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Major League PuzzleA
Major League Puzzle
Foreword: Spring Training
Chapter 1: The Playing Field
Chapter 2: The Gamma-Ray Burst Baseball Card
Chapter 3: Watching The Game
Chapter 4: Pre-Game Analysis
Chapter 6: What Game Are We Playing? Chapter 7: Extra Innings Glossary
The first
week of April 1991 was a busy one in central Florida. Spring training was
winding down. The players had spent the past 6 weeks honing their swing,
working on the curve-ball, taking fielding practice, and playing tune-up
games. Each team had prepared itself, chosen its squad of players, and was
ready to head home. It was time for the season to begin. Time to play the
games for real, to compile the season's statistics, and to begin the long
and difficult 6-month journey towards the World Series. Few people that
Spring would have predicted that the Minnesota
Twins would eventually make it to the top that season and win the World
Series.
On the east coast of central Florida, during that very same week, "spring training" for the gamma-ray burst scientists was also winding down. The flight hardware and experiments had been calibrated and tested. The astronaut crew was trained and ready to fly the shuttle and deliver the payload to orbit. Millions of Earth-bound gamma-rays had been poured through the experiments to practice and simulate the measurements to be made in space. It was time to make these measurements for real. Time to compile the gamma-ray burst statistics and distributions. It was time to begin the long, difficult, and exciting process of understanding the nature of gamma-ray bursts. Like the eventual World Series winners that year, few people could have predicted the eventual outcome in the gamma-ray burst arena either.
BATSE's "season" began on the morning of April 5, 1991 with a spectacular launch aboard the space-shuttle Atlantis as part of mission STS-37. Several days later the GRO spacecraft was deployed from the shuttle to begin the mission of studying the gamma-ray sky. Almost immediately, BATSE began to detect gamma-ray bursts at the rate of approximately one per day. During times when bursts were not in process, BATSE took data on a myriad of other celestial sources that emit gamma-rays. One by one the bursts came in. Over time, the important brightness and angular distributions of the gamma-ray bursts were constructed. Their results were totally unexpected.
The Gamma-Ray
Burst Brightness DistributionIn Chapter 4, we played the part of the good baseball fan, compiling statistics and making analyses to predict what we thought might be seen by BATSE when the more sensitive observations were made. Our analysis rested on the assumption that bursts were created on or near objects called neutron stars that reside in the disk of our own Milky Way Galaxy. Because of the association of bursts with these neutron stars, we made two predictions; one regarding the brightness distribution and the other regarding the angular distribution. If our predictions were confirmed by BATSE, we would obtain definitive proof of the association of gamma-ray bursts with Galactic neutron stars.
Recall the special mathematical relationship in the brightness distribution of the bursts observed by limited-sensitivity, pre-BATSE instrumentation. The number of sources observed with a brightness greater than some value P was observed to be proportional to P raised to the -3/2 power, indicating that the sources being observed possessed a uniform spatial density. The limited volume of space accessible to these pre-BATSE instruments was uniformly filled with gamma-ray burst sources.
We also recall our prediction that BATSE would see beyond the edge of the distribution of objects in the Galactic disk, thereby peering into volumes of space where the density was diminished, for example above and below the disk of the Galaxy. This should result in a deviation of the observed brightness distribution away from our special mathematical relationship, and produce fewer weak bursts in our brightness distribution compared with what would be expected if the number density were constant all the way out to the maximum observable distance.
Our prediction regarding the brightness distribution was right on. Just as we anticipated, BATSE observed fewer weak bursts than expected from the special constant density mathematical relationship. Indeed BATSE, with its dramatic increase in sensitivity over previous instrumentation, had reached the edge of the distribution. Remember, however, that brightness is a quantity that depends on both intrinsic luminosity and distance. Because the intrinsic luminosity of the bursts is not known, BATSE still could not assign a value to the distance for any gamma-ray burst. The deviation in the brightness distribution simply states that the edge of the source distribution is being observed, it does not reveal the exact distance to the edge of this distribution.
The Gamma-Ray
Burst Angular DistributionHaving discovered that the brightness distribution measured by BATSE indicates that the edge of the distribution had been reached, we anticipate a highly non-uniform angular distribution on the sky for the same bursts; the gamma-ray bursts should show a tendency to align themselves with the plane of the Galactic disk. We anticipate the locations of the bursts, especially the weakest ones, to be strung out across the band of the Milky Way like pearls on a necklace. This remaining observation would confirm our prediction of the previous chapter for gamma-ray bursts associated with neutron stars in the disk of the Galaxy.
The angular distribution of bursts detected by BATSE completely contradicts and invalidates our predictions. Instead of finding the gamma-ray bursts with locations aligned with the plane of the Milky Way, the bursts' positions are completely random in the sky. There is no preferred direction to the burst locations and there is no preferred plane such as the plane of the Galaxy. All directions are equally likely to be the direction for a given burst. The angular distribution is consistent with no two bursts repeating and no tendency for bursts to cluster together. No sub-set or grouping of bursts possesses a non-uniform angular distribution. The angular distribution of bursts is almost perfectly isotropic.
This result, especially in light of the observed brightness distribution, was totally unexpected. Given the observed brightness distribution, the angular distribution must be anisotropic if bursts are located in the Galactic plane. Instead of proving that bursts were associated with neutron stars in the plane of the Milky Way, the isotropy in the angular distribution indicates that gamma-ray bursts cannot be associated with any objects in the plane of the Galaxy!
In just a few short months, fifteen years of research involving the associations between gamma-ray bursts and Galactic neutron stars was made largely obsolete. BATSE had just provided astronomers with the latest in a series of great astronomical riddles.
The reactions to these first BATSE results, which were published in Nature in January 1992, ranged from ecstasy to hostility. The gamma-ray bursts, with their combination of angular isotropy and spatial inhomogeneity, possess a distribution unlike that of any other known Galactic object. The results of BATSE indicate that the distribution of gamma-ray bursts appears to be finite in extent, as indicated by the deviation in the brightness distribution, yet also appears to be centered on the Earth, as indicated by the absence of any angular anisotropy in the celestial distribution of the events.
Several hundred years ago, when the Earth was considered to be the center of the universe, with all the planets, stars, and the heavens revolving around it, these results might have been hailed as proof of the geocentric universe. People who espoused differing beliefs were sometimes burned at the stake! However, we live in the 20th century and are now well aware that the Earth is not at the center of anything. We are not at the center of the solar system, and we are not at the center of the Galaxy. Why then do the gamma-ray bursts populate a confined distribution that is apparently centered on the Earth?
Nature has a way of answering the scientist's questions with more fundamental questions. Before BATSE, we may have asked "How are these bursts associated with Galactic neutron stars?". Now we ask questions like "What could these gamma-ray bursts be that they possess such an extremely unusual spatial distribution?" Or, more importantly than "What are they?", "Where are they?"
Foreword: Spring Training
Chapter 1: The Playing Field
Chapter 2: The Gamma-Ray Burst Baseball Card
Chapter 3: Watching The Game
Chapter 4: Pre-Game Analysis
Chapter 6: What Game Are We Playing? Chapter 7: Extra Innings Glossary
Author: Dr. John M. Horack
Curator: Bryan Walls
Responsible Official: John M. Horack