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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 5: The First Pitch
Chapter 6: What Game Are We Playing?
Glossary
The preceding chapters have provided
an overview of the wonderful dilemma scientists have encountered in trying
to explain gamma-ray bursts. Most astronomers feel that the bursts are either
cosmological in nature or are distributed in a large Galactic halo. The
overwhelming evidence of BATSE has forced the abandonment of the hypothesis
that all bursts lie in the disk of our Milky Way Galaxy.
In addition to the angular and brightness distributions, BATSE has uncovered other pieces of evidence that do not necessarily discriminate between scenarios, but still provide quantitative constraints with which any viable scenario of gamma-ray bursts must comply.
Scientists have measured the duration of each gamma-ray burst and constructed a duration distribution. Their findings show that gamma-ray bursts tend to come in two distinct sets, those with durations of approximately 1/2 second, and those with durations of approximately one minute. There is a clear shortage of bursts with durations of around two seconds. This bi-modal nature in the duration distribution of gamma-ray bursts had been hinted at by previous experimentation. With BATSE's superior measurement capability, this bi-modality has now been confirmed. Despite its clear, unambiguous presence, no universally accepted explanation for this feature of gamma-ray burst durations has yet been posited.
Equally interesting is recent research carried out by scientists regarding the luminosity, or intrinsic rate of energy output of the bursts. Although a numerical value cannot be placed on the bursts' luminosity without a correct determination of the distance scale, by analyzing the shape of the brightness distribution, constraints can be placed on the range of luminosities that the detected gamma-ray bursts must have. Astronomers have shown that regardless of the scenario in which the bursts are distributed, it is very likely that approximately 90% of all the gamma-ray bursts observed have luminosities (intrinsic energy output rates) that are within a factor of 10. This is an extraordinarily narrow range of energy output. Despite the widely varying time-profiles, spectral shapes, and burst durations, it is more likely than not that most of the observed gamma-ray bursts are approximately the same strength at the source. We still do not know what that strength is, however, we can only assert that these bursts appear to have about the same luminosity. This result may have important implications for many burst production models, and could not have been determined without the improved measurement capabilities of the BATSE experiment.
Although we have obtained improved angular and brightness distributions for the gamma-ray bursts, along with many other important facts, it is still currently impossible to determine with certainty which of the three scenarios, if any, will be the correct explanation for the bursts. The answer to these questions will not be found without further probing of the skies by BATSE and future gamma-ray burst experimentation. New analyses, such as the temporal and luminosity function analyses mentioned above show great promise for the generation of important clues. The BATSE team has recently established the capability to inform observatories on the ground of the occurrence and location of gamma-ray bursts as they occur. These observers on Earth can turn their telescopes to the burst while it is in progress. This is by far the best "mouse trap" ever built for catching these elusive events in a variety of wavelengths as they are bursting. Despite our current inability to predict the outcome of the burst quandry, we may speculate on each of the candidate's possibilities for success.
With the data acquired from BATSE, cosmological scenarios for the gamma-ray bursts have gained momentum among scientists despite the physical problems regarding the burst creation in this venue. Prior to the launch of BATSE, proponents of a cosmological burst paradigm were considered to be significantly out of the mainstream of scientific thought regarding these events. Now, the same people who once were outsiders are enjoying much more serious consideration of their ideas. This scenario explains the observed angular isotropy and the deviation in the brightness distribution extremely well. By making only minimal (some say conservative) assumptions regarding the structure of our universe, one can generate a model burst population that agrees very well with the observational data.
The Galactic Corona model requires the existence of a tremendously large spherical distribution around our Galaxy which has little, if any, direct observational evidence to support its existance in the size required for bursts. We have mentioned previously that if such a corona does exist, the gravitational forces of the Galaxy would likely warp it into a non-spherical distribution. BATSE has obtained a sufficient number of bursts to require that this halo be so large that it overlap with the Magellanic Cloud galaxies and nearly touch the halo of M31. Because a corona of burst sources is not observed around the Magellanic Clouds, one must invoke some reason why our Galaxy has a corona and these other galaxies do not. However, the irregular nature of these small satellite galaxies may lead to a plausible explanation for why they themselves have no burst halo. With the detection of more bursts, this same explanation will eventually have to be invoked for M31, a spiral galaxy just like our own (except larger) which should have a corona of burst sources if the Milky Way does also.
Astronomers must be objective in their analysis of the data to determine the correct answer for the bursts and not interpret the data to justify their particular favorite scenario. However, among the three scenarios, the cosmological scenario is perhaps the most appealing to our nature as observers, explorers, and inquisitors of the universe we live in. Completely by accident, while looking from space to detect the detonation of nuclear weapons on Earth, we have discovered a wondrous phenomenon in the universe. After further study, we came to the conclusion that these bursts were related in some way to exotic objects called neutron stars, inhabiting the disk of our Galaxy. If this were the end of the story, it would have been an interesting chapter in the history of our exploration of the universe. Instead, the story continues beyond our wildest imagination. BATSE has shown that these accidentally discovered phenomena might actually be among the most violent eruptions anywhere in the universe, releasing incomprehensible amounts of energy. They may occur at distances which our minds cannot even begin to imagine.
At times, science may confuse us with its technical language. Its mathematics may seem completely impenetrable, with no hope for us to understand it. Its concepts may seem too esoteric or too complicated for a person's mind to grasp. However, behind all of the mathematics, laboratory equipment, and technical jargon, science conveys a most powerful underlying message regarding the universe and our place in it. Through the eyes of BATSE, like so many other areas of science, we see the power, the enormity, and the incredibly beautiful and fascinating nature of our universe. As we look outward into the skies, learning more and more about the our universe, we are also looking inward, learning just how small and fragile our spaceship, the Earth, really is when compared to the vastness and hostile environs of space. This is a primary value of science, and a value innate to astrophysics in particular. As we observe and learn more about the space outside ourselves, we cannot help but to learn more about the space within.
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 5: The First Pitch
Chapter 6: What Game Are We Playing?
Glossary
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Author: Dr. John M. Horack
Curator: Bryan Walls
Responsible Official: John M. Horack