A joint Fermilab/SLAC publication

Gamma plus radio equals new view of cosmic jets

04/27/09
The radio jets of several active galaxies mapped by the Very Long Baseline Array (VLBA) are inset into the Fermi Gamma-ray Space Telescope’s map of the gamma-ray sky. Credit: NASA/DOE/Fermi LAT Collaboration and NRAO/AUI/MOJAVE Team/M. Kadler

The radio jets of several active galaxies mapped by the Very Long Baseline Array (VLBA) are inset into the Fermi Gamma-ray Space Telescope’s map of the gamma-ray sky. Credit: NASA/DOE/Fermi LAT Collaboration and NRAO/AUI/MOJAVE Team/M. Kadler

The sky map and list of bright sources based on Fermi Gamma-ray Space Telescope's first three months of operation are already yielding fruit. An international team of researchers participating in the MOJAVE program has correlated radio-wave data with Fermi's gamma-ray findings to move toward a better understanding of the physics behind the universe's most energetic objects.

The team used the Very Long Baseline Array, or VLBA, a set of ten radio telescopes operated by the National Radio Astronomy Observatory. Spanning North America from Hawaii in the west to the US Virgin Islands in the east, the telescopes act as one continent-sized dish, collecting radio-wave data on some of the brightest gamma-ray sources the Fermi telescope sees. These are active galactic nuclei, swirls of gas particles around supermassive black holes that spurt two opposing jets of highly energetic particles. The radio data show that nuclei with radio jets pointed straight at Earth are more likely to be detected by Fermi in gamma-ray wavelengths. The researchers also found that the jets from the brightest gamma-ray sources flare in radio waves at the same time.

Jim Chiang, an astrophysicist with Fermi's Large Area Telescope collaboration at SLAC National Accelerator Laboratory, says the results support a model of active galactic nuclei in which the jets are responsible for both radio waves and gamma rays.

"This confirms that there is a connection between ejection of the blobs seen in radio waves and the production of gamma-ray emission," Chiang says. "Now we actually have quantitative measurements of how the models should predict these effects. We can test these models concretely in terms of numbers."

Stanford astrophysicist Peter Michelson, the lead scientist on the LAT, says he is pleased to see Fermi telescope data being used in a multi-wavelength study. Broadband examination of active galactic nuclei, he says, is one of the goals of the Fermi telescope, and critical to unraveling their mysteries.

"The bright-sources list was really intended primarily to inform the rest of the scientific community as to what we're seeing with Fermi," he says. That way telescopes working in other wavelengths, from radio waves to X-rays, can study these objects, "and we can correlate the emissions across as much of the electromagnetic spectrum as we can.

"We're pretty happy with it, and we'll see a lot more of this in the future," he adds. "This is just the start, really."

MOJAVE, which stands for Monitoring Of Jets in Active galactic nuclei with VLBA Experiments, involves astronomers throughout the United States and Europe. The results are reported in two papers in the May 1 issue of the Astrophysical Journal; they can be found here and here. For more information, see the NASA press release.

by Lauren Schenkman

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