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Gamma Ray Bursts

* Astronomers racing the clock managed to take the first-ever optical images of one of the most powerful explosions in the Universe -- a gamma ray burst --^51c
-- occurring on Saturday, Jan. 23, 1999.

* quick detection by orbiting spacecraft and instant notification to astronomers are critical in order to catch the bursts in the act.

* As the burst was still in progress, computers determined a rough location and radioed the position to the Gamma Ray Burst Coordinates Network (GCN), based at NASA's Goddard Space Flight Center, Greenbelt, MD.
-- The position was immediately forwarded via the GCN to astronomers at ground based observatories throughout the world.
-- Just 22 seconds later the Robotic Optical Transient Search Experiment (ROTSE) in Los Alamos, NM, operated by a team led by Dr. Carl Akerlof of the University of Michigan, was in position and took images of the patch of sky where the burst was reported.
-- Their equipment is assembled from 35 mm camera lenses and parts culled from the amateur astronomy market. The first picture showed a brightening new star within the sky region where the burst was reported.
-- Five seconds later, the burst achieved peak brightness, reaching 9th magnitude, about 16 times fainter than the human eye can see, but easily visible in an amateur telescope.
-- Within eight minutes of the initial detection, the burst had faded by a factor of 100 below its maximum brightness. "I was amazed," Akerlov said. "At best, we expected something really dim optically, at the limit of our sensitivity. Instead we found a whopper."
-- "If this burst had originated in the Milky Way Galaxy, it would have lit up the night sky," said Dr. Alan Bunner, Director of NASA's Structure and Evolution of the Universe science theme at NASA Headquarters.
-- The event was also recorded by instruments aboard the Italian-Dutch BeppoSAX satellite, which obtained a much more accurate position for the burst within a few hours of its onset.
-- It was this more precise location information that the ROTSE team used to find the burst in their images.

* "This is the Holy Grail for the Gamma Ray Burst Coordinates Network," said Dr. Scott Barthelmy, the astronomer at Goddard, who developed and runs the network.
-- "Optical telescopes had seen the afterglow of a burst, but never the burst itself. This observation will help us understand the physical processes behind the bursting."

* Within three hours of the gamma ray burst, a team of astronomers led by Dr. Stephan Odewahn, and Profs. Shri Kulkarni and George Djorgovski of the California Institute of Technology used the 60-inch Mt. Palomar telescope to find a fading optical counterpart to this gamma ray burst, helped by the precise localization provided by BeppoSAX.

* The next night, a joint team led by Dr. D. Kelson of the Carnegie Institution of Washington, using the Keck II 10-meter telescope located at Mauna Kea, HI, found that the distance to the burst is about nine billion light years, more than half way to the edge of the observable Universe.
-- "The optical emission was about 10,000 times brighter than ever observed, something you could see with a pair of good binoculars," said Dr. Neil Gehrels, Project Scientist of the Compton Observatory.
-- Theorists will have a field day trying to explain this phenomenon."

* the simultaneous observation of the burst in optical and gamma ray energies might open the door to a whole new generation of instruments like ROTSE, which is a fully automated telescope that can respond to information about transient celestial sources instantly.

* About three times a day our sky flashes with a powerful pulse of gamma rays^51d
-- The sources of this intense radiation are likely to be emitting, within the span of seconds or minutes, more energy than the sun will in its entire 10 billion years of life.
-- Where these bursts originate, and how they come to have such incredible energies, is a mystery that scientists have been attacking for three decades.
-- The phenomenon has resisted study--the flashes come from random directions in space and vanish without trace--until very recently.

* February 28 of this year, we were lucky.
-- One such burst hit the Italian Dutch Beppo-SAX satellite for about 80 seconds. Its gamma-ray monitor established the position of the burst--prosaically labeled GRB 970228--to within a few arc minutes in the Orion constellation, about halfway between the stars Alpha Tauri and Gamma Orionis.
-- Within eight hours, operators in Rome had turned the spacecraft around to look in the same region with an x-ray telescope.
-- They found a source of x-rays (radiation of somewhat lower frequency than gamma rays) that was fading fast, and they fixed its location to within an arc minute.
-- Never before has a burst been pinpointed so accurately and so quickly, allowing powerful optical telescopes, which have narrow fields of view of a few arc minutes, to look for it.
-- Astronomers on the Canary Islands, part of an international team led by Jan van Paradijs of the University of Amsterdam and the University of Alabama in Huntsville, learned of the finding by electronic mail.
-- time available on the 4.2-meter William Herschel Telescope, which they had been using to look for other bursts.
-- They took a picture of the area 21 hours after GRB 970228.
-- Eight days later they looked again and found that a spot of light seen in the earlier photograph had disappeared.

* On March 13 the New Technology Telescope in La Silla, Chile, took a long, close look at those coordinates and discerned a diffuse, uneven glow.
-- The Hubble Space Telescope later resolved it to be a bright point surrounded by a somewhat elongated background object.
-- Many of us believe the latter to be a galaxy, but its true identity remains unknown as of this writing.

* In November 1996 the High Energy Transient Explorer (HETE) spacecraft, equipped with very accurate instruments for locating gamma-ray bursts, failed to separate from its launch rocket.
-- in December the Russian Mars '96 spacecraft, with several gamma-ray detectors, fell into the Pacific Ocean after a rocket malfunction.
-- These payloads were part of a carefully designed set for launching an attack on the origins of gamma-ray bursts.
-- Of the newer satellites equipped with gamma-ray instruments, only Beppo-SAX--whose principal scientists include Luigi Piro, Enrico Costa and John Heise--made it into space on April 20, 1996.

* Gamma-ray bursts were first discovered by accident, in the late 1960s, by the Vela series of spacecraft of the U.S. Department of Defense.
-- These satellites were designed to ferret out the U.S.S.R.'s clandestine nuclear detonations in outer space--perhaps hidden behind the moon.
-- In 1973 scientists concluded that a new astronomical phenomenon had been discovered.
-- These initial observations resulted in a flurry of speculation about the origins of gamma-ray bursts--involving black holes, supernovae or the dense, dark star remnants called neutron stars.
-- No one knew whether the bursts were coming from a mere 100 light-years away or a few billion. As a result, the energy of the original events could only be guessed at.

* By the mid-1980s the consensus was that the bursts originated on nearby neutron stars in our galaxy.
-- In particular, theorists were intrigued by dark lines in the spectra (component wavelengths spread out, as light is by a prism) of some bursts, which suggested the presence of intense magnetic fields.
-- The gamma rays, they postulated, are emitted by electrons accelerated to relativistic speeds when magnetic-field lines from a neutron star reconnect.
-- A similar phenomenon on the sun--but at far lower energies--leads to flares.

* In April 1991 the space shuttle Atlantis launched the Compton Gamma Ray Observatory, a satellite that carried the Burst and Transient Source Experiment (BATSE).
-- The distribution of gamma-ray bursts clusters of did not trace out the Milky Way
-- nor were the bursts associated with nearby galaxies or galaxies.
-- Instead they were distributed isotropically
-- Theorists soon refined the galactic model: the bursts were now said to come from neutron stars in an extended spherical halo surrounding the galaxy.

* One problem with this scenario is that the earth lies in the suburbs of the Milky Way, about 30,000 light-years from the core.
-- almost 600,000 light-years in outer radius.
-- the halo of the neighboring Andromeda galaxy should be as extended and should start to appear in the distribution of gamma-ray bursts. But it does not.
-- Special models in which the neutron stars beam in the same direction as their motion can, however, overcome this objection.
-- This uniformity has convinced most astrophysicists that the bursts come from cosmological distances, on the order of three billion to 10 billion light-years away.
-- At such a distance the bursts should show the effects of the expansion of the universe.
-- Galaxies that are very distant are moving away from the earth at great speeds; we know this because the light they emit shifts to lower, or redder, frequencies.
-- Likewise, gamma-ray bursts should also show a "redshift," as well as an increase in duration.
-- Unfortunately, BATSE does not see, in the spectrum of gamma rays, bright or dark lines characterizing specific elements whose displacements would betray a shift to the red.
-- Nor does it detect the dark lines found by earlier satellites.

* In April astronomers using the Keck Telescope in Hawaii obtained an optical spectrum of the afterglow of GRB 970228.
-- It is smooth and red, with no telltale lines.
-- Jay Norris of the National Aeronautics and Space Administration Goddard Space Flight Center and Robert Mallozzi of the University of Alabama in Huntsville have statistically analyzed the observed bursts and report that the weakest, and therefore the most distant, show both a time dilation and a redshift.
-- There are, however, other (controversial) ways to interpret these findings.

* One feature that makes it difficult to explain the bursts is their great variety.
-- A burst may last from about 30 milliseconds to almost 1,000 seconds--and in one case, 1.6 hours.
-- Some bursts show spasms of intense radiation, with no detectable emission in between, whereas others are smooth.
-- Also complicated are the spectra--essentially, the colors of the radiation, invisible though they are.
-- The bulk of a burst's energy is in radiation of between 100,000 and one million electron volts, implying an exceedingly hot source.
-- The photons of optical light, the primary radiation from the sun, have energies of a few electron volts.
-- Some bursts evolve smoothly to lower frequencies such as x-rays as time passes.
-- Although this x-ray tail has less energy, it contains many photons.

* If originating at cosmological distances, the bursts must have energies of perhaps 10⁵¹ ergs.
-- About 1,000 ergs can lift a gram by one centimeter.
-- This energy must be emitted within seconds or less from a tiny region of space, a few tens of kilometers across.
-- It would seem we are dealing with a fireball.

* gamma-ray bursts have been the subject of more than 2,500 papers--about one publication per recorded burst.
-- Their transience has made them difficult to observe with a variety of instruments, and the resulting paucity of data has allowed for a proliferation of theories.
-- If one of the satellites detects a lensed burst, astronomers would know for sure that bursts occur at cosmological distances.
-- Such an event might occur if an intervening galaxy or other massive object serves as a gravitational lens to bend the rays from a gamma-ray burst toward the earth.
-- When optical light from a distant star is focused in this manner, it appears as multiple images of the original star, arranged in arcs around the lens.
-- Gamma rays cannot be pinpointed with such accuracy; instead they are currently detected by instruments that have poor directional resolution.
-- bursts are not steady sources like stars.
-- A lensed gamma-ray burst would therefore show up as two bursts coming from roughly the same direction, having identical spectra and time profiles but different intensities and arrival times.
-- The time difference would come from the rays' traversing curved paths of different lengths through the lens.

* To further nail down the origins of the underlying explosion, we need data on other kinds of radiation that might accompany a burst.
-- Even better would be to identify the source.

* Until the fortuitous observation of GRB 970228--we are astonished that its afterglow lasted long enough to be seen--such "counterparts" had proved exceedingly elusive.
-- To find others, we will need to locate the bursts very precisely.

* BATSE consists of eight gamma-ray detectors pointing in different directions from eight corners of the Compton satellite; comparing the intensity of a burst at these detectors provides its location to roughly a few degrees but within several seconds.
-- Often BACODINE can locate the burst even while it is in progress.
-- The location is transmitted over the Internet to several dozen sites worldwide.
-- In five more seconds, robotically controlled telescopes at Lawrence Livermore National Laboratory, among others, slew to the location for a look.
-- Unfortunately, only the fast-moving smaller telescopes, which would miss a faint image, can contribute to the effort.
-- The Livermore devices, for instance, could not have seen the afterglow of GRB 970228 (unless the optical emission immediately after the burst is many times brighter, as some theories suggest).
-- Telescopes that are 100 times more sensitive are required.
-- These mid-size telescopes would also need to be robotically controlled so they can slew very fast, and they must be capable of searching reasonably large regions.
-- If they do find a transient afterglow, they will determine its location rather well, allowing much larger telescopes such as Hubble and Keck to look for a counterpart.
-- The long-lasting, faint afterglow following GRB 970228 gives new hope for this strategy.

* The HETE mission, directed by George Ricker of the Massachusetts Institute of Technology, is to be rebuilt and launched in about two years.
-- It will survey the full sky with x-ray detectors that can localize bursts to within several arc minutes.
-- A network of ground-based optical telescopes will receive these locations immediately and start searching for transients.

* Of course, we do not know what fraction of bursts actually exhibit a detectable afterglow
-- GRB 970228 could be a rare and fortuitous exception.
-- even an observation field as small as arc minutes contains too many faint objects to make a search for counterparts easy.
-- It would be marvelous if we could derive accurate locations within fractions of a second from the gamma rays themselves.
-- Astronomers have proposed new kinds of gamma-ray telescopes that can instantly derive the position of a burst to within arc seconds.

* photons of even higher energy--of about a trillion electron volts--might be captured by special ground-based gamma-ray telescopes.

* At the other end of the spectrum, soft x-rays, which have energies of up to roughly one kiloelectron volt (keV), are helpful for testing models of bursts and also for getting better fixes on position.

* May 8, Beppo-SAX operators located a 15-second burst.
-- Soon after, Howard E. Bond of the Space Telescope Science Institute in Baltimore photographed the region with the 0.9-meter optical telescope at Kitt Peak; the next night a point of light in the field had actually brightened.
-- Other telescopes confirm that after becoming most brilliant on May 10, the source began to fade.

* This is the first time that a burst has been observed reaching its optical peak--which, astonishingly, lagged its gamma-ray peak by a few days.

* Also for the first time, on May 13 the Very Large Array of radio telescopes in New Mexico detected radio emissions from the burst remnant.

* the primarily blue spectrum of this burst, taken on May 11 with the Keck II telescope on Hawaii, showed a few dark lines, apparently caused by iron and magnesium in an intervening galactic cloud.
-- Astronomers at the California Institute of Technology find that the displacement of these absorption lines indicates a distance of more than seven billion light-years.
-- If this interpretation holds up, it will establish once and for all that bursts occur at cosmological distances.
-- In that case, it may not be too long before we know what catastrophic event was responsible for that burst--and for one that might be flooding the skies even as you read.

* Recent discoveries in this field by a collection of international astronomers, including the announcement today of a high-redshift burst, have demonstrated that these bursts are from the most remote parts of the universe^51e.
-- Every burst, of which about one per day is detected, releases perhaps as much energy in 10 seconds as the Sun emits in its entire 10-billion-year lifetime.
-- This week, astronomers from the California Institute of Technology announced yet another breakthrough discovery which adds to the body of research, much of which has been done by scientists working in the Space Sciences Laboratory of the NASA/Marshall Space Flight Center, indicating that these "blasts from the past" are indeed the most powerful explosions in the Universe.

* Since their discovery (by accident) in the late 1960's, several thousand bursts have been detected, most of them with BATSE, the Burst and Transient Source Experiment, on board the Compton Gamma Ray Observatory.
-- Their distribution on the sky is completely uniform.
-- they do not appear to come from the Milky Way.

* most astronomers were convinced that bursts originated in our own Galaxy, on or near objects called neutron stars.
-- Our Milky Way Galaxy contains many neutron stars
-- objects as massive as the Sun (about 300,000 times the mass of the Earth) but no bigger than about 10 kilometers in diameter.
-- Their tremendous gravitational field and magnetic field made them an ideal source for the gamma-ray bursts.

* it became clear from BATSE observations that GRBs are distributed uniformly on the sky, without a concentration to the plane of the Milky Way, or towards its center or in other clumps of or concentrations.
-- the distribution is almost perfectly random.

* the angular distribution could lead one to argue that perhaps we see the bursts coming from quite nearby, from distances small compared to the thickness of the disk of the Milky Way.
-- this would imply, as we saw above, that in no direction do we start to see the edge of the GRB distribution, or in this case, the edge of the Milky Way disk. Such an edge would reveal itself in the distribution of brightnesses of bursts, which would show a deficit of faint bursts.

* The very large galactic halo size is required to avoid an asymmetry in the GRB sky distribution caused by the fact that the Earth is offset from the center of the Galaxy, by about 25,000 light years.

* Finding such counterparts has recently become possible with the detection of GRBs with the Wide Field Camera (WFC) on board of the Italian-Dutch X-ray satellite BeppoSAX
-- can pinpoint a GRB on the sky to within a circle with a diameter of 6 arcminutes (20 percent of the diameter of the full moon)
-- can provide that position within a matter of hours, much faster than was possible before.
-- This error box, if known sufficiently rapidly, is small enough that optical and radio telescopes can be used to search it for the presence of transient emission that may be connected with the GRB.
-- This WFC is sensitive to X rays, and can view a 40 by 40 degrees field of view, within which it can locate X-ray sources with several arcminute accuracy.

* The first time this worked out successfully was on February 28, 1997, when a GRB was detected coming from the constellation Orion.
-- Within 21 hours after this burst a team of astronomers from the University of Alabama in Huntsville and the University of Amsterdam used the 4.2 meter William Herschel Telescope on La Palma (one of the Canary Islands in the Atlantic Ocean, West of the North Africa) to make images of the BeppoSAX GRB location.
-- They did the same about a week later.
-- A comparison of these images immediately revealed one star that was present on February 28, which had disappeared a week later.
-- the Beppo SAX astronomers had used a more precise X-ray instrument on their satellite to also observe the GRB location in detail
-- they found that it contained a weak decaying X-ray source, with a position (accurate to somewhat less than an arcminute) that coincided with that of the optical star that disappeared.
-- the X-ray and optical afterglow of a GRB had been detected for the first time.
-- Further conformation that the decaying X-ray and optical signals came from one and the same source was obtained from X-ray images taken with the ROSAT satellite, which located the X-ray afterglow with an accuracy of some 10 arcseconds: it still coincided with the optical afterglow.

* As soon as the disappearing optical afterglow was discovered
-- deep optical images were made of it with the ESO New Technology Telescope and the Keck Telescope, which showed that at the location of the optical transient there was a very weak object that looked slightly extended, quite possibly a weak galaxy.
-- In a matter of weeks two observations were made with the Hubble Space Telescope, which showed that the fading optical transient consists of a point source (its fading between two HST observations shows it is the GRB counterpart) and an extended source ('fuzz') at whose edge it appears to be located.
-- Although they don't constitute solid proof, these observations naturally fit the idea that the GRB of February 28, 1997 went off in a faraway faint galaxy (the fuzz).

* The next major step forward occurred with a burst that was observed with the WFC on May 8, 1997.
-- A faint optical variable source was detected at the GRB location by H. Bond of the STScI, using a 36 inch telescope at Kitt Peak National Observatory.
-- A spectrum of this object taken by M. Metzger of Caltech and his collaborators with the Keck telescope showed that the presence of absorption lines, which are redshifted by 0.83 (i.e., their wavelengths are 1.83 times their value measured in the laboratory).
-- The case for the long distance scale appears now to be settled.

* for this GRB of May 8, D. Frail of the National Radio Astronomical Observatory discovered the first radio emission associated with any GRB.

* In May 7, 1998 Shrinivas Kulkarni of Caltech and his colleagues used the 10-meter Keck Telescope in Hawaii, and NASA's Hubble Space Telescope to study the position of a similar burst detected by BATSE and BeppoSAX on the 14th of December 1997.
-- What they have concluded is that this object possesses a redshift of over 3.4 - more than 12 billion light years away.
-- This is over a million times farther than astronomers had theorized the bursts might lie when the BATSE experiment was built and launched in 1991. See the NASA press release for more information.

* Gamma ray bursters are transient, one-time events that appear on the cosmic stage and are never seen again^51f.
-- This week, scientists announced that they had found yet another soft gamma ray repeater - SGR - to add to the cast of three that have been discovered since 1979.

Editor's note (July 7, 1998): The science team that discovered and located this object has decided not to confirm it as an SGR. Although it appeared to be an SGR, the scientists later decided that the data were too tentative to warrant confirmation at this time.

* in May the science team announced that extensive observtions of another SGR allowed them to confirm its identity as the first known magnetar, or highly magnetized neutron star.

* "This is the most intense SGR seen by BATSE," said Dr. Chryssa Kouveliotou of the Universities Space Research Association.

* Unlike gamma ray bursts, which emit large amounts of high energy gamma radiation, SGRs have a larger proportion of lower-energy X-ray radiation.
-- in contrast to gamma ray bursts, which can rumble on for many minutes, SGRs pop off - like a cap gun - in as short a time as it takes to snap your fingers.

* The first SGR was discovered by a Russian spacecraft in 1979.
-- Two others were not recognized as a new phenomenon until 1986, when several scientists gathered in Toulouse, France, to decide how to classify and name this newly found class of objects.
-- Spurred by Dr Kevin Hurley, of the University of California at Berkeley, the scientists reviewed these unusual burst sources, and decided they had discovered a new phenomenon.
-- SGR research remained quiet until 1993, when BATSE captured two of the sources emitting again.
-- In late 1996, Kouveliotou was able to use BATSE to alert other scientists to point the Rossi X-ray Timing Explorer at the expected position.

* And so the population sat at three until this summer.

* "On June 29, BATSE triggered on an extremely intense burst that had all the characteristics of SGR emission," said Kouveliotou. "Immediately we alerted Kevin for additional information from the Ulysses spacecraft."

* This extra burst was also seen by the Konus detector aboard the Wind geoscience satellite.
-- An International Planetary Network (IPN) arc between BATSE and Ulysses and between
-- Ulysses and Konus placed these two events on unknown grounds: no other SGR source was detected from their location.

* "Still we decided to pay it safe," said Kouveliotou. "We stood by and waited for the next burst to clinch the source location.
-- On Friday (September 12), we got it!
-- We heard from the All Sky Monitor on the Rossi X-ray Timing Explorer that they had seen a very bright event that literally shut their instrument off.
-- Dan Smith, an ASM team member at the Massachusetts Institute of Technology, immediately informed all other spacecraft teams.
-- When data from BATSE came in, we realized we had another intense SGR burst; the event was also seen by Ulysses, and Hurley immediately put all three events on the map."
-- "The locations agreed," he said. "We concluded that we are observing a new SGR."

* on Thursday, even more good news arrived: Fred Vrba of the U.S. Naval Observatory at Flagstaff, Arizona, announced that the observatory had found an object that is glowing very bright in infrared.
-- It is not yet confirmed as the optical counterpart, but nothing else has been nominated.

* "It's very hot now," Kouveliotou said. "Everybody's looking at it, scanning the error box."
-- The error box is the small section of sky where instruments aboard other spacecraft indicate the source can be found.
-- The June 29 observation by Ulysses, BATSE and Konus produced a long, thin error box that included too much sky, while BATSE showed that the burst was similar to past SGRs.
-- "We decided to wait and get a better measurement when the two spacecraft had moved apart," Kouveliotou said.
-- September 12, when the second burst went off, Ulysses had moved far enough away that their combined detection produced a much smaller error box.

* "Kevin computed the position and it was clearly a new location," Kouveliotou said.

* They recorded bursts of radiation in gamma rays, the highest part of the electromagnetic spectrum, but not lower down. And the bursts appeared to be coming from outside the solar system.
-- Their appearances and locations were random and not associated with any known object.
-- Instead, scientists got an even deeper mystery. Bursts flashed and faded in a matter of seconds or minutes, too quick to aim a telescope for follow-up observations.
-- they appeared to be distributed outside the galaxy and probably deep in the universe.

* The big break came in 1997 when Dr. Jan van Paradijs of the University of Amsterdam, using observations by the Beppo SAX satellite and ground-based observatories, tied a burst on Feb. 28, 1997 to a source deep in space.
-- The Feb. 28 burst was the big news at the 4th Huntsville Gamma Ray Burst Symposium in September 1997.

* "The sense of the community is that the doubt is over," said Dr. Chip Meegan, a BATSE coinvestigator at NASA/Marshall, before the 1997 symposium.
-- "Gamma ray bursts are cosmological." That means that instead of coming from within our galaxy or even immediately around the galaxy, they are deep in space, probably more than 8 billion light years away (by comparison, our galaxy is about 150,000 light years across).
-- "But they're still very strange," Meegan continued. "All of the questions about them being cosmological are still there." The principal question is, what produces so much energy?
-- The questions still stand and will be discussed, along with findings from the past two years of observations at the 5th Huntsville Gamma Ray Burst Symposium.

* "The supernova crowd will be there since we have a tentative association between supernovae and gamma-ray bursts," Connaughton said. "There's been lots of speculation by astronomers active in supernova research as well as those active in gamma-ray bursts."
-- If the two are associated, then what special conditions lead to a supernova expending such a phenomenal amount of energy mainly in the gamma-ray spectrum yet hiding or muting itself in visible light until weeks later?

* In January, the Robotic Optical Transient Search Experiment at Los Alamos National Laboratory, cued by BATSE, caught the optical flash within 20 seconds of a gamma-ray flash being recorded.
-- many other attempts to catch optical transients coincident with the gamma-ray emissions have been fruitless.

* Cosmic gamma-ray bursts have been called the greatest mystery of modern astronomy^51g.
-- Space satellites indicate that Earth is illuminated by 2 to 3 bursts every day.

*Until recently we didn't even know if they came from the neighborhood of our own solar system or perhaps from as far away as the edge of the universe.
-- The first vital clues began to emerge in 1997 when astronomers detected an optical counterpart to a gamma-ray burst. In February 1997 the BeppoSAX X-ray astronomy satellite pinpointed the position of a burst in Orion to within a few arcminutes.
-- They detected a rapidly fading star, probably the aftermath of a gigantic explosion, next to a faint amorphous blob believed to be a very distant galaxy.

* Since then seven more optical counterparts have been discovered.
-- A recent discovery makes gamma-ray bursts seem more fantastic than ever. Shri Kulkarni of Caltech and his colleagues found that a gamma-ray burst recorded in December 1997 came from a faint galaxy with a redshift of 3.4.
-- That means that the burst originated over 12 billion light years away.
-- Kulkarni noted that "The energy released by this burst in its first few seconds staggers the imagination." Indeed, it was one of the biggest explosions since the Big Bang itself.

* The light curves of the few known optical and X-ray counterparts are consistent with that of an expanding fireball that is glowing because of a "Synchrotron Shock".
-- Dr. Robert Preece, a gamma-ray astrophysicist at the University of Alabama in Huntsville, likened the shock wave to a wave on the beach. "A shock forms when the wave crest starts to fall over, and scud from the wave shoots out ahead."
-- In the cosmic shock wave, the 'scud' is electrons and protons.
-- They accelerate ahead of the wave and spiral around magnetic fields lines, producing a form of radiation called synchrotron emission.
-- Synchrotron emission is seen all the time here on Earth as a blue glow in particle accelerators, and radio astronomers detect it coming from the Milky Way.

In a recently published edition of the Astrophysical Journal Letters Rob Preece and his collaborators from the University of Alabama examined over 100 bright bursts collected by the BATSE instrument on the Compton Gamma Ray Observatory and measured the slopes of their low-energy spectra. The figure at left shows their data. They plotted the slope of the low energy part of the spectra (vertical axis) vs. the peak burst energy (horizontal axis). The red line is the so-called "Line of Death", corresponding to a spectral slope greater than -2/3. If a data point falls above the line, that gamma-ray burst cannot have been caused by a synchrotron shock, and thus the Synchrotron Shock Model is "dead" for that burst. Preece et al. found that 44% of the bursts fell above the Line of Death. If we assume that all gamma-ray bursts are caused by the same thing, this means that none can be due to a synchrotron shock.

* BATSE scientists found that gamma ray bursts are randomly distributed across the sky, indicating that they are peppered throughout the heavens, rather than clustered along the plane of our Milky Way galaxy^51h.

* Since 1997, a few bursts have been observed with optical, X-ray, or radio counterparts that are thought to be at cosmological distances up to 10 to 12 billion light years away.

* burst and a supernova both on April 25, 1998, both in the same region of the sky.
-- The gamma-ray burst was observed by BATSE and the Beppo SAX satellite.
-- The precise position of the burst provided by instruments on Beppo SAX allowed ground-based optical telescopes to discover that the burst was coincident with a new supernova - SN1998bw - within the same small section of the sky.

* the gamma-ray burst was average in its properties.
-- Nothing distinguished it from the other bursts routinely detected by BATSE.
-- sn1998bw was extraordinary - the intrinsically brightest supernova ever observed in its category.

* It could be a coincidence.
-- BATSE has recorded more than 2,000 bursts since 1991, about one a day.
-- only 1 supernova in 10 is actually detected.
-- a burst and a supernova are bound to coincide in time and apparent location.
-- if they're related, it could send astrophysicists back to rethink the mystery.

* "I looked to see if there were any more of these coincidences," Kippen said.
-- "The chances of observing a supernova within a small bit of sky over the span of a few days is pretty small, about 1 in 10,000."
-- Kippen divided the BATSE burst catalog into two groups. First he looked for bursts that were seen both by BATSE and Ulysses.
-- BATSE has eight detector modules. Measuring the brightness of a burst as seen by the three or four modules that are actually triggered will describe a large error box in the sky.
-- Triangulating the arrival time of the burst with the time of arrival at Ulysses, located deep within our solar system, reduces the error box to a short, thin arc across the sky.
-- Kippen then compared these with supernovas that had been detected at about the same time as the burst.
-- He allowed a generous margin - up to a month - since the dates of supernova explosions aren't always precisely known.
-- He came up empty handed. The 415 bursts and 585 supernovas all had separate locations.
-- there is the possibility that the supernovas might cause weaker bursts that were detected by BATSE and not by Ulysses which carries a much smaller instrument. This gave him a set of 1,222 bursts.

* "At some level you expect gamma rays to come from a supernova," Kippen said.
-- they should be less powerful than the events that cause gamma-ray bursts at cosmological distances
-- their light profiles also should be different.

* NASA Hubble Space Telescope Imaging Spectrograph views of the rapidly fading visible-light fireball from the most powerful cosmic explosion recorded to date.
-- the light from the blast was equal to the radiance of 100 million billion stars. The initial explosion began as an intense burst of gamma-rays which happened on Jan. 23, 1999.

* The blast had already faded to one four-millionth of its original brightness when Hubble made observations on February 8 and 9.
-- the fading fireball embedded in a galaxy located 2/3 of the way to the horizon of the observable universe.

* Hubble’s resolution shows the galaxy is not the classic spiral or elliptical shape.
-- The galaxy might be distorted by a collision with another galaxy.
-- This would induce rapid starbirth as gas clouds were heated and compressed, precipitating millions of newborn stars.

* The presence of this so-called starburst activity is strongly supported by Hubble and Keck telescope images that show the host galaxy is exceptionally blue.
-- This means it contains a large number of blue newborn stars.

* Space Telescope’s observations further support the idea that these mysterious powerful explosions happen where vigorous star formation takes place.
-- Gamma-ray bursts may be created by the mergers of a pair of neutron stars or black holes, or a hypernova, a theorized type of exceptionally violent exploding star.

* unlike visible light, gamma-rays are exceedingly difficult to observe with a telescope, and the bursts' short duration exacerbates the problem⁵¹ⁱ.
-- The Italian/Dutch satellite BeppoSAX had the ability to localize the bursts on the celestial sphere with a sufficient precision to permit follow-up observations with the world's most powerful ground-based telescopes.

* This breakthrough led to the discovery of long-lived "afterglows" of bursts in X-rays, visible and infrared light, and radio waves.
-- While gamma-ray bursts last only a few seconds, their afterglows can be studied for several months.
-- This led to the discovery that the bursts do not originate within our own galaxy, the Milky Way, but rather are associated with high-redshift, extremely distant galaxies in the universe.

* The gamma ray burst was detected on December 14, 1997, by the BeppoSAX and CGRO satellites.
-- BeppoSAX and NASA's Rossi X-ray Timing Explorer spacecraft detected an X-ray afterglow.
-- BeppoSAX precision led to the detection of a visible light afterglow, found by a team from Columbia University, New York, N.Y., and Dartmouth College, Hanover, N.H., including Professors Jules Halpern, David Helfand, John Torstensen, and their collaborators, using a 2.4-meter telescope at Kitt Peak, Az., but no distance could be measured from these observations.

* A team of astronomers from the California Institute of Technology announced today that a recently detected cosmic gamma ray burst was as bright as the rest of the universe, releasing a hundred times more energy than previously theorized.

* The team has measured the distance to a faint galaxy from which the burst, designated GRB 971214, originated.
-- It is about 12 billion light years from the Earth. (One light year is approximately 5.9 trillion miles.)

* "The energy released by this burst in its first few seconds staggers the imagination," said Caltech professor Shrinivas Kulkarni, one of the two principal investigators on the team.
-- The burst appears to have released several hundred times more energy than an exploding star, called a supernova, until now the most energetic phenomenon in the universe known to scientists.

* "For about one or two seconds, this burst was as luminous as all the rest of the entire universe," said Caltech professor George Djorgovski, the other principal investigator on the team.

* Finding such large energy release over such a brief period of time is unprecedented in astronomy, except for the big bang itself.

* "In a region about a hundred miles across, the burst created conditions like those in the early universe, about one millisecond (1/1,000 of a second) after the big bang," said Djorgovski.

* "Most of the theoretical models proposed to explain these bursts cannot explain this much energy," said Kulkarni. "However, there are recent models, involving rotating black holes, which can work.
-- On the other hand, this is such an extreme phenomenon that it is possible that we are dealing with something completely unanticipated and even more exotic."

* the Caltech team detected an extremely faint galaxy at its location, using one of the world's largest telescopes, the W.M. Keck Observatory 10-meter Keck II telescope on Mauna Kea, Hawaii.
-- The galaxy is about as faint as an ordinary 100 watt light bulb would be as seen from a distance of a million miles.

* Subsequent images taken with the Hubble Space Telescope confirmed the association of the burst afterglow with this faint galaxy.

* The Caltech team succeeded in measuring the distance to this galaxy, using the light-gathering power of the Keck II telescope.
-- The galaxy is at a redshift of z=3.4, or about 12 billion light years distant (assuming the universe to be about 14 billion years old).

* From the distance and the observed brightness of the burst, astronomers derived the amount of energy released in the flash.

* Although the burst only lasted a few seconds, the energy released was hundreds of times larger than the energy given out in supernova explosions, and it is about equal to the amount of energy radiated by our entire Galaxy over a period of a couple of centuries.

* This is only the energy seen in the gamma-rays; it is possible that other forms of radiation, such as neutrinos or gravity waves, which are extremely difficult to detect, carried a hundred times more energy than that.

Still under development ! This should change soon, though !