds, is equal to all of the Sun's energy generated over its 10 billion year lifetime.
The false-color image was taken Sept. 5, 1997 with the Space Telescope Imaging Spectrograph.
Credit: Andrew Fruchter (STScI), NASA
NASA Headquarters, Washington, DC
Space Telescope Science Institute, Baltimore, MD
Marshall Space Flight Center, Huntsville, AL
April 1, 1997
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 February 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, 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.
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.
Agenzia Spaziale Italiana
March 12, 1997
The scientific community is divided over their intepretation. Some believe that these phenomena originate inside our own galaxy, in explosions on the surface of neutron stars (extremely dense objects formed in the last stage of the life of massive stars), and others believe instead that the origin is to be found at very large distances in the universe, for instance in galaxies, where two neutron stars may collide or large explosions in their very central regions may occur.
BeppoSAX for the first time has detected X-ray emission from the source producing a Gamma-Ray Burst, localizing the source in the sky with an unprecedented precision provoking enormous interest in the international scientific community. In consequence the most important worldwide observatories, during the last few days, have been observing the zone from which the gamma-rays and X-rays have been observed in the hope of discovering the type of astronomical object that produced the burst and thus resolve the mistery of Gamma-Ray Bursts.
The BeppoSAX satellite for X-ray astronomy was produced by ASI in collaboration with the Dutch Agency for Space Programs NIVR, and was successfully launched on 30th april 1996.
The gamma ray burst was discovered simultaneously in the data from the Gamma Ray Burst Monitor and Wide Field Cameras onboard by the team of italian/dutch scientists at 05:00 am on Friday 28th february who continually monitor the satellite data at the Scientific Operations Center in Nuova Telespazio, Rome.
On the basis of experience gained in january (when a similar gamma-ray burst was observed in the constellation of Serpens by BeppoSAX, and studied in detail 16 hours later, in comparison with the previous record for a X-ray observation of about 18 days) the Italian scientists responsible for the gamma-ray burst detector onboard the satellite (E. Costa, CNR, and F. Frontera, University of Ferrara), in conjunction with the Mission Scientist (L. Piro, CNR) and the Mission Director (R. C. Butler, ASI), were able to reschedule the satellite observations and point the BeppoSAX narrow field X-ray telescopes in only 8 hours at the gamma-ray burst source. In consequence a X-ray source never before seen was discovered and localized with an accuracy of one hundreth of a degree. The source is actually in the constellation of Orion.
Useful links:
Gamma Ray Bursts
The Paczynski-Lamb Debate in 1995: The Distance Scale to Gamma Ray Bursts
BeppoSax
Gamma Ray Burster
February 28, 1997
UCLA graduate student Samuel B. Larson, working with astronomy professors Ian S. McLean and Eric E. Becklin, reported at the American Astronomical Society meeting in Toronto that the regions of the sky where bright gamma-ray bursts have occurred contain twice as many bright galaxies than one would expect to find randomly.
"Until recently, gamma-ray bursts were thought to originate within our own Milky Way galaxy," Larson said. "Our results suggest that other galaxies may be responsible for the bursts."
The UCLA study focuses on a celestial event that is among the most perplexing mysteries in astronomy. Gamma-ray bursts are explosions that last several seconds and release energy of almost unimaginable intensity, possibly more than a supernova. The bursts occur in seemingly arbitrary directions in the universe, with no evidence of their origin. And, in spite of their tremendous release of energy, gamma-ray bursts leave no measurable aftereffects from their explosion.
Gamma-ray bursts are not visible, and because gamma rays are absorbed by the Earth's atmosphere, the bursts cannot be detected from the Earth's surface. The bursts were first identified by military satellites that were launched in the 1960s to monitor illegal nuclear testing. Today, the bursts are detected primarily by the Compton Gamma-Ray Observatory, a satellite launched from the Space Shuttle in April 1991.
The observatory contains several research instruments, including BATSE (the Burst and Transient Source Experiment). BATSE identifies an average of one gamma-ray burst per day, but because the instrument's coverage of the sky is incomplete, astronomers estimate that two or more bursts each day go undetected. In spite of the size and frequency of the gamma-ray bursts, and the ability of astronomers to record them daily, less is known about gamma-ray bursts than any other significant astronomical phenomenon.
"Astronomical theorists are not short of ideas to describe what might cause gamma-ray bursts -- in fact, the problem is quite the opposite: there are few observational clues to constrain researchers," Larson said. "The very nature of gamma-ray bursts defy traditional methods of study; they typically last only a few seconds, their locations cannot be predicted in advance, and most of their light comes from the very high energy region of the spectrum, making them difficult to detect."
Also intriguing is the lack of knowledge about the distance of the bursts from earth. Random gamma-ray bursts may occur on the edge of our own solar system or the far reaches of the universe, a range that spans billions of light years.
"Astronomers lack a smoking gun -- after a burst, no telltale evidence remains that it has occurred," Larson said. "From evidence gathered by deep-space probes, the only thing astronomers understand about the location of the bursts is that they are not coming from within our solar system."
Further complicating the study of gamma-ray bursts is that the burst locations are not known precisely, and dozens of objects can be found at a single gamma-ray burst location. "It's a classic needle in a haystack problem, with the added twists that we don't know what the needle looks like or where the haystack is located," Larson said.
In the effort to understand gamma-ray bursts, more than 100 different explanations have been proposed. They range from colliding comets to superconducting cosmic strings, but no theory has yet proved conclusive. Until the UCLA study, most astronomers looked for unusual objects at gamma-ray burst locations. No related objects have been identified, and researchers also reported a lack of galaxies near gamma-ray bursts, increasing suspicions that the bursts were coming from within our own Milky Way Galaxy. However, to many astronomers, the random distribution of gamma-ray bursts, combined with a scarcity of faint bursts, suggested that they originate from outside the Milky Way.
UCLA research proceeds using the premise that the bursts come from outside the Milky Way galaxy. If gamma-ray bursts come from outside of our galaxy, another galaxy should be present that contains the exploding object.
The UCLA team embarked on a three-year investigation that focuses on small regions in the heavens where bursts have occurred previously. Using the 3-meter telescope at the University of California Observatory near San Jose, Calif., and the 1.5-meter telescope at the Cerro Tololo Inter-American Observatory near La Serena, Chile, the UCLA team has recorded the most sensitive infrared images of gamma-ray burst locations thus far -- revealing objects up to 100 times fainter than those used in previous studies.
"We used infrared light because galaxies are brighter and less obscured by the dust in our own galaxy than when viewed in visible light," said McLean, who developed the infrared camera at UCLA.
The infrared images reveal significantly more bright galaxies near gamma-ray burst locations than is considered normal. "The presence of these galaxies eliminates one of the most critical arguments against the notion that gamma-ray bursts come from outside of our galaxy," Larson said. "Although it may be coincidental, the profusion of galaxies suggests they may have been responsible for the bursts. These galaxies are close enough to imply modest bursts of energy, yet they are far enough away to produce the random pattern of bursts that we see."
The UCLA team reports that it is becoming clear that information distinct from the gamma-ray explosion itself must be found to unravel the mystery of the bursts. Finding a conclusive association between gamma-ray burst locations and another class of objects would be a particularly significant breakthrough, immediately revealing the source and our distance from the bursts.
"No one is yet certain how to identify the source of gamma-ray bursts," said Larson, "but these findings may have brought us one step closer."
Two images are available on the Internet:
Galaxies with overlay drawing of gamma-ray bursts location
Galaxies without overlay drawing
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