The discovery of gamma ray emission from the distant galaxy known as Markarian 501 is the first discovery of such a 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 Trevor C. Weekes, an astrophysicist at the Smithsonian's Fred Lawrence Whipple Observatory in Arizona. He presented details of the discoveries May 1 at the American Astronomical Society meeting in San Diego.
Gamma rays are highly energetic photons. 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 generated by extremely energetic physical processes. On Earth, gamma rays this energetic are created only in the beams of the most powerful particle accelerators.
Although gamma rays cannot penetrate Earth's atmosphere, they can be detected from the ground by their secondary interactions with the Earth's atmosphere. These interactions produce a cascade of particles that cause the atmosphere to emit a faint blue light.
The Whipple collaboration has pioneered the 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.
The Whipple group detected the first extragalactic source of very-high-energy gamma rays, active galaxy Markarian 421, in 1992 at the Smithsonian's Whipple Observatory in southern Arizona. This source had been detected earlier by NASA's Compton Gamma Ray Observatory.
The gamma-ray telescopes on the Compton Observatory, however, have not been able to detect the new source, Markarian 501. The Whipple collaboration observed this galaxy in 1995, using arrays of mirrors and light detectors.
It now appears that Markarian 501 may be one of a new class of galaxies, a subset of blazars, which are among the brightest gamma-ray-emitting objects in the sky, says John Finley, assistant professor of physics at Purdue University and a member of the Whipple collaboration.
Blazars are violently active distant galaxies believed to have enormous black holes at their cores. They emit jets of material at nearly the speed of light.
It always has been assumed that the gamma-ray sources would be weaker and less bright at the high end of the spectrum, and hence easier to detect at lower energies, Finley says.
"This discovery to the contrary implies that these newly discovered galaxies are natural particle accelerators on a scale previously thought impossible," he says.
Markarian 501 is similar to Markarian 421 in many ways: It is an active galactic nucleus at the center of a giant elliptical galaxy located some 400 million light-years from the solar system.
Because blazars emit gamma rays in narrow beams jetting from opposite poles, their detection relies on the chance pointing of the 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 Purdue physics Professor Jim Gaidos, who also is a member of the Whipple collaboration.
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 the very high energies. The short time-scale of the variations -- about one day -- implies that the emission region has dimensions less than that of the solar system, Gaidos says.
"It's 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," he says.
Already the Whipple researchers believe that they have tentative evidence for emission from another blazar that is not seen in the satellite gamma-ray telescopes. Collectively, these three objects are the closest members of the sub-class of blazar known as BL Lac-type objects.
The discoveries are the result of collaborative efforts of scientists from Purdue, the Smithsonian Astrophysical Observatory, Iowa State University, the University of Michigan, the University of Leeds, United Kingdom, and University College in Dublin, Ireland.
Members of the Purdue team in addition to Finley and Gaidos are Glenn Sembroski and Charles Wilson, senior research scientists. The studies are funded by the U.S. Department of Energy.
Sources: John Finley, (765) 494-5048; Internet, firstname.lastname@example.org
Jim Gaidos, (765) 494-5171; Internet, email@example.com
Trevor C. Weekes, Harvard-Smithsonian Center for Astrophysics, (602) 670-5726
Writers: Dan Brocious, Smithsonian/Whipple Observatory, (602) 670-5706
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