(Originally published by SLAC)
April 28, 2011
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 of the pulsars and binary objects that have high energy densities in our Galaxy, as well as the diffuse glow from our own Galaxy's and several others' legacy of supernova particle acceleration. 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. All of these developments have been catalogued in these research highlights.
Now Fermi has brought together the astronomically near and far with observations of yet another celestial high energy process. In an analysis led by KIPAC's Igor Moskalenko and Elena Orlando, and Italian colleagues Monica Brigida and Nicolo Giglietto, the first 18 months of Fermi's Large Area Telescope (LAT) observations of the Sun have been combed through. The team reports that there are two distinct components of emission from the direction of the Sun. One component is that which is coincident with the solar disk, and the other extends outward, diminishing significantly 5 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 rays with the photons the Sun puts out, through a process called inverse-Compton scattering. In the inverse-Compton process, photons are 'kicked' to higher energies by fast moving charged particles, which is exactly what cosmic rays are. The level of such inverse-Compton gamma-ray emission from the Sun was first predicted by in a paper by Moskolenko and colleagues in 2006. The LAT observations indicate that the total energy flux of the extended component is roughly equal to that of the disk component of the Sun, and also roughly equal to that of the isotropic gamma-ray background.
The cosmic rays responsible for donating energy to solar photons to kick them up into gamma-ray energies are the tie-in for this observation to more distant parts of space. While the Sun itself is a major source of cosmic rays, the cosmic rays that can kick the solar light into gamma-rays mostly originate from supernova remnants in distant parts of the Galaxy, along with a few from distant galaxies.
The details of how cosmic rays near the Sun interact are complicated because the solar wind - the Sun's own output of charged particles and magnetic fields - can deflect external cosmic rays in the neighborhood of the Sun. The solar wind itself fluctuates with the solar cycle and flaring activity, and is the cause of the 'space weather' that affects communications near Earth. As there are no direct measurements of the cosmic ray flux nearer to the Sun than Earth, future Fermi LAT observations of the extended gamma-ray emission from the Sun will be important in understanding both solar physics and the distribution and spectrum of cosmic rays, and the interaction of the two.
This work is based on a paper to be published in the Astrophysical Journal and available from astro-ph at arXiv:1104.2093