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Major League PuzzleA
Major League Puzzle
Foreword: Spring Training
Chapter 1: The Playing Field
Chapter 3: Watching The Game Chapter 4: Pre-Game Analysis Chapter 5: The First Pitch Chapter 6: What Game Are We Playing? Chapter 7: Extra Innings Glossary
Chapter 2 - The Gamma-Ray Burst Baseball Card
Baseball
cards, like this one of Pittsburgh
Pirate great Honus Wagner, are the collectable accessory of every young
fan. On the front, a picture of the player is shown, sometimes in an action
pose. Usually the position and team affiliation of the player is provided
as well. On the back of the baseball card, one finds a litany of statistics
and other information regarding the player. Date of birth, height, weight,
batting average, number of home runs, throws left, bats right; all of these
and more are provided in a synopsis of the player's credentials. The card
provides the collector and fan with a handy characterization of the player's
facets. Cards with pictures and statistics of good players can fetch a high
price at collector's conventions, while other cards are more suitable as
noise-makers in the spokes of a young person's bicycle. Like baseball players,
some gamma-ray bursts are more noteworthy than others for a variety of reasons.
Before we attempt to learn more about the nature of gamma-ray bursts, a
look at a few of their general properties, their baseball cards if you will,
is in order.
The gamma-ray bursts are among the most enigmatic phenomena observed in the sky today. Approximately once per day, at an apparently random time and in an unpredictable location of the sky, a gamma-ray burst occurs. If your eyes could see these events, they would resemble giant flashbulbs; a point-source blast of gamma-radiation which temporarily overpowers every other source in the sky.
Scientists have many methods of characterizing the gamma-ray bursts that are observed. The most commonly examined feature of a burst is its time-profile. The time-profile is a graph of the burst's brightness as a function of time. We are all familiar with the seismograph, the instrument that draws wildly fluctuating lines on a piece of paper during an earthquake. The output of the seismograph, the piece of paper with the jerky lines on it, is also a time-profile. It is a measure of the earthquake's intensity as a function of time. A gamma-ray burst time profile is essentially no different. However the intensity that is plotted for a burst is not a measure of how much the ground is shaking, but is instead a measure of how many gamma-rays are being detected from the source.
The time-profiles of bursts are as unique as fingerprints. Over 1,500 gamma-ray bursts have been detected since their initial discovery, and no two burst time-profiles are alike. This has severely curtailed scientists' attempts to classify the bursts into groups or families based on the appearance of their time-profiles. Some burst profiles are extremely smooth. The brightness of these bursts does not change very rapidly. Other burst profiles look more like the seismograph output we are familiar with, displaying large fluctuations in brightness over a short period of time. In some bursts, significant fluctuations in brightness can occur on time scales as short as 1/1000 of a second.
The time-profile shown above is for gamma-ray burst #143, a rather bright burst seen early in the BATSE mission. This time-profile, as well as those for other BATSE gamma-ray bursts, are available at the CGRO Science Support Center. Interestingly, this burst has the property that it appears to have turned on for awhile, then turn off for a period of time, and then turn back on again. This hiatus in emission is not uncommon in bursts and can last for just a few seconds, or for several hundred seconds. A burst time-profile viewed in different gamma-ray energies can have significant structural and temporal differences in each of the observed energy ranges.
After the occurrence of a gamma-ray burst, its position on the sky can often be determined with reasonable accuracy. Many regions of the sky where a burst has been detected have been extensively scrutinized with large optical telescopes on the ground. These observations are performed to search for some identifiable object that might be the source of the gamma-ray burst. If the object that created the burst could be identified, it might offer important clues towards figuring out just how and why these bursts occur. Time after time these burst positions are completely void of any viable candidate objects that may have produced the burst we have observed. Astronomers have not found one known object that is located in a position consistent with that of an observed gamma-ray burst and that is capable of producing the observed eruption of the gamma-rays.
Astronomers have also attempted to detect bursts in progress in other ranges of the electromagnetic spectrum, such as optical, infrared, and radio waves. To date, no such radiation has been observed. It appears that gamma-ray bursts emit radiation only in the gamma-ray regime of the electromagnetic spectrum. This is a most unusual property for any celestial object. The Sun, for example, can be observed in nearly every range of the electromagnetic spectrum, from radio-waves up to gamma-rays. Bursts, however, show themselves only in the high-energy region of gamma-rays.
Perhaps the most fundamental question regarding the bursts is their distance. Astronomers still have no way to determine how far away these gamma-ray bursts are occurring. The closest and farthest possible distances currently estimated for gamma-ray bursts are a factor of 12 trillion apart. This is the equivalent difference between one inch and 190,000,000 miles. We really do not even have a good estimate, much less know for certain how far away the gamma-ray bursts are.
Without a distance scale for the gamma-ray bursts, the cause of these events remains a mystery. It is clear, however, that the gamma-ray bursts represent a release of energy that is so large, the Sun may require 1,000 years or more to emit the same amount of energy that gamma-ray bursts can release in about 10 seconds.
The discovery of these perplexing events was completely by accident. In the late 1960's, the United States' government orbited a series of satellites called the Vela Network. Their purpose was to serve as a monitor and detector of any clandestine nuclear weapons tests or explosions which other nations might perform in the Earth's upper atmosphere. These satellites were equipped with detectors to collect the gamma-radiation emitted in such nuclear weapons detonations. However, instead of detecting bursts of gamma-radiation from man-made sources, the universe was serendipitously found to be producing gamma-ray bursts of its own.
The announcement of the discovery of gamma-ray bursts was made by a group of scientists from Los Alamos, New Mexico in 1973. Thus when virtually all of today's current gamma-ray burst scientists were born (or in many cases were finished with graduate school), the phenomenon they are intensively studying was not even known to exist! Aspiring young scientists need not always worry about their particular field of research, as it may yet remain to be discovered.
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In the quarter-century since their discovery, many satellites and experiments have been built to detect and analyze these events. The United States is only one of several nations that have a scientific program to study gamma-ray bursts. Pioneer Venus Orbiter (PVO) is one example of a U.S. spacecraft which carried a gamma-ray burst detector. Other U.S. detectors were flown aboard Apollo missions to the moon, Spacelab missions on the space-shuttle, and aboard interplanetary spacecraft such as Ulysses and the Mars Observer. Scientists at NASA, government laboratories, as well as many leading universities are all actively exploring these bursts. In addition to the recent efforts of American science teams and their experiments, the Japanese have orbited an astrophysics satellite named GINGA that carried a gamma-ray burst detector. The former Soviet Union and the nation of Russia have launched many gamma-ray burst detectors aboard spacecraft such as Venera 11, Venera 12, Prognoz, and the astrophysics satellite GRANAT. Canadian, French, German, and Italian scientists have also played major experimental and theoretical roles in the study of gamma-ray bursts. The study of this phenomenon is truly an international effort.
Despite the large number, all of these experiments are somewhat limited in their sensitivity to detect gamma-ray bursts. They are rather small, and only the brightest bursts are therefore visible to these experiments. The Burst and Transient Source Experiment (BATSE), currently in operation aboard NASA's Compton Gamma Ray Observatory, is the latest entry to the list of gamma-ray burst detectors. BATSE has a sensitivity which is approximately a factor of ten better than any previously flown experiment, and also has the capability to individually localize each burst detected to position on the sky. Because of its tremendously improved sensitivity, BATSE detected more bursts in its first three years of operation than all previous instrumentation combined. As we shall see later, the data obtained by NASA's BATSE experiment has single-handedly revolutionized the field of gamma-ray burst astrophysics.
SUPPLEMENT II. More About BurstsGamma-ray bursts hold the unusual distinction of being among the most widely observed yet most poorly understood astrophysical processes. It is almost impossible to think of another phenomenon that can be observed on a daily basis without divulging at least some of the secrets behind its origin.
Most of the photons associated with a gamma-ray burst are observed to have energies between about 100 and 300 keV. Bursts have been observed at gamma-ray energies as low as a few keV and also at energies exceeding several hundred million-eV (MeV). However, the bulk of the radiation is observed to be in the range of a few hundred keV. This is one of the most unusual properties of gamma-ray bursts. They do not appear to release any other form of radiation other than gamma-rays. One would naively expect that if gamma-ray bursts are really some form of explosion or violent release of energy, they should be visible in many different regions of the electromagnetic spectrum. Most explosions we are familiar with on Earth can release other forms of energy in addition to light, such as heat and noise. There is currently no viable explanation for the paucity of radiation at other wavelengths accompanying a gamma-ray burst.
As we have observed, photon energies are usually measured in units of electron-Volts. Burst energies, however, are measured in units of ergs. The conversion between ergs and electron-Volts is rather easy. One erg is equivalent to approximately 624 million keV. In any given second when a gamma-ray burst is in process, a detector with a collection area of 1 square centimeter can collect approximately between one ten-millionth and one one-hundred-thousandth ergs worth of gamma-rays. Although the amount of energy that is actually collected during a gamma-ray burst is small, the great distances at which the bursts must occur imply a tremendous amount of energy actually being released at the source. We are only able to intercept a small amount of that energy with our detectors because we are so far away from the actual event.
Astronomers characterize the brightness of a gamma-ray burst in terms of its peak flux, or the maximum amount of energy per unit area and unit time collected during the burst. The "per unit area and unit time" stipulation is required, because larger detectors will collect more energy per second from a burst than will smaller detectors. The concept is analogous to collecting rain with a bucket. If you have a larger bucket, you will collect more water in a given amount of time during a rainstorm. To measure just how hard it is raining, you need not only count how much water you have collected, but also factor in the size of your bucket as well as how long you stood out in the rain. The units of peak flux are therefore ergs per square centimeter per second. The smallest peak fluxes observed by BATSE are in the range of a few times 0.00000001 ergs per square centimeter per second, while the largest peak flux ever observed in a burst is approximately 0.0015 ergs per square centimeter per second. This is a large range, nearly a factor of 100,000 between the brightest burst ever observed and the weakest bursts observed.
The brightness of the gamma-ray bursts is not the only property that has a large range of possible values. Gamma-ray burst durations, the lengths of time over which the bursts occur, are also quite variable. The shortest burst ever detected lasted for only about 16/1000 of a second, roughly the time it takes to snap your fingers. The longest burst, on the other hand, was observed to last over 1,000 seconds. This burst is nearly 100,000 times longer than the shortest burst observed.
Other celestial phenomenon can sometimes possess observational features that appear similar in nature to a gamma-ray burst. Solar flares, tremendous magnetic storms on the surface of the Sun, are one example of an event with a time-profile that can, in some cases, look like a true gamma-ray burst. Usually, however, these intervening events are easily distinguished from real gamma-ray bursts through some other observed property. Solar flares obviously come from the Sun. If an event is detected with a time-profile similar to that of a gamma-ray burst or solar flare, and its direction is consistent with coming from the Sun, one can probably deduce that this event was solar in origin, not a true cosmic gamma-ray burst. The gamma-ray emission of solar flares is also often accompanied by a very soft X-ray glow that is not present in the time-profiles of gamma-ray bursts.
Foreword: Spring Training
Chapter 1: The Playing Field
Chapter 3: Watching The Game Chapter 4: Pre-Game Analysis Chapter 5: The First Pitch Chapter 6: What Game Are We Playing? Chapter 7: Extra Innings Glossary