FIRST DETECTION OF >100 MeV GAMMA-RAYS ASSOCIATED WITH A BEHIND-THE-LIMB SOLAR FLARE

Abstract

Here, we report the first detection of >100 MeV gamma-rays associated with a behind-the-limb solar flare, which presents a unique opportunity to probe the underlying physics of high-energy flare emission and particle acceleration. On 2013 October 11 a GOES M1.5 class solar flare occurred ~9°.9 behind the solar limb as observed by <i>STEREO</i>-B. <i>RHESSI</i> observed hard X-ray (HXR) emission above the limb, most likely from the flare loop-top, as the footpoints were occulted. Surprisingly, the <i>Fermi</i> Large Area Telescope (LAT) detected >100 MeV gamma-rays for ~30 minutes with energies up to 3 GeV. The LAT emission centroid is consistent with the <i>RHESSI</i> HXR source, but its uncertainty does not constrain the source to be located there. The gamma-ray spectra can be adequately described by bremsstrahlung radiation from relativistic electrons having a relatively hard power-law (PL) spectrum with a high-energy exponential cutoff, or by the decay of pions produced by accelerated protons and ions with an isotropic pitch-angle distribution and a PL spectrum with a number index of ~3.8. Furthermore, we show that high optical depths rule out the gamma-rays originating from the flare site and a high-corona trap model requires very unusual conditions, so a scenario in which some of the particles accelerated by the CME shock travel to the visible side of the Sun to produce the observed gamma-rays may be at work.