TeV flare from the blazar Mrk421 observed by ARGO-YBJ

ATel #4272; B. Bartoli, P. Bernardini, X. J. Bi, I. Bolognino, P. Branchini, A. Budano, A. K. Calabrese Melcarne, P. Camarri, Z. Cao, R. Cardarelli, S. Catalanotti, C. Cattaneo, S. Z. Chen, T. L. Chen, Y. Chen, P. Creti, S. W. Cui, B. Z. Dai, G. D'Alì Staiti , A. D'Amone, Danzengluobu, I. De Mitri, B. D'Ettorre Piazzoli, T. Di Girolamo, X. H. Ding, G. Di Sciascio, C. F. Feng, Zhaoyang Feng ,Zhenyong Feng, F. Galeazzi, E. Giroletti, Q. B. Gou, Y. Q. Guo, H. H. He, Haibing Hu, Hongbo Hu, Q. Huang, M. Iacovacci, R. Iuppa, I. James, H. Y. Jia, Labaciren, H. J. Li, J. Y. Li, X. X. Li, G. Liguori, C. Liu, C. Q. Liu, J. Liu, M. Y. Liu, H. Lu, L. L. Ma, X. H. Ma, G. Mancarella, S. M. Mari, G. Marsella, D. Martello, S. Mastroianni, P. Montini, C. C. Ning, A. Pagliaro, M. Panareo, B. Panico, L. Perrone, P. Pistilli, F. Ruggieri, P. Salvini, R. Santonico, S. N. Sbano, P. R. Shen, X. D. Sheng, F. Shi, A. Surdo, Y. H. Tan, P. Vallania, S. Vernetto, C. Vigorito, B. Wang, H. Wang, C. Y. Wu, H. R. Wu, B. Xu, L. Xue, Q. Y. Yang, X. C. Yang, Z. G. Yao, A. F. Yuan, M. Zha, H. M. Zhang, Jilong Zhang, Jianli Zhang, L. Zhang, P. Zhang, X. Y. Zhang, Y. Zhang, J. Zhao, Zhaxiciren, Zhaxisangzhu, X. X. Zhou, F. R. Zhu, Q. Q. Zhu and G. Zizzi
on 24 Jul 2012; 11:35 UT
Credential Certification: Silvia Vernetto (vernetto@to.infn.it)

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Following the detection by Fermi LAT (ATel#4261) of a GeV gamma ray flare from Mrk421 on July 16, we report on the observation by ARGO-YBJ during the same day. ARGO-YBJ is a full coverage air shower detector located at the Yangbajing Cosmic Ray Laboratory in Tibet (China) at 4300 m a.s.l. It continuously monitors the northern sky at energies above 0.3 TeV with a field of view of about 2 sr and a duty cycle of about 85%. Integrating the data from 05:21 UT to 13:26 UT on July 16, when the zenith angle of the source was less than 50 degrees, we observed an excess of 4 standard deviations from a direction consistent with Mrk421, corresponding to a flux of about 7 Crab units at a median energy of 1 TeV. The analysis of the data of the following days is in progress.