Search for Gamma-Ray Bursts with the ARGO-YBJ Detector in Shower Mode

The ARGO-YBJ detector, located at the Yangbajing Cosmic Ray Laboratory (4300 m a. s. l., Tibet, China), was a "full coverage" (central carpet with an active area of similar to 93%) air shower array dedicated to gamma-ray astronomy and cosmic-ray studies. The wide field of view (similar to 2 sr) and high duty cycle (> 86%), made ARGO-YBJ suitable to search for short and unexpected gamma-ray emissions like gamma-ray bursts (GRBs). Between 2007 November 6 and 2013 February 7, 156 satellite-triggered GRBs (24 of them with known redshift) occurred within the ARGO-YBJ field of view (zenith angle theta <= 45 degrees). A search for possible emission associated with. these GRBs has been made in the two energy ranges 10-100 GeV and 10-1000 GeV. No significant excess has been found in time coincidence with the satellite detections nor in a set of different time windows inside the interval of one hour after the bursts. Taking into account the EBL absorption, upper limits to the energy fluence at a 99% confidence level have been evaluated, with values ranging from similar to 10(-5) erg cm(-2) to similar to 10(-1) erg cm(-2). The Fermi-GBM burst GRB 090902B, with a high-energy photon of 33.4 GeV detected by Fermi-LAT, is discussed in detail.