Search for Gamma-Ray Emission from the Sun during Solar Minimum with the ARGO-YBJ Experiment

The hadronic interaction of cosmic rayswith solar atmosphere can produce high energy gamma rays. The gamma-rayluminosity is correlated both with the flux of primary cosmic raysand the intensity of the solar magnetic field. The gamma raysbelow 200 GeV have been observed by $Fermi$ without any evident energy cutoff. The bright gamma-rayflux above 100 GeV has been detected only during solar minimum. The only available data in TeV range come from the HAWC observations, however outside the solar minimum. The ARGO-YBJ dataset has been used to search for sub-TeV/TeV gamma raysfrom the Sun during the solar minimumfrom 2008 to 2010, the same time period covered by the Fermi data. A suitable model containing the Sun shadow, solar disk emission and inverse-Compton emission has been developed, and the chi-square minimization method was used to quantitatively estimate the disk gamma-raysignal. The result shows that no significant gamma-raysignal is detected and upper limits to the gamma-rayflux at 0.3$-$7 TeV are set at 95\% confidence level. In the low energy range these limits are consistent with the extrapolation of the Fermi-LAT measurements taken during solar minimumand are compatible with a softening of the gamma-rayspectrum below 1 TeV. They provide also an experimental upper bound to any solar disk emission at TeV energies. Models of dark matter annihilation via long-lived mediators predicting gamma-rayfluxes > $10^{-7}$ GeV $cm^{-2}$ $s^{-1}$ below 1 TeV are ruled out by the ARGO-YBJ limits.