SEARCH FOR GAMMA RAY BURSTS WITH THE ARGO-YBJ DETECTOR IN SCALER MODE

We report on the search for Gamma Ray Bursts (GRBs) in the energy range 1-100GeV in coincidence with the prompt emission detected by satellites, using the Astrophysical Radiation Ground-based Observatory at YangBaJing (ARGO-YBJ) air shower detector. Thanks to its mountain location (Yangbajing, Tibet, People's republic of China, 4300m above ssea level), active surface (6700m^2 of Resistive Plate Chambers) and large field of view (about 2sr, limited only by atmospheric absorption), the ARGO-YBJ air shower detector is particularly suitable for the detection of unpredictable and short duration events such as GRBs. The search is carried out using the "single particle technique" i.e. counting all the particles hitting the detector without measurement of the eneregy and arrival direction of the primary gamma ray. Between 2004 december 17 and 2009 April 7, 81 GRBs detected by satellites occurred within the field of view of ARGO YBJ (zenith angle theta<=45°). It was possible to examine 62 of these for >1GeV counterpart in the ARGO-YBJ data finding no statistically significant emission. with a lack of detected spectra in this energy range fluence upper limits are profitable, especially when the redshift is known and the correction for the extragalactic absorption can be considered. The obtained fluence upper limits reach values as low as 10^-5ergxcm^-2 in the 1-100GeV energy region. Besides this individual search for a high energy counterpart, a statistical study of the stack of all the GRBs both in time and in phase was made, looking for a common feature in GRB high energy emission. No significant signal has been detected.