Enhanced TeV gamma ray flux from the Crab Nebula observed by the ARGO-YBJ experiment

ATel #4258; 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 16 Jul 2012; 09:16 UT
Credential Certification: Songzhan Chen (chensz@ihep.ac.cn)

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Following the detection by Fermi (ATel#4239) of enhanced gamma ray emission from the Crab Nebula region on July 3rd, 2012, we report on the observation performed in the same day by the ARGO-YBJ experiment. 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 0.3TeV with a field of view of about 2 sr and a duty cycle of about 85%. The Crab Nebula region is observed every day for about 5.5 hours. A preliminary analysis of the data shows an excess of events with statistical significance of about 4 standard deviations from a direction consistent with the Crab Nebula on July 3rd, corresponding to a flux about 8 times higher than the average emission at a median energy of about 1 TeV. The monitoring of the Crab Nebula by ARGO-YBJ is still under way, while no significant excesses are detected in the following days from 4th to 6th , July.