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Fermi Telescope Unites the Near and Far With Gamma Rays From the Sun

(Image - Fermi LAT data)
The left panel shows the gamma rays observed by the Fermi LAT per pixel from near the sun; the right panel shows the same for another patch of sky. There is a clear large flux from the solar disk, and a less dense but extended flux surrounding it. (Image courtesy the Fermi LAT team.)

In its revolutionary study of the gamma-ray sky, the Fermi Gamma-ray Space Telescope has thus far seen hundreds of distant active galaxies, and nearly 100 pulsars and binary objects in our galaxy, as well as the diffuse glow from particles accelerated in the aftermaths of supernovae. It has even brought the study of high-energy physics closer to home by capturing the gamma-ray output of a solar flare on the surface of the sun.

Now the Fermi Telescope has brought together the astronomically near and far with observations of yet another celestial high-energy process. A recent analysis by the telescope team, led by Igor Moskalenko and Elena Orlando of the Kavli Institute for Particle Astrophysics and Cosmology at SLAC and Stanford, and Italian colleagues Monica Brigida and Nicolo Giglietto, combed through the first 18 months of observations of the sun from Fermi's Large Area Telescope, or LAT. The team found two distinct components of gamma-ray emission from the direction of the sun. One is coincident with the solar disk; the other extends outward, diminishing significantly by five degrees on the sky away from the sun—corresponding to roughly the distance from the sun to the orbit of Mercury—but not completely disappearing until 20 degrees away.

This second, extended component is almost certainly due to the interactions of cosmic ray electrons with photons from the sun, through a process called inverse-Compton scattering. In this process, photons are "kicked" to higher energies by fast-moving charged particles—which is exactly what cosmic rays are. The LAT observations indicate that the total energy flux of this extended component is roughly equal to that of the disk component, and also roughly equal to that of the isotropic gamma-ray background.

While the sun itself is a major source of cosmic rays, the cosmic rays that can boost solar light to gamma-ray energies originate mostly from supernova remnants in distant parts of the galaxy. The details of how these cosmic rays behave near the sun are complicated because the solar wind—the sun's own output of charged particles and magnetic fields—can deflect incoming cosmic rays in the neighborhood of the sun. Future Fermi LAT observations of the extended gamma-ray emission from the sun will be important in understanding more about both solar physics and cosmic rays, and the interaction of the two.

This work is based on a paper to be published in the Astrophysical Journal.

—adapted from a research highlight by Jack Singal
SLAC Today, May 2, 2011