Prompt emission from GRB 150915A in the GeV energy range detected at ground by the New-Tupi detector

C. R. A. Augusto, C. E. Navia, M. N. de Oliveira, A. A. Nepomuceno, V. Kopenkin, T. Sinzi

Published 2016-05-13Version 1

Since 2014, a new detector (New-Tupi) consisting of four plastic scintillators ($150 \times 75 \times 5 cm^3$) placed in pairs and located in Niteroi, Rio de Janeiro, Brazil, has been used for the search of transient solar events and photomuons from gamma-ray bursts (GRBs). On September 15, 2015, at 21:18:24 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 150915A (trigger 655721). The GRB light curve shows a weak complex structure of long duration $T_{90}=164.7 \pm 49.7 $ sec, and a fluence in the 15-150 keV band of $8.0 \pm 1.8 \times 10^{-7}erg/cm^2$. GRB 150915A was fortuitously located in the field of view of the New-Tupi detector, and a search for prompt emission in the GeV energy range is presented here. The analysis was made using the "scaler" or "single-particle" technique. The New-Tupi detector registered a muon excess peak of 6.1s duration with a signal significance $6.9\sigma$, the signal was within the T90 duration of the Swift BAT GRB, with an estimated fluence $4.8 \times 10^{-6} erg/cm^2$ (10-100 GeV). The Poisson probability of the event to be a background fluctuation is $6.9 \times 10^{-10}$ and it appears in the counting rate of the New-Tupi detector with an annual rate $\sim 2.6$.