TeV Solar Gamma Rays From Cosmic-Ray Interactions

TL;DR

This paper predicts very high-energy gamma-ray flux from the Sun due to cosmic-ray interactions, proposing new observational tests with HAWC and LHAASO to explore cosmic-ray spectra and solar environment effects.

Contribution

It provides the first theoretical predictions of TeV gamma-ray flux from the Sun and halo, and suggests observational tests with upcoming experiments to study cosmic-ray interactions.

Findings

Predicted gamma-ray flux up to 1000 TeV from the Sun and halo.

Proposed observational tests with HAWC and LHAASO.

Insights into cosmic-ray spectra and solar magnetic effects.

Abstract

The Sun is a bright source of GeV gamma rays, due to cosmic rays interacting with solar matter and photons. Key aspects of the underlying processes remain mysterious. The emission in the TeV range, for which there are neither observational nor theoretical studies, could provide crucial clues. The new experiments HAWC (running) and LHAASO (planned) can look at the Sun with unprecedented sensitivity. In this paper, we predict the very high-energy (up to 1000 TeV) gamma-ray flux from the solar disk and halo, due to cosmic-ray hadrons and electrons ($e^++e^-$), respectively. We neglect solar magnetic effects, which is valid at TeV energies; at lower energies, this gives a theoretical lower bound on the disk flux and a theoretical upper bound on the halo flux. We show that the solar-halo gamma-ray flux allows the first test of the $\sim 5$--70 TeV cosmic-ray electron spectrum. Further, we show that HAWC can immediately make an even stronger test with nondirectional observations of cosmic-ray electrons. Together, these gamma-ray and electron studies will provide new insights about the local density of cosmic rays and their interactions with the Sun and its magnetic environment. These studies will also be an important input to tests of new physics, including dark matter.