We present results of a search for a possible signal from small scale solar transient events (such as flares and interplanetary shocks) as well as possible counterparts to gamma-ray burst (GRB) observed simultaneously by the Tupi muon telescope (Niteroi, Brazil, $22.9°\mathrm{S}$, $43.2°\mathrm{W}$, 3 m above sea level) and the Pierre Auger Observatory surface detectors (Malargue, Argentina, $69.3°\mathrm{S}$, $35.3°\mathrm{W}$, altitude 1400 m). Both cosmic ray experiments are located inside the South Atlantic Anomaly region. Our analysis of several examples shows similarities in the behavior of the counting rate of low energy (above 100 MeV) particles in association with the solar activity (solar flares and interplanetary shocks). We also report an observation by the Tupi experiment of the enhancement of muons at ground level with a significance $8\sigma$ in the 1-sec binning counting rate (raw data) in close time coincidence (T-184 sec) with the Swift-BAT GRB110928B ($\mathrm{\text{trigger}}=504307$), according to the GRB Coordinate Network report. At the same time this event was not included in the Swift-XRT products for GRBs. The GRB 110928B coordinates are in the field of view of the vertical Tupi telescope, and the burst was close to the MAXI source J1836-194. The 5-min muon counting rate in the vertical Tupi telescope as well as publicly available data from Auger (15 min averages of the scaler rates) show small peaks above the background fluctuations at the time following the Swift-BAT GRB 110928B trigger. In accordance with the long duration trigger, this signal can possibly suggest a long GRB, with a precursor narrow peak at T-184 sec, or a transient Galactic source.

• Received 12 January 2012

DOI:https://doi.org/10.1103/PhysRevD.86.022001