Results from the ARGO-YBJ experiment

The full coverage extensive airshower detector ARGO-YBJ, located in Tibet at 4300 m of altitude, has monitored the sky at gamma ray energy E > 0.6 TeV in the declination band from -10° to +70°. In 424 days the Crab Nebula and Mrk421 have been detected with a significance, respectively, of 7.0 and 8.0 standard deviations. The analysis of the cosmic ray background in the same sky band, has revealed the existence of a significant excess of the cosmic ray flux in two localized regions of angular size 10°–30°, confirming previous indications. The origin of such excesses is still unexplained. During 2008 the observed Mrk421 flux was highly variable, with the strongest flares in March–June, in good correlation with X-ray data. One of the most intense flares occurred in the first half of June and has been deeply studied by different detectors in the energy range from optical to 100MeV gamma rays, but only partially up to TeV energies, since the moon lighth ampered the Cherenkov telescope measurements during the second and most intense part of the emission. Our data complete these observations, with the detection of a signal of intensity of about 7 Crab units on June 11–13, with a statistical significance of 4.2 standard deviations. The observed flux is consistent with a prediction made in the frame work of the Synchrotron Self-Compton model, in which the flare is caused by a rapid acceleration of leptons in the jet