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Gamma Rays Provide Detailed Energy Spectrum of AGN

An excess of TeV-energy gamma rays from galaxy Markarian 421 may oblige astronomers to revise their models of active galactic nuclei (AGN). The discovery challenges existing concepts of the acceleration processes close to black holes and how radiation is absorbed in space, and indicates that the universe is not as opaque as previously thought.

Last year Mrk421 rewarded patient observers with the most explosive gamma display ever, with a flux ten times higher than that of the much closer Crab Nebula, the strongest known steady gamma source in the sky. At a Friday afternoon session at the APS/AAPT Spring meeting in Washington, DC, Trevor Weekes of the Whipple Observatory presented the first detailed spectrum for Mrk421. He reported that the flux of gammas falls off at the highest energies (up past 6 TeV), but not nearly as fast as one would have expected. Weekes suggested that the anticipated effect of two sources of attenuation, dust near the AGN and the amorphous population of infrared photons in intergalactic space, may have been overestimated.

The very-high-energy gamma rays from galaxies are produced in the interaction of cosmic particles of even greater energy with ambient particles or photons in the jets apparently emanating from each pole. These cosmic particles are accelerated by a process derived from the enormous gravity of a black hole to energies in excess of those attained by man-made particle accelerators. Theoretical astrophysicists have yet to explain the processes involved.

Even more difficult to explain is how the gamma rays, once produced, can escape from the jet without interacting with lower energy photos and degrading in energy as a result. Once clear of the galaxy, the gamma rays must traverse vast regions of intergalactic space, where they could be absorbed by interacting with infrared radiation from galaxies formed in the early universe. However, the recent observations suggest that the density of infrared photons is much less than previously predicted.

"We saw many more very high energy gamma ray photons from this source than we thought we would," said James Gaidos, a professor of astrophysics at Purdue University and a member of the research team. "We had believed there were more low-energy photons out there to absorb the gamma rays, but so many are getting through to us from such a large distance that it appears there's much less interaction taking place." Since low energy photons were created in the universe at the time of galaxy formation, the number of observed photons imparts information on how the galaxies formed. A reduced number of such photons thus has direct implications for current theories of the history of the universe, particularly for galaxy formation.

Many suspect that AGNs, quasars, and indeed all the most violent celestial objects in the universe share a common energy-production architecture: a black hole, supplied by a surrounding accretion disk, broadcasting powerful jets of matter in two polar directions. Mrk421 (400 million light years away) is the closest such object whose jet axis is aimed directly at us.

Because of the shielding effect of the earth's atmosphere, gamma rays must generally be detected by earth-orbiting gamma ray telescopes such as the Compton Gamma Ray Observatory. However, if the energy is sufficiently great (TeV) they can be seen indirectly with sensitive telescopes on mountain tops. Gamma ray collision with an air molecule generates a cascade of light-emitting particles which can be detected by large optical detectors such as the Whipple Observatory's 10- meter optical reflector.

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Editor: Barrett H. Ripin