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
"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
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