Localization of the Gamma-Ray Emission Region in the 1 September 2014 Behind-the-Limb Solar Flare According to the Fermi/LAT Data

Abstract

Since the launch of the Fermi mission in 2008, it has become possible to study high-energy solar \(\gamma\) -rays with an unprecedented imaging capability. In particular, the position of the \(>100\text{ MeV}\) \(\gamma\) -ray source can shed light on the origin of high-energy protons that is still controversial. However, the imaging of solar \(\gamma\) -ray sources with the Fermi Large Area Telescope (LAT) is a complex multi-stage process influenced by a number of factors and instrumental effects, which is difficult to fully comprehend a priori. The SOL2014-09-01 behind-the-limb event was significant, for which the \(\gamma\) -ray source position was not firmly established at once. Following the methodology outlined by the Fermi/LAT team, we estimated the proton power-law indices and \(\gamma\) -ray centroid positions at two temporal intervals of this event, separated by one hour. Our estimates for the first interval are comparable to estimates recently updated by the Fermi/LAT team, thereby confirming the consistency of the analysis applied. Although, in the second interval, corresponding to the decay phase of the flare, the proton power-law index clearly hardened, the presumable position of the fading \(\gamma\) -ray source remained unchanged. Its constancy in both temporal intervals and its proximity to the bases of long coronal loops connected to the flare site support the flare origin of high-energy protons injected into these loops along with electrons and trapped there for a long time. Our experience analyzing Fermi/LAT data clarifies their complex handling and will hopefully benefit the solar community in their wider use.