David Aguilar
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Christine Pulliam
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CfA Press Release


(May 1, 1996 HEAD-AAS Meeting)

Using simple ground-based methods, an international group ofscientists has discovered a new class of active galaxy. Thesegalaxies, believed to be powered by massive black holes, radiategamma-rays at shorter wavelengths than can be seen by anyEarth-orbiting gamma-ray satellite.

The discovery of gamma-ray emission from the distant galaxy knownas Markarian 501 marks the first discovery of a gamma-ray sourcebased solely on observations from a ground-based gamma-rayobservatory.

"This marks the birth of a new branch of astronomy, one that canbe used to explore an exotic universe populated by explodingstars and massive black holes," says Iowa State Universityphysics professor, Richard Lamb.

The ground-based collaboration detected the first extragalacticsource of very-high-energy gamma-rays, active galaxy Markarian421, in 1992 at the Smithsonian's Whipple Observatory in southernArizona. This source was one of a number of such objectsdetected by NASA's Compton Gamma Ray Observatory (in orbit sinceApril 1991).

The gamma-ray telescopes on the Compton Observatory, however, have not been able to detect the new source, Markarian 501. Butwith arrays of mirrors and light detectors, on a mountain top insouthern Arizona, the Whipple collaboration observed this galaxyin 1995. Now it appears that it may be one of a new class ofgalaxies, a subset of Blazars, which are unique in being amongthe brightest objects in the sky at the shortest gamma-raywavelengths.

Blazars are violently active distant galaxies believed to haveenormous black holes at their cores, and, characteristically toemit jets of material moving at nearly the speed of light. Because of the difficulty of accelerating the very-high-energyparticles necessary to produce gamma rays, it has always beenassumed that gamma-ray sources would be weaker at very-highenergies and hence easier to detect at lower energies. Thisdiscovery to the contrary implies that these galaxies are naturalparticle accelerators on a scale hitherto thought impossible.

Markarian 501 is similar to Markarian 421 in many ways: it is anactive galactic nucleus at the center of a giant ellipticalgalaxy located some 400 million light-years from the solarsystem.

Because the gamma ray emission is in the form of narrow beams ofvery-high-energy particles, their detection relies on the chancepointing of the beam in the direction of the solar system.

"These two galaxies may be the tip of the iceberg with many moresystems undetected because they do not happen to point in ourdirection," says Smithsonian physicist, James Buckley.

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

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

"It is incredible that the major fraction of the energy emittedby these galaxies comes from such a small region of space andmost of it at these very short wavelengths," says University ofMichigan physicist, Michael Schubnell.

Already the Whipple research team believes that they havetentative evidence for emission from another Blazar (which is not seen in the satellite gamma-ray telescopes). Collectively, these three objects are closest to the sub-class ofBlazar known as BL Lac-type objects.

"It is extraordinary that the high-energy-particle activity inthese distant galaxies can be seen as a faint blue glow in theEarth's atmosphere, 400 million light-years away," saysSmithsonian Astrophysicist, Trevor Weekes.

The latest discoveries are the result of a detector developmentprogram funded by the U.S. Department of Energy. The WhippleGamma Ray Collaboration involves research groups at theSmithsonian Astrophysical Observatory, Iowa State University, theUniversity of Michigan and Purdue University as well asresearchers at the University of Leeds, U.K., and UniversityCollege, Dublin, Ireland. The Collaboration has also workedclosely with gamma-ray astronomers from the EGRET experiment onCGRO to correlate their observations.

Detecting gamma rays

With energies hundreds of billions of times greater than photonsof visible light and wavelengths much smaller than the nucleus ofan atom, the very-high- energy gamma rays are in the portion ofthe electromagnetic spectrum generated by the most violentphysical processes. In general such high-energy gamma rays areproduced in the interaction of even higher energy particles; onEarth they can only be seen in the beams of the most powerfulparticle accelerators.

Although gamma rays cannot penetrate into the Earth's atmospherethey can be detected from the ground by their secondaryinteractions with the Earth's atmosphere. The gamma-rayinteraction high in the atmosphere produces a cascade ofparticles which cause the atmosphere to emit a faint blue light.The Whipple Observatory Gamma Ray Collaboration has pioneered thedetection technique that can detect these gamma rays with highsensitivity. It uses giant optical reflectors, arrays of lightdetecting devices and fast pulse counting electronics.

T.C.Weekes, Whipple Observatory

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