Search For VHE Grbs Using Yangbajing Cosmic-Ray Experiment

Gamma-Ray Bursts(GRBs) have been studied by space experiments and ground-based experiments since their discovery in 1967.Until now, We have got rich informations on the propeties(time profile, energy spectrum and spatial distribution) of GRBs,but GRBs are still the most mysterious astronomical phenomenon.The origin of GRBs remains unclear.even the distance is unsolved.Upto now, about three thousands of GRBs have been observed by satellite experiments and some of them have been observed afterglows.The results confirmed that the spatial distribution of GRBs is highly isotropyical and the cosmological model was commonly considered .The energy of the GRBs observed all are less than 30GeV for the small effective area of satellite experiments. Searching for VHE (E>100GeV) GRBs is very important and help determine whether or not the GRBs are distributed cosmologically.Because these gamma rays are strongly attenuated by infrared background photons and 2.7K microwave background radiation photons during their travel towards the earth.Is VHE GRBs exist? Many theoretical models predict or at least allow VHE gamma-rays emission in the process of GRBs;In GeV energy region.EGRET experiment on CGRO had detected a few GRBs which coincident with the BATSE bursts;The results of BATSE bursts show many bursts have very hard spectra and there is no cut exist at high energy. So it is possible to use ground-based experiment with large area and high trigger rate to search for VHE GRBs.Many ground-based experiments were studying on TeV GRBs, such as Milagro,HEGRA .Tibet AS Y array.But no positive VHE GRBs are observed by now. Compare with the same kind of other instrument in the world.the Tibet cosmic ray observation experiment(located at Yangbajing in Tibet,4,310 m a.s.l.) has the lowest threshold energy and the highest trigger rate.Especially for the Tibet III AS Y array with the threshold energy about 1.5TeV and trigger rate about 680Hz,it has many advantages in searching for TeV GRBs.Searching for TeV GRBs was made by using the ASy(Tibet III) data, which were taken from Nov. 1999 to July 2002. In this analysis, two methods have been applied. First is searching for TeV GRB's signals independently and got the lowestchance probability( which is due to the fluctuation of background) about 7.2×10-12; Second is searching for TeV burst-like events coincident with satellite GRBs data and got the lowest chance probability about 7.5×10-8. No convincing signal was detected .To these results, the flux upper limit at the 95% confidence level was estimated to be about 10-9~10-7ph.cm~2.s-1 by Monte Carlo simulation.The sensitivity of YBJ-ARGO to GRBs(E>10GeV) was estimated by Monte Carlo simulation.The minimun flux to make a GRB observable(with a significance of 5 o ) was estimated to be about 5×10ph.cm-2.s-1.