David Aguilar
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Christine Pulliam
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CfA Press Release
 Mount Hopkins, Arizona--The detection of very-high-energy gamma rays from X-ray emitting galaxies may signal the existence of a new constituent of the cosmos--"extreme" galaxies.

This discovery, based on data taken this past winter at the Fred Lawrence Whipple Observatory by an international team of astrophysicists, confirms the hypothesis of an Italian group that galaxies which emit strongly in the highest energy X-rays would also emit the most energetic gamma rays.

The Whipple group detected the first of these galaxies, Markarian 501, in 1996. This detection was confirmed by several other observatories during a dramatic outburst in 1997. The second such galaxy, 1ES2344+514, gave hints of its nature during observations in 1998, but was not confirmed until recent months.

The third "extreme" galaxy, 1H1426+428, was the first selected for observation in gamma rays because of its strong X-ray signal.

"Its discovery is a confirmation of our understanding of these powerful emissions," says Whipple Gamma Ray Project spokesperson Trevor C. Weekes.

With energies many billions of times greater than photons of visible light and wavelengths smaller than the nucleus of an atom, these gamma rays are at the top end of the electromagnetic spectrum. Only the most violent physical processes in the universe can produce them. Gamma rays of these tera-electron-volt (TeV) energies can be created on Earth only by the collision of particles in the most powerful particle accelerators.

"The 'cosmic particle accelerators' in these extreme galaxies are more efficient than previously thought and may be important sources of cosmic radiation," says Washington University physicist James H. Buckley.

The conventional explanation for these gamma-ray galaxies, dubbed "extreme" by Italian theorists at the Osservatorio Astronomico di Brera, Milan, is that the high energy emissions arise in jets of high energy particles originating in the vicinity of supermassive black holes. The gamma-rays are emitted in narrow beams and, in the case of these detections, the beams are pointed in our direction.

High energy gamma-ray telescope observations made with NASA's Compton Gamma Ray Observatory over nine years produced a comprehensive catalog which included some 70 gamma-ray emitting galaxies. Surprisingly, these three "extreme" sources were not in the catalog, indicating these are galaxies which are most luminous at the very high energies.

What is unusual about these galaxies is that the energies of the gamma rays are ten to 100 times greater than the GRO galaxies. In fact, these are among the highest energy photons ever detected from cosmic sources.

"Although more than a billion light-years away, variations on time-scales of days are clearly seen," says Smithsonian Predoctoral Fellow Deirdre R. Horan.

The discovery of the TeV gamma-ray emission from these galaxies is the culmination of the development of an unusual detection method. At the Smithsonian Institution's Fred Lawrence Whipple Observatory, the Whipple team has developed a unique telescope which combines techniques borrowed from high-energy particle physics, solar energy research, and optical astronomy.

"When high-energy gamma-ray photons collide with molecules of oxygen or nitrogen 20 km up in the atmosphere, they produce a shower of electrons and positrons, which we observe as a flash of light lasting a few billionths of a second," explains Whipple physicist Hussein Badran.

"By focusing the light collected with a large optical reflector onto a camera made of fast, sensitive, light detectors, the gamma rays are identified and their source in the sky pinpointed."

Over the past two decades, the sensitivity of these detectors has improved significantly and the technique has been copied at a number of overseas observatories.

The Whipple teams consists of U.S. scientists from Iowa State University, Kansas State University, Purdue University, the Smithsonian Astrophysical Observatory, the University of Californian at Los Angeles, the University of Chicago, the University of Utah, and

Washington University, as well as scientists from the National University of Ireland, Dublin, Ireland, and the University of Leeds, United Kingdom. This team is now engaged in the construction of VERITAS, an array of seven large reflectors which will give a factor of twenty improvement in sensitivity over the present instrument.

A photo of the Whipple Observatory 10-meter gamma-ray reflector, a figure depicting the technique used in ground-based gamma-ray astronomy, and a model of a gamma-ray emitting AGN are available at http://egret.sao.arizona.edu/agnpressrelease.

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