Iowa State University


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



AMES, Iowa -- Astronomers are having a difficult time explaining how a distant galaxy can emit gamma rays at extremely high energies.

The galaxy, called Markarian 421, is challenging conventional theories of particle acceleration processes driven by black holes, said David Carter-Lewis, an Iowa State University physics and astronomy professor. Carter-Lewis is a member of an international team that made observations of Markarian 421. The observations also indicate that the universe is not as opaque at these energies as previously thought.

The very high-energy gamma rays were detected last May from an unusually powerful flare coming from Markarian 421. Gamma- ray emissions from this galaxy were first detected by the space-based Compton Gamma Ray Observatory and confirmed by a ground-based gamma-ray telescope at the Whipple Observatory in southern Arizona. Although the galaxy emits radiation at radio, optical, and x-ray wavelengths, during this flare it emitted most of its power in very high-energy gamma rays.

"The flare is of unprecedented magnitude," said Carter-Lewis. "It's a factor of 30 brighter than normal and a factor of three larger than anything we've ever seen."

Carter-Lewis, ISU graduate students Jeff Zweerink, Frank Samuelson and Gora Mohanty, and research associates Frank Krennrich and Mike Catanese are part of the team that made the observations. Other team members are from the Smithsonian Astrophysical Observatory, Amado, Ariz.; Purdue University, West Lafayette, Ind.; University College, Dublin, Ireland; and University of Leeds, United Kingdom.

Zweerink will present the team's results today (April 18) at the annual spring meeting of the American Physical Society in Washington, D.C.

With energies trillions of times greater than visible light and wavelengths smaller than the nucleus of an atom, gamma radiation is in the portion of the electromagnetic spectrum generated by nature's most violent physical processes. Because of the shielding effect of Earth's atmosphere, gamma rays are usually detected by Earth-orbiting telescopes such as Compton. However, if the energy is sufficiently great it can be seen indirectly with sensitive telescopes located on mountain tops.

Markarian 421 is an elliptical galaxy some 400 million light- years from Earth. (A light year is the distance light travels in one year's time, more than 5.8 trillion miles.) It belongs to a sub-class of galaxies that have an active galactic nucleus theorized to be driven by a rotating super-massive black hole.

"These are remarkable objects," Carter-Lewis says. "All models of these have a black hole at its center that's eating nearby material to get its energy, and then it spews material out into intergalactic space in the form of a jet."

A jet of material is emanating from each of the two poles of Markarian 421. The axis of one of the jets is pointing in our direction.

"What's going on inside Markarian 421 is fundamentally different than what goes on in a star," Carter-Lewis said. "Most astronomical objects give off thermal radiation. This is extremely non-thermal."

Theorists think that these very high-energy gamma rays are produced in the interaction of cosmic particles at even greater energy with ambient particles or photons in these jets. The cosmic particles may be electrons or protons and they must be accelerated by a process derived from the enormous gravity of a black hole. The energies they achieve are greater than those attained by human-built atom smashers, like at Fermilab near Chicago.

If the primary particles are protons, their energies must be millions of times greater than the observed very high-energy gamma rays. In addition, the extremely short duration of the flares seen in Markarian 421 (one was less than 30 minutes long) imply that the emission region is very small, comparable in size to our solar system. However, the energy it emitted is at levels normally associated with whole galaxies.

Even more difficult to explain is how the gamma rays, once produced, can escape from the environs of the jet without interacting with lower energy photons and degrading in energy.

"These gamma rays can collide with starlight between galaxies," Carter-Lewis said. He added that an intriguing possibility is to use the spectrum obtained from these flares as a probe of intergalactic starlight.

The observation team pioneered a technique to identify these gamma rays from various types of background radiation. The gamma rays are detected as distinct events as they strike Earth's upper atmosphere. Their 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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News releases announcing the Whipple Observatory Gamma Ray Collaboration's discovery are being issued simultaneously by the Harvard-Smithsonian Center for Astrophysics, Iowa State University and Purdue University


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