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UNH Astronomers Find Gamma-Ray Flare Activity Near Unusual White Dwarf
DURHAM, N.H. -- University of New Hampshire astronomers say they may have found evidence of never-before-seen gamma-ray flare activity on a white dwarf star.

Until now, scientists have only detected similar flaring activity on our own Sun, but this source appears to be a white dwarf, the end-stage in the life of a star.

The findings by Mark McConnell, UNH research associate professor in the Institute for the Study of Earth, Oceans and Space, are being presented this week to the American Astronomical Society meeting in Washington, D.C.

"This result provides further support for the idea that solar flares are a prototype for large-scale phenomena elsewhere in the galaxy," says McConnell. "Our understanding of solar flares may therefore prove extremely useful in understanding other astrophysical phenomena."

The existence of stellar flares has been known for many years, but these are typically observed at optical, radio or x-ray wavelengths. This is the first time that evidence has been found for gamma-ray flares on an object other than the Sun.

"Our understanding of the solar flare phenomena may be directly applicable in our efforts to better understand the environment of a more exotic object like a white dwarf," says James Ryan, UNH professor of physics and McConnell's co-investigator.

The apparent source of the gamma-ray flaring in this case is white dwarf RE J0317-853. A white dwarf is achieved after a star exhausts all its nuclear fuel. White dwarfs are very compact, usually the size of the Earth, but with a mass comparable to the Sun.

RE J0317-853 is highly unusual, however. Roughly 114 light years away from Earth (by comparison, Pluto, our solar system's outermost planet, is 5.5 light hours away), its surface temperature is an extreme 50,000 degrees-Kelvin (the Sun's temperature is 10,000 degrees K), and its magnetic field is 3 million times that of the Sun. McConnell points out its temperature is the highest and magnetic field is the strongest ever recorded for a white dwarf. It also has a very rapid rotation, once every 12 minutes, compared to the Sun's 27-day rotation.

McConnell and Ryan based their results on work with the Imaging Compton Telescope, or COMPTEL, on NASA's Compton Gamma-Ray Observatory, launched in 1991 aboard the space shuttle Atlantis. Ryan directs the UNH portion of the COMPTEL project.

McConnell and Ryan concentrated on searching for line emission at an energy of 2.22 MeV (million-electron volts), which corresponds to the energy of a photon released when a free neutron recombines with a proton to produce a nucleus of heavy hydrogen. This process, called neutron capture, is a major contributor to gamma-ray emissions.

McConnell's data suggests that RE J0317-853 appears to be producing a continuous or quasi-continuous series of flares. "The data are consistent with a steady source of emission," says McConnell, "but some level of time variability cannot be ruled out. We might be witnessing several very large flares that erupt only occasionally, or we might be witnessing a continuous or nearly continuous eruption of smaller flares across the surface of the white dwarf."

In either case, the flares would be large by solar standards. The measured level of gamma-ray emission implies an amount of energy corresponding to the energy of a typical solar flare being released once every second.

Gamma-rays are the most energetic form of radiation. Among the known sources of gamma rays are supernova explosions, neutron stars and pulsars, black holes and quasars. In investigating the gamma-ray source found by COMPTEL, McConnell at first suspected a black hole or binary system as the source, but there were no known binary systems in that position in the sky.

In order to search for neutron capture emission, the COMPTEL researchers assembled a map of the entire sky spanning a small energy range centered on 2.2 MeV. The map was generated using data accumulated over the first five years of the CGRO mission.

Additional observations of white dwarf RE J0317-853 are planned for this spring, using both the COMPTEL and OSSE (Oriented Scintillation Spectrometer Experiment) experiments aboard the Compton Gamma-Ray Observatory. Observations also are being planned for the near future with the Rossi X-Ray Timing Explorer to determine the level of hard X-ray emission.

Since its launch in 1991, COMPTEL has relayed vital information to UNH researchers. It has been used to map the distribution of radioactive aluminum in the galaxy, work which sheds light on the origins of heavy elements in the universe.

There was the so-called "Super Bowl Burst," Jan. 31, 1993, the highest energy gamma rays ever detected. In 1995, the observatory detected the sudden appearance of a never-before-seen object that bursts and pulses at the same time, a bursting pulsar. And the UNH COMPTEL instrument has detected gamma rays coming off a supernova that is an estimated 15 million light years away.

Contact: Carmelle Druchniak, 603-862-1462

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