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 s , 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 counting rate excess peak of duration T _ { 90 } = ( 6.1 \pm 0.6 ) s with a signal significance ( 4.4 \pm 0.5 ) \sigma , ( and not 6.9 \sigma as reported in the previous version ) .
The signal is within the T90 duration of the Swift BAT GRB , with an estimated ” excess ” fluence of F _ { S } ( E > 0.1 GeV ) = 1.3 \pm 0.3 \times 10 ^ { -6 } erg / cm ^ { 2 } .
This value can be considered the lower limit of the gamma ray fluence in the GeV energy region .
However , the Poisson probability of the event to be a background fluctuation is 5.0 \times 10 ^ { -6 } and it appears in the counting rate of the New-Tupi detector with an annual rate \sim 76 .
In addition , the signal has a significance of only 2 \sigma in the time profiles with a bin above 2 seconds .
Thus we conclude that the event has a high probability to be background fluctuation .