Enhanced TeV gamma ray flux from the Crab Nebula observed

ATel #2921; G. Aielli, P. Camarri, R. Iuppa, R. Santonico (INFN and Tor Vergata Univ.), C. Bacci, P. Branchini, A. Budano, S. Bussino, P. Celio, M. De Vincenzi, I. James, S. M. Mari, P. Montini, P. Pistilli (INFN and Roma III Univ.), B. Bartoli, S. Catalanotti, B. D'Ettorre Piazzoli, T. Di Girolamo, M. Iacovacci, L. Saggese (INFN and Napoli Univ.), P. Bernardini, C. Bleve, I. De Mitri, G. Mancarella, G. Marsella, D. Martello, M. Panareo, L. Perrone(INFN and Salento Univ.), X. J. Bi, Z. Cao, S. Z. Chen, Y. Chen, ZhaoyangFeng, Q. B. Gou, Y. Q. Guo, H. H. He, Hongbo Hu, X. X. Li, C. Liu, H. Lu, X. H.Ma, P. R. Shen, X. D. Sheng, F. Shi, Y. H. Tan, B. Wang, H. Wang, C. Y. Wu, H. R.Wu, Z. G. Yao, M. Zha, H. M. Zhang, JiLong Zhang, JianLi Zhang, Y. Zhang, Q. Q. Zhu (IHEP, Beijing), A. K. Calabrese Melcarne, G. Zizzi (INFN-CNAF), A. Cappa, P. Vallania, S. Vernetto (INFN and INAF, Torino), R. Cardarelli, G. Di Sciascio, B. Liberti (INFN, Tor Vergata), C. Cattaneo, P. Salvini (INFN, Pavia), P. Creti, A. Surdo (INFN, Lecce), S. W. Cui (HebeiUniv.), B. Z. Dai, C. Q. Liu, J. Liu, Q. Y. Yang, X. C. Yang, L. Zhang, P. Zhang(Yunnan Univ.), G. D'Ali' Staiti (INFN and Palermo Univ.), T. L.Chen, Danzengluobu, X. H. Ding, Haibing Hu, Labaciren, H. J. Li, M. Y. Liu, C. C. Ning, A. F. Yuan, Zhaxisangzhu, Zhaxiciren (TibetUniv.), M. Dattoli, C. Vigorito (INFN, INAF and Torino Univ.), C. F. Feng, J. Y. Li, X. B. Qu, L. Xue, X. Y. Zhang (Shandong Univ.), Zhenyong Feng, Q. Huang, H. Y. Jia, B. Xu, X. X. Zhou, F. R.Zhu (South West Jiaotong Univ.), F. Galeazzi, R. Gargana, F. Ruggieri, C. Stanescu (INFN, Roma III), P. Galeotti (INFN andTorino Univ.), E. Giroletti, G. Liguori (INFN and Pavia Univ.), S. Mastroianni, E. Rossi(INFN, Napoli), A. Pagliaro (INFN and INAF, Palermo)
on 11 Oct 2010; 11:42 UT
Credential Certification: Silvia Vernetto (vernetto@to.infn.it)

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Following the discovery by AGILE (ATEL #2855) and confirmation by Fermi/Lat (ATEL #2861) of enhanced gamma ray emission from the Crab Nebula region in the time interval from September 18 to September 22 2010, we report on the observation performed during the same days by ARGO-YBJ.
ARGO-YBJ is a full coverage air shower detector located in the Yangbajing Cosmic Ray Laboratory in Tibet (China) at 4300 m a.s.l. working with an energy threshold of a few hundred GeV. The field of view of about 2 sr and the duty cycle of about 90%, allow a continuous monitoring of a large part of the sky. 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 in the time interval from September 17 to September 22, corresponding to a flux about 3-4 times higher than usual, at the median energy of about 1 TeV. Moreover, the ARGO-YBJ analysis in a time window extended up to September 27, suggests that at TeV energies the enhanced emission lasts more than that observed by lower energy experiments.
The monitoring of the Crab Nebula by ARGO-YBJ is still under way and the analysis of more data is in progress.