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Gamma-ray Astronomy

Gamma-ray Astronomy

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Gamma-ray astronomy is the study of gamma rays within the universe. Beginning in the 1940s and 1950s astronomers and other physicists were studying the processes that were taking place in the universe.

From their research, they were confident that gamma-ray emissions were taking placeeven thought they could not detect any of them. From the scientific research of Eugene Feenberg, Henry Primakoff, Satio Hayakawa, Franklin Hutchinson, Philip Morrison, and others, the field of gamma-ray astronomy developed.

Gamma rays, often symbolized with the Greek letter γ, are a highly energetic form of electromagnetic radiation. The wavelength of a gamma ray is very shortless than the radius of an atomthe energy they carry can be millions of electron volts (a unit of energy that is equal to one volt times the charge of a single electron). Gamma rays originate in the nucleus of an atom, and are created when cosmic rays collide with atoms in molecules of gas. In the collision, the nucleus of the atom is destroyed, and gamma rays are emitted.

Gamma rays are emitted from a variety of sources, including neutron stars, black holes, supernovas, and even the sun. Observations at gamma-ray energies allow astronomers to study objects that are not highly visible in other spectral regions. For example, Geminga, a pulsar located in Orion, is more visible in the gamma ray region than at any other wavelength. Because gamma rays identify locations of extreme particle acceleration processes, and are emitted by the interaction of interstellar gas with cosmic rays, they provide scientists with a tool to study both phenomena. Gamma rays can also help scientists learn more about active galactic nuclei and the process of star formation.

Gamma rays are as perplexing as they are informative, however. In 1979, instruments aboard several satellites recorded an ultra-high intensity burst of electromagnetic radiation passing through the Earths solar system. When astronomers monitoring the satellites discovered this phenomenon, they tried to explain it. All that was known for certain was that the radiation was caused by gamma rays.

Since the 1979 incident, gamma rays have been observed occurring in short bursts several times a day as brief high-energy flashes. Most astronomers believed their origin was from within the Milky Way galaxy. In 1991, NASA launched its Compton Gamma Ray Observatory satellite. For more than two years the Compton Observatory detected gamma ray bursts at a rate of nearly one a day for over 600 days. The energy of just one of these bursts has been calculated to be more than one thousand times the energy that the sun will generate in its entire 10-billion-year lifetime.

Gamma ray bursts appear uniformly across the sky, surrounding the Earth in a spherical shell of fireworks. Because of the shape of the Milky Way and the Earths location within it, the bursts would appear to be concentrated in just one area in the sky if they were coming from within the galaxy. This perfectly symmetrical distribution tells astronomers that these gamma rays originate far outside the Milky Way.

The late 1990s turned gamma ray astronomy on its ear. For years, it was accepted that gamma ray bursts never appeared in the same location twice, which led to theories that the pulses of radiation were generated by colliding neutron stars, or other catastrophic cosmic events. Then in October of 1996, the Compton observatory captured two bursts from the same region of the sky: a 100-second (s) pulse followed 15 minutes later by a 0.9-s pulse. Two days later, gamma rays flared again in the same spot, in a 30-s burst followed by a 23-minute burst 11 minutes afterward. Although scientists are still unclear on the cause of the radiation, many are certain that the same stellar object generated more than one of the bursts. If they are correct, then annihilation-based theories of gamma ray burst generation are invalid, and science must look elsewhere for answers to the riddle.

In 1996, an Italian and Dutch collaboration launched the Beppo-SAX orbiting observatory, designed to pinpoint the location of gamma ray bursts. In 1998, the investigators hit pay dirtBeppo-SAX registered a burst that was determined to be larger than any other cosmic explosion yet detected, except for the big bang. At the time, though, no one was particularly excited. The intensity of the burst, as measured by the Compton observatory, appeared to be nothing unusual. As the gamma rays faded into an afterglow that included lower-energy radiation such as x rays, astronomers worldwide continued to monitor the output. Then, two weeks after the initial burst, a faint galaxy was discovered in the spot from which the gamma ray burst emerged.

Calculations showed that the galaxy is more than 12 million light-years away from Earth. This data, combined with the burst intensity measured by the Compton observatory, allowed scientists to calculate the total energy released by the event. The numbers were stupefyingthe gamma ray burst released 3 x 1053 ergs of energy, several hundred times the amount released by a supernova. If the calculations are accurate and the faint galaxy really was the source of the gamma ray burst, the 1998 event was the largest cosmic explosion ever detected, except for the big bang.

In January 1999, astronomers made a giant leap forward in the study of gamma ray bursts when a complex net of observatories captured a gamma ray burst as it took place. Previously, gamma ray bursts

KEY TERMS

Black hole A supermassive object with such a strong gravitational field that nothing, not even light, can escape it.

Neutron star The remnant of an extinct supernova. Next to black holes, neutron stars are the most dense objects in the universe.

Pulsar A rapidly spinning neutron star with its magnetic axis inclined relative to its rotation axis. Radiation streams continuously from the pulsar along its magnetic axis, so if the magnetic axis passes through the Earths line of sight as the pulsar rotates, astronomers see a flash.

Supernova The final collapse stage of a super-giant star.

had only been observed after the fact. The Burst and Transient Source Experiment, aboard the Compton observatory, captured a burst of gamma rays, simultaneously notifying a computer at NASAs Goddard Space Flight Center in Greenbelt, Maryland. The computer passed a message across the Internet to activate an observatory in Los Alamos, New Mexico, which automatically began making observations. Meanwhile, scientists at Beppo-SAX were called in to identify the location of the gamma ray source.

NASA and the scientific community have proposed a new orbital gamma ray telescope. The high-sensitivity gamma-ray large area space telescope (GLAST), a joint venture of the U.S. Department of Energy and NASA, will feature a wide field-of-view, high-resolution positional accuracy, and long-life detectors. Slated for launch in the fall of 2007, GLAST will provide astronomers with a new tool to study gamma-ray bursts, pulsars, active galactic nuclei, dark matter, diffuse background radiation, and a host of other high-energy puzzles. GLAST will carry the large area telescope (LAT), which is an imaging gamma-ray detector to find photons with energy from 30 million to 300 billion electron volts. It will also carry the burst monitor (GBM), which contains 14 scintillation detectors to find bursts of photons from 5 thousand to 25 million electron volts.

See also Nuclear fission; Neutron star.

Resources

BOOKS

Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw-Hill, 2006.

Aveni, Anthony F. Uncommon Sense: Understanding Natures Truths Across Time and Culture. Boulder, CO: University Press of Colorado, 2006.

Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.

Chaisson, Eric. Astronomy: A Beginners Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.

Cheng, K.S., and Gustavo E. Romero. Cosmic Gamma-Ray Sources. Dordrecht, Netherlands, and Boston, MA: Kluwer Academic, 2004.

Schonfelder, Volker. The Universe in Gamma Rays. Berlin, Germany, and New York: Springer, 2001.

Johanna Haaxma-Jurek

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