Possible evidence of a ground level enhancement of muons in association with a SWIFT Trigger

Abstract

Starting from April 2007, a search for solar daily variation of the muon intensity ($E_μ>0.2$ GeV) at sea level and using two directional muon telescopes is in progress. In this survey, several ground level enhancements (GLEs) on the muon counting rate background have been found. Here, we highlight one of them, observed in the vertical telescope on 07 August 2007 for the following reasons: The GLE consists of a single narrow peak, with a statistical significance of 4.4$σ$. The GLE is in temporal coincidence with a SWIFT trigger $N^0287222$, at 21:16:05 UT according to the Burst Alert Telescope (BAT) on board of the SWIFT spacecraft. However, the Swift StarTracker had lost stellar lock minutes before that and the resulting improper s/c attitude information caused BAT to "trigger" on a known source. Even so, the SWIFT trigger coordinates are inside the effective field of view of the vertical Tupi muon telescope. The temporal and directional coincidences between this GLE and the SWIFT satellite unknown event strongly suggest that they may be physically associated. Details and implications of this possible association are reported in this work.