Search For GeV～TeV Gamma-Ray Bursts With ARGO Experiment

Gamma-ray bursts (GRBs) are the brightest explosions of gamma rays coming from the universe in a short period of time. Since their discovery by Vela satellite in 1967, gamma-ray bursts have remained one of the most enigmatic astronomical phenomena. Therefore, gamma-ray bursts have been studied by several satellite experiments and ground-based experiments.Upto now, more than four thousands of GRBs have been observed by satellite experiments.At the same time,theoretical studies are going on very actively. Though much progress has been achieved from both experimental and theoretical efforts, lots of basic questions such as the origin and the emission mechanism of GRBs still remain unresolved.However, most up-to-date known GRBs were observed in energy range from keV to MeV and the only several GRBs were detected by EGRET and Fermi which emitted GeV photons. In order to understand the complete picture of the process, it is important to detect the GeV-TeV emissions from GRBs. At the same time, searching for GeV-TeV GRBs is very important to give some information on the origin and the emission mechanism of GRBs, and also give constrain on the models of GRBs.The ARGO experiment, located at Yangbajing, Tibet, China,have many advantages such as full coverage, wide field of view, high duty cycle and the lowest threshold energy. So, ARGO experiment is expected to have higher sensitivity in detecting GRBs than other ground-based experiments. In this work, we study the ARGO sensitivity in detecting GRBs with photon energies greater than 10 GeV. If the redshift of GRBs are less than 0.5 and the f luence are more than 10-5 erg. cm-2. s-1, ARGO will be sensitive enough to discover them.Searching for GeV-TeV GRBs coincidence with satellite GRBs data is carried out by using the ARGO Shower mode data and Scaler mode data. Between June 2006 and December 2008,32 GRBs detected by satellites occurred within the field view of ARGO(zenith angleθ<45°). Using the Shower mode data, the lowest chance probability (Pb) is 5.2X 10-7 which is related with GRB081102B. Using the Scaler mode data, the highest significance is 5.34σwhich is also related with GRB081102B. The results are discussed in text.After considering the number of trials, no significant GeV-TeV GRBs was observed. The EBL absorption was considered, then the fluence upper limits which depends on GRBs'time duration, spectral index, redshift and the zenith angle at the observation were estimated at the 95% confidence level by Monte Carlo simulation. If the redshift z=0. and the spectral index a=2.5,the fluence upper limits for ARGO to detect E>10GeV GRBs reach values as low as 10-6erg. cm-2...