Gamma-ray Astronomy

Resources

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 place—even 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 short—less than the radius of an atom—the 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 Earth’s 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 Earth’s 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 dirt—Beppo-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 stupefying—the 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 Earth’s 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 NASA’s 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 Nature’s 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 Beginner’s 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

Gamma-ray astronomy

Gamma rays are a highly energetic form of electromagnetic radiation . The wavelength of a gamma ray is very short—less than the radius of an atom—the energy they carry can be millions of electron volts. 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 our 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 our own 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 a total of over 600. The energy of just one of these bursts has been calculated to be more than one thousand times the energy that our sun will generate in its entire 10-billion-year lifetime.

Gamma ray bursts appear uniformly across the sky, surrounding Earth in a spherical shell of fireworks. Because of the shape of the Milky Way and our location within it, the bursts would appear to be concentrated in just one area in the sky if they were coming from within our galaxy. This perfectly symmetrical distribution tells us 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 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 more than one of the bursts were generated by the same stellar object. 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 dirt—Beppo-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 intial 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 stupefying—the 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 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 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) will feature a wide field-of-view, high-resolution positional accuracy , and long-life detectors. Slated for launch in the first decade of the twenty-first century, GLAST will provide astronomers with a new tool to study gamma ray bursts, pulsars, active galactic nuclei, diffuse background radiation, and a host of other high-energy puzzles.

See also Nuclear fission; Neutron star.

Resources

books

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

periodicals

Cowen, Ron. "Catching Some Rays." Science News 139 (11 May 1991).

Folger, Tim. "Bright Fires Around Us." Discover (August 1993).

Taubes, Gary. "The Great Annihilator." Discover (June 1990).

Johanna Haaxma-Jurek

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 our line of sight as the pulsar rotates, we see a flash. The rate of the

Supernova

—The final collapse stage of a supergiant star.

Gamma-ray burst

Gamma-ray bursts are unexplained intense flashes of light that occur several times a day in distant galaxies. The bursts give off more light than anything else in the universe and then quickly fade away. They were first detected in the late 1960s when instruments on orbiting satellites picked them up. No known explosion besides the big bang is more powerful than a gamma-ray burst. (The big bang is a theory that explains the beginning of the universe as a tremendous explosion from a single point that occurred 12 to 15 billion years ago.) Gamma-ray bursts are mysterious because scientists do not know for sure what causes them or where in the sky they will occur.

Types of bursts

There are two types of gamma-ray bursts: short and long. Short bursts last no more than two seconds. Long bursts can last up to just over fifteen minutes. The shorter life of a short gamma-ray burst makes it more difficult for astronomers to study. Short bursts leave no trace of light because they have no detectable afterglow (a gleam of light that remains briefly after the original light has dissipated). In addition, weaker gamma-ray

bursts tend to be observed as shorter, since only the higher parts of the emission are observable.

Astronomers believe that each type of burst may come from a different type of cosmic explosion. To learn more about the sources of gamma-ray bursts, scientists at the National Aeronautics and Space Administration (NASA) studied the time histories of short and long bursts. They did this by counting the number of gamma-ray pulses (particles of light, called photons, that arrive at about the same time) in each burst, and by measuring the arrival time of lower-energy and high-energy pulses. The astronomers learned that short bursts had fewer pulses than long bursts and that their lag times were twenty times shorter than those of longer bursts. This suggested that both long and short bursts were produced in physically different objects.

Words to Know

Big bang theory: Theory that explains the beginning of the universe as a tremendous explosion from a single point that occurred 12 to 15 billion years ago.

Black hole: Single point of infinite mass and gravity formed when a massive star burns out its nuclear fuel and collapses under its own gravitational force.

Gamma ray: Short-wavelength, high-energy radiation formed either by the decay of radioactive elements or by nuclear reactions.

Neutron star: The dead remains of a massive star following a supernova. It is composed of an extremely dense, compact, rapidly rotating core composed of neutrons that emits varying radio waves at precise intervals.

Supernova: The explosion of a massive star at the end of its lifetime, causing it to shine more brightly than the rest of the stars in the galaxy put together.

Theories of gamma-ray burst origin

Scientists' theories about the source of gamma-ray bursts are many. Some believe that they are a result of a fusion of black holes or neutron stars. (A black hole is the remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity. A neutron star is a dead remnant of a massive star; a star dies when it uses up all of its nuclear fuel.) Others believe that supernovae or hypernovae are the cause of a gamma-ray burst. (A supernova is a typical exploding star; a hypernova also is an exploding star, but with about 100 times more power as that of supernova.) In 2000, two sets of astronomers found evidence of an iron-rich cloud near gamma-ray bursts. Since stars at the supernova stage produce iron, the scientists theorized that a supernova emitted the iron cloud just before the gamma-ray burst.

The power of a gamma-ray burst is astounding. A satellite launched by NASA in 1991 detected gamma-ray bursts at a rate of nearly one a day for almost two years. The energy of just one burst was calculated to be more than 1,000 times the energy that the Sun would generate in its almost 10-billion-year lifetime.

The most distant gamma-ray burst measured so far is one that scientists detected on January 31, 2000. To determine how far the gamma-ray burst had traveled, astronomers measured the burst's spectrum (the range of individual wavelengths of radiation produced when light is broken down by the process of spectroscopy). Astronomers estimated that the explosion that caused the burst took place near the time the Milky Way

(a galaxy that includes a few hundred billion stars, the Sun, and our solar system) was formed, or 6 billion years before our solar system was born. Viewed another way, this particular gamma-ray burst has traveled through 90 percent of the age of the universe.

Scientists study gamma-ray bursts as a way of helping to better understand the evolution of the universe.

[See also Gamma ray; Star ]