ARGO-YBJ detects a VHE gamma-ray flare from Mrk501
ATel #3801; The ARGO-YBJ Collaboration: B. Bartoli, P. Bernardini, X. J. Bi, C. Bleve, 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'AlÃÂ Staiti, Danzengluobu, M. Dattoli, 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, 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, X. B. Qu, F. Ruggieri, P. Salvini, R. Santonico, P. R. Shen, X. D. Sheng, F. Shi, C. Stanescu, 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, Y. X. Yan, 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, Zhaxiciren, Zhaxisangzhu, X. X. Zhou, F. R. Zhu, Q. Q. Zhu, G. Zizzi
on 9 Dec 2011; 18:28 UT
Credential Certification: Silvia Vernetto (vernetto@to.infn.it)
We report on the observation of a strong gamma-ray flare from Mrk501 at energies above 300 GeV by the ARGO-YBJ experiment, following the discovery by MAXI/GSC (ATEL #3752) of X-ray brightening starting on November 1, 2011.
According to the Swift/BAT light curve at 15-50 keV, this flare started on October 17 and lasted at least until December 9. Integrating the flux from Mrk501 detected by ARGO-YBJ over the period from October 17 to November 21 (more data are still in analysis), an excess with statistical significance of 6.1 standard deviations is found. The average flux is about 3 Crab-unit. From November 1 to November 13, when Swift/BAT measured the peak flux, the ARGO-YBJ data show an excess with statistical significance of 4.8 standard deviations from Mrk501, indicating that the flux is enhanced by a factor of 10, corresponding to a flux of about 4 Crab-unit.
The observed flare is probably the strongest outburst from Mrk501 since its last huge emission observed in 1997. Multiwavelength observations of this source are encouraged.
ARGO-YBJ is a full-coverage air-shower detector located at the Yangbajing Cosmic Ray Laboratory at 4300 m a.s.l. in Tibet, China. With an energy threshold of a few hundred GeV, the detector has a field of view of about 2 sr and a duty cycle of about 90%, which allow a continuous monitoring of a large part of the sky. The Mrk501 region is observed for about 6 hours every day. The monitoring of the Mrk501 by ARGO-YBJ is still under way and the analysis with more data is in progress.
