From a NASA Press Release
A new gamma ray burst mission, the High-Energy Transient Explorer (HETE-2), made its entrance into space on October 10, 2000, from the Kwajalein Missile Range in the Marshall Islands.
"Gamma ray bursts are stupendous explosions. They are the most energetic events since the Big Bang, yet one occurs about once a day somewhere in the sky," said Dr. George R. Ricker from the Massachusetts Institute of Technology (MIT) in Cambridge, Mass., HETE-2 principal investigator. "The successful launch of HETE-2 means that for the first time we can locate with pinpoint accuracy hundreds of these bursts. Also, HETE-2's ability to relay the accurate location of each burst in real-time to space- and ground-based optical and radio observatories will surely revolutionize this exciting new area of high energy astrophysics."
Gamma ray bursts are a mystery to scientists, and very little is known about their fundamental origin. HETE-2 will embark upon a gamma ray burst fact-finding mission during the four years it is slated to operate. In addition to detecting hundreds of bursts during its mission, it will provide detailed information on the location and light characteristics of many of these bursts.
Within seconds of a burst, HETE-2 will be able to calculate a precise location for that burst. On the ground, a dedicated network of 12 listen-only burst alert stations will relay the data to the MIT control center. From there, information will be transmitted to the Gamma Ray Burst Coordinate Distribution Network at the Goddard Space Flight Center, which can send the information to other observatories worldwide in 10-20 seconds, significantly faster than previously possible. HETE-2 will allow astronomers to see a burst while it is still occurring and allow scientists to study its development at various wavelengths.
The spacecraft carries three main instruments and is supported by a computer network that transmits data to other observatories. The French Gamma Telescope (FREGATE), built by CESR, will detect gamma ray bursts and very bright (higher energy) X-ray transients. The Wide-Field X-ray Monitor (WXM), built by RIKEN and Los Alamos National Laboratory, detects photons slightly lower in energy than the FREGATE does. The WXM therefore will detect fewer gamma ray bursts than FREGATE, but because of its superior resolution, will be able to locate the FREGATE-detected bursts to within 10 arc minutes (an area of sky about equal to 1/10 the size of the full Moon). The oft X-Ray Camera (SXC), built by MIT, covers the lowest energy band of the three instruments. It also provides the best angular resolution, resulting in a location accuracy of about 10 arc seconds, more than an order of magnitude finer than any previous GRB instrument.
HETE-2 is a collaboration between NASA; MIT; Los Alamos National Laboratory, New Mexico; France's Centre National d'Etudes Spatiales (CNES), Centre d'Etude Spatiale des Rayonnements (CESR), and Ecole Nationale Superieure de l'Aeronautique et de l'Espace (Sup'Aero); and Japan's Institute of Physical and Chemical Research (RIKEN). The science team includes members from the University of California (Berkeley and Santa Cruz) and the University of Chicago.
More information on the HETE-2 mission can be found at:
http://space.mit.edu/HETE
The Compton Gamma-Ray Observatory was launched in 1991. There had been a hope that all NASA's "Great Observatories" would be up simultaneously to send back data across the spectrum about given objects. But since the Space Infrared Telescope Facility won't be working before 2002, that hope is dashed. Astronomers will be blind to gamma rays for some time. A solar gamma-ray spcecraft, NASA's High Energy Solar Spectroscopic Imager (HESSI), is to observe solar flares in x-rays and gamma rays, something particularly important now, at the peak of the sunspot cycle. But it was damaged in testing in March 2000 and its launch has been delayed until 2001. For non-solar observations, NASA's Gamma-ray Large Area Space Telescope (GLAST) is scheduled for launch in 2005. It will have 30 times CGRO's sensitivity for the highest-energy gamma rays and will have 10 times CGRO's angular resolution. The European Space Agency's International Gamma-Ray Astrophysics Laboratory (INTEGRAL), with spectroscopc capabilities, is scheduled for launch in April 2002.
Perhaps the most significant set of discoveries of Compton GRO dealt with gamma-ray bursts. It detected thousands of them and showed that they were randomly distributed across the sky. Its teamwork with the BeppoSAX spacecraft allowed several gamma-ray bursts to be pinpointed as extremely distant, and therefore extremely powerful, objects. A NASA High Energy Transient Explorer-2 (HETE-2), to be launched in July 2000, will be less sensitive than CGRO for these bursts but will pinpoint them in the sky better. The NASA Swift mission, to be launched in 2003, will also have good spatial resolution while being five times more sensitive than Compton to the bursts. These smaller missions have less general capability than the dear departed Compton Gamma-Ray Observatory.
NASA's extremely productive and long-lived Compton Gamma-Ray Observatory mission -- which exceeded its mission by four years and completely changed ideas on the most important unsolved puzzles in astrophysics -- has come to end with the failure of one of the satellite's three gyroscopes.
NASA plans to safely direct the satellite back into Earth's atmosphere no earlier than June 1 with the remaining two gyroscopes, which are used to steer the craft. As an extra precaution, Compton engineers are also developing a method to control the satellite without any gyroscopes, for use as backup during the reentry maneuvers in case an anomaly is encountered with the gyroscopes. Compton's four instruments are still in working order.
"Compton has been a workhorse for nine years, far exceeding our expectations for a two- to five-year mission," said Dr. Alan Bunner, director of NASA's Structure and Evolution of the Universe science theme, NASA Headquarters, Washington, DC. "New discoveries made by Compton changed our view of the Universe in fundamental ways."
Compton's lasting legacy will be its impact on gamma ray astronomy. The telescope detected more than 400 gamma ray sources, 10 times more than were previously known. Compton recorded more than 2,500 gamma ray bursts; before Compton, only about 300 had been detected.
"NASA must have a controlled reentry to direct Compton towards an uninhabited area in the Pacific ocean, " said Dr. Ed Weiler, Associate Administrator for the Office of Space Science, NASA Headquarters. "NASA decided before Compton was launched that, due to its size, it would be returned to Earth by controlled reentry when the mission was over. This was always NASA's plan. "
The propulsion system on Compton lacks sufficient fuel to boost the spacecraft to a higher, longer-lived orbit. Left alone, Compton will eventually fall from orbit due to a minute drag from the Earth's tenuous atmosphere at Compton's orbital height. Unlike most other satellites, Compton is too large to burn up entirely in the atmosphere during reentry. An uncontrolled reentry would expose some area under its orbital path (28.5 degrees north and south latitude) to the risk of falling debris.
The decision to reenter Compton before a second gyroscope fails, even though the satellite is functioning normally, was made at NASA Headquarters on March 23, 2000, after extensive study to consider all options. Research showed it was significantly safer to perform a controlled reentry than any other method of dealing with the satellite. "We actively pursued the option that provided the lowest risk to human lives," said Weiler.
Debris from the reentry will be scattered over an area estimated to be 16 miles wide and 962 miles long. The center of the reentry area is on the equator approximately 2,500 miles southeast of Hawaii (about 120 degrees west longitude). A large portion of the satellite will vaporize as it transits the atmosphere, and most of the pieces that survive will be tiny, about the size of a pea or a grain of sand. However, Compton contains structures made of titanium, which are expected to fall as larger pieces.
"Enough will survive to present a small but still unacceptable risk to populated areas if Compton were allowed to reenter in an uncontrolled manner," said Preston Burch, Deputy Program Manager for Space Science Operations at NASA's Goddard Space Flight Center, Greenbelt, MD. "NASA will work closely with aviation and maritime authorities to ensure the impact area is free from traffic during reentry."
Compton flight controllers, stationed at Goddard, will fire Compton's propulsion system thrusters in the direction opposite to its orbital motion, which will slow the spacecraft down and cause its orbital height to decrease so that it reenters the atmosphere. There will be four separate firings of the propulsion system thrusters, each about a day apart. After each firing, Compton's new orbit will be determined precisely, and the performance of the thrusters will be evaluated. The thruster performance varies according to the pressure of the propellant, so the thrusters will not perform the same way because each firing consumes propellant, which decreases its pressure.
NASA and international space agencies plan several upcoming missions to continue where Compton left off. The Compton Gamma Ray Observatory was the second of NASA's Great Observatories and the gamma-ray equivalent to the Hubble Space Telescope and the Chandra X-ray Observatory. Compton was launched aboard the Space Shuttle Atlantis in April 1991, and, at 17 tons, was the largest astrophysical payload ever flown at that time.
More information is available on the Internet at:
http://pao.gsfc.nasa.gov/gsfc/spacesci/structure/cgro.htm
The Compton Gamma Ray Observatory, launched in 1991, is down to its last two gyroscopes. Though it could work another half dozen years, NASA is considering crashing it in the spring of 2000 to prevent it going out of control if another gyro fails.
The European Space Agency's International Gamma-Ray Astrophysics Laboratory (in collaboration with the U.S. and Russia) is scheduled for launch in 2001. NASA's SWIFT mission is to record gamma-ray bursts after its 2003 launch. HETE-II is to be launched in spring 2000. It is a low-cost MIT satellite funded by NASA.
International teams of astronomers are now busy working on new and exciting data obtained during the last week with telescopes at the European Southern Observatory (ESO).
Their object of study is the remnant of a mysterious cosmic explosion far out in space, first detected as a gigantic outburst of gamma rays on May 10. Gamma-Ray Bursters (GRBs) are brief flashes of very energetic radiation - they represent by far the most powerful type of explosion known in the Universe and their afterglow in optical light can be 10 million times brighter than the brightest supernovae [1]. The May 10 event ranks among the brightest one hundred of the over 2500 GRB's detected in the last decade.
The new observations include detailed images and spectra from the VLT 8.2-m ANTU (Unit Telescope 1) telescope at Paranal, obtained at short notice during a special Target of Opportunity programme. This happened just over one month after that powerful telescope entered into regular service and demonstrates its great potential for exciting science. In particular, in an observational first, the VLT measured linear polarization of the light from the optical counterpart, indicating for the first time that synchrotron radiation is involved. It also determined a staggering distance of more than 7,000 million light-years to this GRB.
The astronomers are optimistic that the extensive observations will help them to better understand the true nature of such a dramatic event and thus to bring them nearer to the solution of one of the greatest riddles of modern astrophysics.
A prime example of international collaboration
The present story is about important new results at the front-line of current research. At the same time, it is also a fine illustration of a successful collaboration among several international teams of astronomers and the very effective way modern science functions.
It began on May 10, at 08:49 hrs Universal Time (UT), when the Burst And Transient Source Experiment (BATSE) onboard NASA's Compton Gamma-Ray Observatory (CGRO) high in orbit around the Earth, suddenly registered an intense burst of gamma-ray radiation from a direction less than 100 from the celestial south pole. Independently, the Gamma-Ray Burst Monitor (GRBM) on board the Italian-Dutch BeppoSAX satellite also detected the event (see GCN GRB Observation Report 304 [2]). Following the BATSE alert, the BeppoSAX Wide-Field Cameras (WFC) quickly localized the sky position of the burst within a circle of 3 arcmin radius in the southern constellation Chamaeleon. It was also detected by other satellites, including the ESA/NASA Ulysses spacecraft, since some years in a wide orbit around the Sun.
The event was designated GRB 990510 and the measured position was immediately distributed by BeppoSAX Mission Scientist Luigi Piro to a network of astronomers. It was also published on Circular No. 7160 of the International Astronomical Union (IAU).
From Amsterdam (The Netherlands), Paul Vreeswijk, Titus Galama, and Evert Rol of the Amsterdam/Huntsville GRB follow-up team (led by Jan van Paradijs) immediately contacted astronomers at the 1-meter telescope of the South African Astronomical Observatory (SAAO) (Sutherland, South Africa) of the PLANET network microlensing team, an international network led by Penny Sackett in Groningen (The Netherlands). There, John Menzies of SAAO and Karen Pollard (University of Canterbury, New Zealand) were about to begin the last of their 14 nights of observations, part of a continuous world-wide monitoring program looking for evidence of planets around other stars. Other PLANET sites in Australia and Tasmania where it was still nighttime were unfortunately clouded out (some observations were in fact made that night at the Mount Stromlo observatory in Australia, but they were only announced one day later).
As soon as possible - immediately after sundown and less than 9 hours after the initial burst was recorded - the PLANET observers turned their telescope and quickly obtained a series of CCD images in visual light of the sky region where the gamma-ray burst was detected, then shipped them off electronically to their Dutch colleagues [3]. Comparing the new photos with earlier ones in the digital sky archive, Vreeswijk, Galama and Rol almost immediately discovered a new, relatively bright visual source in the region of the gamma-ray burst, which they proposed as the optical counterpart of the burst, cf. their dedicated webpage at http://www.astro.uva.nl/~titus/grb990510/ .
The team then placed a message on the international Gamma-Ray Burster web-noteboard (GCN Circular 310), thereby alerting their colleagues all over the world. One hour later, the narrow-field instruments on BeppoSax identified a new X-Ray source at the same location (GCN Circular 311), thus confirming the optical identification.
All in all, a remarkable synergy of human and satellite resources!
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 -- as it was occurring on Saturday, Jan. 23, 1999. Gamma ray bursts produce more energy in a very short period than the rest of the entire Universe combined.
Because such bursts occur with no warning and typically last for just a few seconds, quick detection by orbiting spacecraft and instant notification to astronomers are critical in order to catch the bursts in the act.
The gamma-ray-burst detectors of the Burst and Transient Source Experiment (BATSE) onboard NASA's orbiting Compton Gamma Ray Observatory detected the beginning of a bright gamma ray burst. 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," Akerlof 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.
Astronomers are not certain what produces gamma ray bursts, but possible causes include the mergers of two neutron stars, two black holes, or a neutron star and a black hole, or the explosion of a so-called hypernova. A hypernova is a theorized type of supernova or exploding star.
"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."
Dr. Gehrels said 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. Orbiting telescopes detect several hundred gamma ray bursts each year.
The ROTSE project is designed and operated by a collaboration of astrophysicists from the University of Michigan and the Department of Energy's Los Alamos and Lawrence Livermore National Laboratories. The Principal Investigator for BATSE is Dr. Gerald Fishman at NASA's Marshall Space Flight Center, Huntsville, AL. The National Science Foundation provided funding for observations at Keck II.
A recently detected cosmic gamma ray burst released a hundred times more energy than previously theorized, making it the most powerful explosion since the creation of the universe in the Big Bang.
"For about one or two seconds, this burst was as luminous as all the rest of the entire universe," said Caltech professor George Djorgovski, one of the two principal investigators on the team from the California Institute of Technology, Pasadena, CA.
The team measured the distance to a faint galaxy from which the burst originated at about 12 billion light years from the Earth. The observed brightness of the burst despite this great distance implies an enormous energy release. The team's findings appear in the May 7 issue of the journal Nature.
The burst was detected on Dec. 14, 1997, by the Italian/Dutch BeppoSAX satellite and NASA's Compton Gamma Ray Observatory satellite. The Compton observatory provided detailed measurements of the total brightness of the burst, designated GRB 971214, while BeppoSAX provided its precise location, enabling follow-up observations with ground-based telescopes and NASA's Hubble Space Telescope.
"The energy released by this burst in its first few seconds staggers the imagination," said Caltech professor Shrinivas Kulkarni, the other principal investigator 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 known phenomenon in the universe. Finding such a 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.
This large amount of energy was a surprise to astronomers. "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 we are dealing with something completely unanticipated and even more exotic."
Gamma-ray bursts are mysterious flashes of high-energy radiation that appear from random directions in space and typically last a few seconds. They were first discovered by U.S. Air Force Vela satellites in the 1960s. Since then, numerous theories of their origin have been proposed, but the causes of gamma-ray bursts remain unknown. The Compton observatory has detected several thousand bursts so far.
The principal limitation in understanding the bursts was the difficulty in pinpointing their direction on the sky. Unlike visible light, gamma rays are exceedingly difficult to observe with a telescope, and the bursts' short duration exacerbates the problem. With BeppoSAX, scientists now have a tool to localize the bursts on the celestial sphere with 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. Study of the afterglows indicated that the bursts do not originate within our own galaxy, the Milky Way, but rather are associated with extremely distant galaxies.
Both 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, NY, and Dartmouth College, Hanover, NH, 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.
As the visible light from the burst afterglow faded, the Caltech team detected an extremely faint galaxy at its location, using one of the world's largest telescopes, the 10-meter Keck II telescope at 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 and provided a more detailed image of the host 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 lasted approximately 50 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. Scientists say it is possible that other forms of radiation from the burst, such as neutrinos or gravity waves, which are extremely difficult to detect, carried a hundred times more energy than that.
NASA is planning two missions to further investigate gamma-ray bursts: the High Energy Transient Experiment II (HETE II), scheduled to launch in the fall of 1999, and the Gamma Ray Large Area Space Telescope (GLAST), scheduled to launch in 2005. HETE II will be able to precisely locate gamma-ray bursts in near real-time and quickly transmit their locations to ground-based observatories, permitting rapid follow-up studies. GLAST will detect only those gamma-ray bursts that emit the highest energy gamma rays, and will be able to locate them with sufficient precision to permit coordinated observations from the ground. Because not much is known about the bursts at these high energies, the observations may permit researchers to choose among competing theories for the origin of gamma-ray bursts.
NOTE: Images of the GRB 971214 field are available at:
FTP://PAO.GSFC.NASA.GOV/newsmedia/GRB/
Information on the BeppoSAX spacecraft is available at:
Information on the Compton Gamma Ray Observatory is available at:
http://cossc.gsfc.nasa.gov/cossc/descriptions/cgro.html
Information on Gamma Ray Bursts is available at:
http://cossc.gsfc.nasa.gov/cossc/nasm/VU/overview/bursts/bursts.html
A huge cloud of high-energy gamma-rays forms a halo extending into outer space from the Milky Way, a phenomenon that cannot be accounted for by known celestial bodies, scientists from the University of California, Riverside, Clemson University and University of Chicago reported today (Tuesday, Nov. 4). The new, unexpected distribution of gamma rays, mapped by NASA's Compton Gamma Ray Observatory, forms an aurora many thousands of light years thick and possibly surrounding the entire Milky Way, the galaxy containing Earth, according to Dave Dixon, an assistant research physicist at UCR. The discovery was presented today at the meeting of the High Energy Astrophysics Division of the Astronomical Society in Estes Park, Colo. "Looking in any other wavelength, there is nothing out there that should be obviously making gamma rays. These gamma rays are providing the first evidence that some high energy process is occurring out there," said Dixon, who made the discovery with Dieter Hartmann, an astrophysicist at Clemson University, and Eric Kolaczyk, a statistician at the University of Chicago.
The three scientists analyzed data collected by the Energetic Gamma-Ray Experiment Telescope (EGRET), one of four instruments aboard the Compton Gamma Ray Observatory, which is orbiting Earth on a satellite to measure and record invisible gamma rays which cannot be detected on the ground because Earth's atmosphere absorbs them.
The visible light from stars and galaxies seen with optical telescopes represents only a fraction of the radiated energy from celestial bodies and other space phenomenon. Gamma rays are photons, or particles of light, with the highest energies of all forms of radiation, higher even than X-rays. For instance, a single gamma-ray photon seen in the newly discovered halo has about 1 billion times the energy of a photon of ordinary, visible light. Gamma rays are of great interest to astrophysicists because they may offer clues to some of the most violent events in the universe, such as the process of a dying star becoming a supernova and the birth of a galaxy.
What is so curious about the newly discovered gamma-ray cloud, Dixon said, is that the photons do not appear to be coming from any compact sources, like other galaxies or a black hole. "The reason this is interesting is that there isn't any obvious source for these gamma-rays, based on astronomical observations in other wavelengths of light," Dixon said. "That is, as far as we can tell using other telescopes, the space around our galaxy is rather empty of the kinds of things which we would expect to generate gamma rays in the observed brightness distribution."
Though no particular explanation is singled out by the current data, Dixon and Hartmann offer three possibilities -- that gamma rays are created when high-energy cosmic rays collide with photons of lower energy light, such as visible or infrared light, that they are being emitted by rapidly spinning neutron stars, or that the gamma-ray distribution is providing evidence of dark matter, the missing mass of the universe that scientists have not been able to observe directly.
The high-energy gamma rays seen in the halo could be the result of collisions of high-energy cosmic rays, in the form of electrons, traveling at near the speed of light and colliding with low energy photons they encounter is space, Dixon said. Under this scenario, the electrons transfer some of their energy to the photons of visible or infrared light, boosting them to gamma-ray energies, in a process called the "inverse Compton effect" by astrophysicists.
It has recently been reported that some other spiral galaxies that are similar to the Milky Way have a dim halo of infrared photons surrounding them, providing the "seed photons" that could be converted to gamma rays by interacting with high-energy cosmic rays.
Some galaxies are also seen to be undergoing "starbursts" -- rapid formation and destruction of massive stars in their centers. These massive stars are short-lived, and die in giant explosions called supernovae. The shock wave of energy from supernovae leads to even more star formation activity, making the center of such galaxies a caldron of violent activity. Such a starburst would generate massive amounts of cosmic rays, providing the high-energy electrons needed to generate gamma rays.
The cloud of gamma-rays detected may provide evidence that the Milky Way, too, was once a starburst galaxy, Dixon said. "That is sort of an open question right now," he said. "There seems to be unexplained evidence for such past activity in the center of the Milky Way." From Earth, the center of the Milky Way is located about 25,000 light years away, in the direction of the constellation Sagittarius. It is also possible, he said, that neutron stars, incredibly dense objects which are left over from some supernova explosions, are emitting the gamma rays. It is known that certain pulsars -- rapidly spinning neutron stars which shoot out beams of radiation like a lighthouse -- emit almost all of their energy in gamma rays and are basically invisible otherwise. These pulsars, however, would have to exist in great numbers to account for the gamma-ray halo seen by Dixon and his colleagues. "Though it is unlikely that neutron stars were formed in the galactic halo, their massive star progenitors could have lived in the plane of the Milky Way, with the neutron stars being shot out by the violent supernova explosions," he said.
Another, more intriguing, possibility is that the cloud is providing indirect evidence of dark matter. The visible galaxies of stars and planets account for only a small percentage of the total mass of the universe, according to scientists who have calculated that a far greater total mass is needed to hold celestial objects in their gravitational orbits. Physicists have described the missing matter as "dark matter," since it doesn't absorb or emit light.
A number of theories to explain dark matter have been advanced, one attributing the missing mass to weakly interacting massive particles, or WIMPs. These heavy particles -- theoretical entities not yet detected in earthbound experiments -- would not interact with light, according to the theory. But it is possible, Dixon said, that two WIMPs could occasionally collide with one another, generating gamma rays or other particles of matter and antimatter which subsequently annihilate into gamma rays. "If you look at the wide distribution of where the gamma-rays are coming from, it is very suggestive of a dark matter distribution," he said. Dixon cautioned, however, that the scientific explanation for the phenomenon is still a wide open question. The Gamma Ray Large Area Space Telescope (GLAST), planned for a future NASA mission, may help answer the puzzle, he said. "These are just three viable options, and no doubt others will be advanced in the future," he said. "The only way to nail this thing down is to get better data with a more advanced gamma-ray telescope such as GLAST, or to find some smoking gun which can be observed in another wavelength of light."
EGRET, one of the instruments aboard the Compton Gamma Ray Observatory, was designed to study high-energy gamma-ray emission. The instrument records both the direction and energy of gamma-ray photons, but this "picture" of the sky in gamma rays is noisy, similar to a television picture with a lot of static. The source of this noise is not interference of any sort, but occurs because relatively few gamma-ray photons are detected, compared with an optical telescope, for example.
Kolaczyk and Dixon developed a technique using wavelets, a relatively new signal processing tool, to remove some of this noise and get a clearer picture of the gamma-ray sky. Dixon and Hartmann, both visiting scientists at the Max-Planck Institute in Germany last summer, used the technique to map the diffuse cloud of gamma rays that appears to form a corona surrounding the Milky Way.
Their research was supported by NASA's Compton Gamma Ray Observatory Guest Investigator Program and the Max Planck Gesellschaft, Germany, with the assistance of the Compton Observatory Science Support Center. Those involved in the project in addition to Dixon, Hartmann and Kolaczyk were: Jalal Samimi of Sharif University, Tehran; Parameswaran Sreekumar of the Laboratory for High Energy Astrophysics, Goddard Space Flight Center; and Roland Diehl, Gottfried Kanbach, Hans Mayer-Hasselwander and Andy Strong of the Max Planck Institut fur Extraterrestriche Physik in Germany.
