Observation of a muon excess following a gamma-ray burst event detected at the International Space Station

Abstract

On April 24, 2012, at 16:47:14 UT, the Gas Slit Camera (GSC) of the Japanese Monitor of All-sky X-ray Image (MAXI) instrument on the International Space Station detected a short x-ray transient lasting about 34 seconds. The MAXI/GSC transient was most likely a gamma-ray burst (GRB), because of the high Galactic latitude, spectral hardness ratio, and the absence of known bright x-ray sources at the detected position. In addition, the MAXI/GSC transient GRB 120424A coordinates were in the field of view of the inclined Tupi muon telescope located at ground level (3 m above sea level) at ($22.9\ifmmode^\circ\else\textdegree\fi{}\text{ }\mathrm{W}$, $43.2\ifmmode^\circ\else\textdegree\fi{}\text{ }\mathrm{S}$) in the South Atlantic Anomaly region. We report here that the Tupi telescope registered a muon excess with a signal significance $6.2\ensuremath{\sigma}$ within the MAXI/GSC transient time period. Assuming a power law function with a spectral index of $\ensuremath{\gamma}=\ensuremath{-}1.54$ in the tail of the primary gamma-ray energy spectrum, we can conclude that the fluence obtained from the muon excess detected by the Tupi telescope is consistent with the preliminary value obtained by the MAXI team. This result agrees with an assumption that the muons were produced in photonuclear reactions in the Earth's atmosphere. In addition, we show also that the South Atlantic Anomaly region can be a favorable place at ground for the detection of the tail of the energy spectrum (the GeV counterpart) of some GRBs.