2005ApJ...621.1137N

In a recent study of a search for enhancements from the Galactic center with muons at sea level using the TUPI muon telescope, we have found several ground-level enhancements (GLEs) as very sharp peaks above the count rate background. This paper reports a consistent analysis of two GLEs observed in 2003 December and detected after an upgrade of the data acquisition system, which includes a noise filter and allows us to verify that the GLEs are not mere background fluctuations. The main target of this study is a search for the origin of the GLEs. The results show that one of them has a strong correlation with a solar flare, while the other has an unknown origin, because there is no satellite report of a solar flare, no prompt X-ray emission, and no excess of nuclei during the raster scan in which the GLE was observed. Even so, two possibilities are analyzed: the solar flare hypothesis and the gamma-ray burst (GRB) hypothesis. We show, by using the FLUKA Monte Carlo results for photoproduction, that under certain conditions there is the possibility of an enhancement of muons at sea level from GeV GRBs.