Jan. 6, 1994
Vol. 13, No. 9

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    Evidence shows gamma ray bursts originate in Milky Way

    University scientists have found new evidence that cosmic gamma ray bursts originate from within the Milky Way -- our own galaxy -- and not from the edge of the universe, as many astronomers had believed. Jean Quashnock, Research Scientist in the Enrico Fermi Institute, and Don Lamb, Professor in Astronomy & Astrophysics, announced their results in two papers published in the Monthly Notices of the Royal Astronomical Society on Dec. 15.

    For 25 years scientists have observed these mysterious bursts of extremely high energy radiation. But because the bursts are so brief -- from less than a second to a few minutes in duration -- and because they appear randomly, no one has been able to determine what causes them, or even how far away they are. They never seemed to appear in the same place twice, Lamb said. "You didn't know when they were going to happen or where they were going to happen, and you didn't get a second chance."

    Scientists have hotly debated whether the bursts are cosmological -- from the edge of the universe -- or galactic, from within the Milky Way. "Now we have strong evidence that most gamma ray bursts come from within our galaxy," Lamb said. "The 25-year-old question about where the bursts originate may finally be answered."

    Lamb said the evidence that the bursts are galactic in origin was a surprise, "and certainly not something I was confident of initially."

    Quashnock said gamma ray bursts have been one of the most tantalizing problems in astronomy. "You've got these incredibly high energy bursts; if they were cosmological, these would have been the biggest events in the universe," he said. "And it's gamma rays and nothing else -- here and there and then gone again."

    Quashnock said that when he began studying the problem, he approached it from a decidedly cosmological perspective. But when he saw the patterns fall into place and realized the bursts were galactic, he said, "It was very exciting. For a small part of the day, I knew something that no one else in the world knew."

    Prior to the launch of NASA's Compton Gamma Ray Observatory (CGRO) satellite in April 1991, many astronomers believed that the bursts came from within the galaxy. But when astronomers looked at the data the satellite had collected, they saw a random distribution of bursts on the sky. Scientists had expected to see a concentration of bursts in the plane of the Milky Way, which would have confirmed the galactic hypothesis. When the distribution of the bursts suggested sources far beyond the realm of our galaxy, scientific opinion shifted toward a cosmological explanation.

    Quashnock and Lamb started looking more closely at the CGRO data to try to make sense of it. At first, the distribution of the bursts did appear to be totally random. But when the researchers took into account the statistical uncertainties inherent in determining the exact positions of the bursts in the sky, they began to see clumps of bursts. In fact, some were clustered so closely together they could not be distinguished from each other. In most cases, there was one bright burst with one or more fainter bursts around it.

    Lamb said these bursts are almost certainly coming from the same sources. About a third of the 38 brightest bursts the researchers studied appeared to repeat within the 10 months of observations they analyzed.

    "No one in 25 years had ever seen anything like this," Lamb said. Lamb and Quashnock analyzed data produced by the CGRO's Burst and Transient Source Experiment (BATSE), an instrument that can "see" bursts up to 10 times fainter than those seen with any previous detector.

    Lamb said that in order for the energies seen in the bursts to reach the earth, much greater energies must be generated at the source. If the bursts were cosmological, the explosions would have been so large -- on the order of 1052 ergs -- they would have completely consumed the mass of any known compact object, such as a neutron star or a black hole, and thus eliminated the possibility of a repeat burst of energy from the same source. Galactic sources would require explosions on the order of 1038 ergs, making it quite plausible that mass could be left over to generate one or more additional bursts from the same source. "The fact that sources repeat knocks out all of the cosmological models discussed to date," Lamb said.

    To probe the question further, Quashnock said he and Lamb divided the bursts into three categories based on brightness. They found that the brightest bursts were uniformly distributed across the sky. The very faintest bursts appeared to be repeats of the bright ones. But bursts of medium brightness were concentrated in the plane of, and toward the center of, the galaxy.

    Quashnock likened the distribution to that of the stars in the night sky. The ones that are nearest to the earth show up brightest and most evenly distributed on the sky. "You look at the night sky and you see stars all around you. But on a dark night you also see the Milky Way, which is made up of the rest of the stars in the plane of the galaxy, only they are much farther away," he said. When Quashnock saw the same distribution pattern show up with the gamma ray bursts, he said, "I never considered the cosmological model again after that."

    The medium-bright bursts didn't appear to repeat, Quashnock said, simply because they are bright bursts that are farther away. The instrument can't see the very faint gamma ray bursts that might be expected to be the repeats of these distant, but still galactic, bursts, he said.

    The most plausible source of the bursts, said Lamb, are galactic neutron stars. These are dense, highly evolved stars, in which 1.4 to 2 times the mass of the sun is compressed into a sphere only about 15 or 20 kilometers across. No one knows exactly how the gamma ray bursts might be generated, but one theory suggests that they might come from material (such as a comet) crashing to the surface of a neutron star. The gravitational pull of neutron stars is so great that a tremendous amount of energy would be released when the material collided with the star's surface.

    Quashnock said no one had seen the burst patterns before because the positions of the bursts were so poorly known. "If there were no ambiguity in the positions, the repeating would have been obvious," he said.

    The method of measuring of the bursts makes the exact origin of the bursts difficult to pinpoint. A burst's position is determined by measuring the ratio of photons to each of BATSE's eight detectors, which are arrayed on the corners of the satellite. To further complicate these measurements, some gamma ray bursts are reflected into BATSE's detectors by the earth's atmosphere. To correct for this effect, scientists at the Marshall Space Flight Center, BATSE's command center, use sophisticated modeling techniques. But even with these corrections, the locations of the brightest bursts can be determined only to within about four degrees.

    Lamb and Quashnock performed their analyses on 260 bursts cataloged by BATSE between April 1991 and March 1992.

    Quashnock said the next step will be to try to pin down more precisely where the burst sources are located and to try to find radiation of lower energy that may be emanating from the same sources. In early 1995, the High Energy Transient Experiment (HETE) will be launched. This may allow scientists to correlate gamma ray bursts with ultraviolet or visible radiation, for example. "HETE has a good chance of finally linking up gamma ray bursts with the rest of astronomy," Lamb said.

    -- Diana Steele