Iowa State University

Richard Lamb, Physics and Astronomy, (515) 294-3873
David Carter-Lewis, Physics and Astronomy, (515) 294-8269
Skip Derra, News Service, (515) 294-4917


AMES, Iowa -- Using simple ground-based methods, an international group of scientists has discovered a new class of active galaxy believed to be powered by massive black holes. These galaxies radiate gamma rays at shorter wavelengths than can be seen by any Earth-orbiting gamma ray satellite.

The discovery of gamma ray emissions from a distant galaxy called Markarian 501 is the first discovery of a gamma ray source based solely on observations from a ground-based observatory.

"This marks the birth of a new branch of astronomy, one that can be used to explore an exotic universe populated by exploding stars and massive black holes," says Richard Lamb, an ISU physics and astronomy professor and collaborator on the project.

Lamb and fellow ISU physics and astronomy professor David Carter-Lewis are part of the Whipple Gamma Ray Collaboration, a team of researchers from Iowa State; the Smithsonian Astrophysical Observatory, Amado, Ariz.; University of Michigan, Ann Arbor; Purdue University, West Lafayette, Ind.; University of Leeds, United Kingdom; and University College, Dublin, Ireland. The team presented their findings at the American Astronomical Society meeting in San Diego, Calif., on April 30.

This team in 1992 detected the first extragalactic source of very-high-energy gamma rays, from a galaxy called Markarian 421, using the Smithsonian's Whipple Observatory. This source was one of a number of such objects detected by NASA's Compton Gamma Ray Observatory (in orbit since April 1991).

The gamma ray telescopes on the Compton Observatory, however, have not been able to detect the new source, Markarian 501. But with arrays of mirrors and light detectors, on a mountain top in southern Arizona, the researchers observed this galaxy in 1995. Now it appears that it may be one of a new class of galaxies, a subset of Blazars, which are among the brightest objects in the sky at the shortest gamma ray wavelengths.

Blazars are violently active, distant galaxies believed to have enormous black holes at their cores. They characteristically emit jets of material approaching the speed of light. Because of the difficulty of accelerating the very-high-energy particles necessary to produce gamma rays, it has always been assumed that gamma ray sources would be weaker at very-high energies and hence easier to detect at lower energies. This discovery to the contrary implies that these galaxies are natural particle accelerators on a scale that had been thought impossible.

Markarian 501 is similar to Markarian 421 in many ways: it has an active galactic nucleus at the center of a giant elliptical galaxy located some 400 million light-years from the solar system. (A light year is the distance light travels in a year, more than 5.8 trillion miles.)

Because the gamma ray emission is in the form of narrow beams of very-high-energy particles, their detection relies on the chance pointing of a galaxy's beam in the direction of the solar system.

"These two galaxies may be the tip of the iceberg with many more systems undetected because they do not happen to point in our direction," says Smithsonian physicist and collaborator James Buckley.

The energy released by these Blazars varies over time. A striking feature of the new observations is that this variability is even more pronounced at very high energies. Interestingly, these gamma ray variations appear to correlate with changes in the longer X-ray and ultra-violet wavelengths.

The short time scale of the variations (about one day) implies that the emission region has dimensions less than that of the solar system.

"It is incredible that the major fraction of the energy emitted by these galaxies comes from such a small region of space and most of it at these very short wavelengths," says ISU's Carter-Lewis.

The Whipple research team believes that they have tentative evidence for emission from two more Blazars (neither of which has been detected by satellite gamma ray telescopes). Collectively, these four objects are closest to the sub-class of Blazar called BL Lac-type objects.

"It is extraordinary that the high-energy particle activity in these distant galaxies 400 million light-years away can be seen as a faint blue glow in the Earth's atmosphere," says Smithsonian astrophysicist Trevor Weekes.

The latest discoveries are the result of a detector development program funded by the U.S. Department of Energy. The Whipple Collaboration researchers have also worked closely with gamma ray astronomers from the EGRET experiment on the Compton Gamma Ray Observatory to correlate their observations.

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Dick Lamb, ISU professor of physics and astronomy, is taking part in the American Astronomical Society presentation by the Whipple Collaboration team. His hotel's phone number in San Diego is (619) 488-1081.

Detecting gamma rays

With energies hundreds of billions of times greater than photons of visible light and wavelengths much smaller than the nucleus of an atom, the very-high-energy gamma rays are in the portion of the electromagnetic spectrum generated by the most violent physical processes. In general such high- energy gamma rays are produced in the interaction of even higher energy particles; on Earth they can only be seen in the beams of the most powerful particle accelerators.

Although gamma rays cannot penetrate into the Earth's atmosphere they can be detected from the ground by their secondary interactions with the Earth's atmosphere. The gamma ray interaction high in the atmosphere produces a cascade of particles which cause the atmosphere to emit a faint blue light. The Whipple Observatory Gamma Ray Collaboration has pioneered the detection technique that can detect these gamma rays with high sensitivity. It uses giant optical reflectors, arrays of light detecting devices and fast pulse counting electronics.

Trevor Weekes, Whipple Observatory

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