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.