Implications of a Loop-top Origin for Microwave, Hard X-Ray, and Low-energy Gamma-Ray Emission from Behind-the-limb Flares

Abstract

Fermi has detected hard X-ray (HXR) and gamma-ray photons from three flares, which according to STEREO  occurred in active regions behind the limb of the Sun as delineated by near-Earth instruments. For two of these flares, RHESSI  has provided HXR images with sources located just above the limb, presumably from the loop-top (LT) region of a relatively large loop. Fermi -GBM has detected HXRs and gamma-rays, and the Radio Solar Telescope Network has detected microwave emissions with similar light curves. This paper presents a quantitative analysis of these multiwavelength observations assuming that HXRs and microwaves are produced by electrons accelerated at the LT source, with emphasis on the importance of the proper treatment of particle escape from the acceleration-source region and the transrelativistic nature of the analysis. The observed spectra are used to determine the magnetic field and relativistic electron spectra. It is found that a simple power law in momentum (with a cutoff above a few 100 MeV) agrees with all observations, but in energy space, a broken power-law spectrum (steepening at ∼ mc 2 ) may be required. It is also shown that the production of the >100 MeV photons detected by Fermi -LAT at the LT source would require more energy than photospheric emission. These energies are lower than that required for electrons, so that the possibility that all the emissions originate in the LT cannot be ruled out on energetic grounds. However, the differences in the light curves and emission centroids of HXRs and >100 MeV gamma-rays favor a different source for the latter.