Let the Sun Shine (In Gamma Rays)

May 5, 2015

The Fermi Gamma-ray Space Telescope, famous for probing the Galaxy and distant reaches of the Universe, has now seen its first flare from our own Sun.

Ultraviolet light image of a solar flare, seen as a bright patch by NASA's Solar Dynamics Observatory (SDO) satellite. One of the three main instruments on SDO was built at Stanford.

Since gamma rays are the highest energy photons of light, whatever is emitting them necessarily involves high energy physics. When it comes to astrophysical gamma rays, the emitting processes are often at energy scales and sizes way beyond what is achievable in manmade laboratories. The sizes and energies of gamma ray emitters in the Universe are, in every sense of the word, astronomical. For this and other reasons, the Fermi Gamma-ray Space Telescope has been one of the major bridges between and - unifiers of - astrophysics and high energy physics.
Fermi has thus far seen hundreds of distant active galaxies where interactions of matter and supermassive black holes result in enormously energetic jets of particles and light pointed right at us, as well as several more galaxies where it sees the collective effect of many supernovae that have accelerated particles to very high energies. It has also seen nearly 100 of the pulsars and binary objects that have high energy densities in our Galaxy, as well as the glow from our own Galaxy's legacy of supernova particle acceleration. But beyond these sources that tantalize us with their unimaginable distances in light years and megaparsecs, can Fermi learn about high energy physics by observing something closer to home? The answer is now yes.
Solar flares are dramatic releases of energy on the surface of the Sun, involving the interaction of strong magnetic fields with charged particles, and are sometimes energetic enough to be detected in gamma rays. Flares come and go in cycles that last several years, and in its roughly two years of operation thus far, Fermi was observing during a relatively calm period of solar activity. Now, however, that gamma-ray slumber has been broken, and Fermi has observed gamma rays from a solar flare. KIPAC graduate student Alice Allafort and colleagues including KIPAC affiliate Yasuyuki Tanaka have revealed that on March 8 2011 the Sun underwent a precipitous increase in its gamma ray output. This gamma ray increase coincided with an intense period of flare activity seen by instruments that observe the Sun in X-ray light.
The increase in output was observed by Fermi's Large Area Telescope (LAT), which is sensitive to gamma rays of energies ranging from 20 MeV to 300 GeV, but not by Fermi's Gamma-ray Burst Monitor or the RHESSI satellite, which are sensitive to lower energy gamma rays. Yet the flaring is quite distinctive when observed in X-rays which have still lower energies. This already provides some interesting data for scentists to consider as they strive to further understand the plasma physics at work in our Sun.  As solar flares can send out charged particles and X-rays that effect the Earth's atmosphere, communications, and satellite orbits, study of the plasma physics of the Sun is not only an academic question.  Such a meeting of Fermi and solar science will no doubt be the catalyst for further collaboration between KIPAC and Stanford's solar physics group.

Science Contact:
Alice Allafort
Email: allafort@stanford.edu