Long-term monitoring of the TeV emission from Mrk 421 with the ARGO-YBJ experiment
astro-ph.HE
/ Authors
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
and 98 more authors
Z. Cao, A. Cappa, R. Cardarelli, S. Catalanotti, C. Cattaneo, P. Celio, S. Z. Chen, T. L. Chen, Y. Chen, P. Creti, S. W. Cui, B. Z. Dai, G. D'Alí 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
/ Abstract
ARGO-YBJ is an air shower detector array with a fully covered layer of resistive plate chambers. It is operated with a high duty cycle and a large field of view. It continuously monitors the northern sky at energies above 0.3 TeV. In this paper, we report a long-term monitoring of Mrk 421 over the period from 2007 November to 2010 February. This source was observed by the satellite-borne experiments Rossi X-ray Timing Explorer and Swift in the X-ray band. Mrk 421 was especially active in the first half of 2008. Many flares are observed in both X-ray and gamma-ray bands simultaneously. The gamma-ray flux observed by ARGO-YBJ has a clear correlation with the X-ray flux. No lag between the X-ray and gamma-ray photons longer than 1 day is found. The evolution of the spectral energy distribution is investigated by measuring spectral indices at four different flux levels. Hardening of the spectra is observed in both X-ray and gamma-ray bands. The gamma-ray flux increases quadratically with the simultaneously measured X-ray flux. All these observational results strongly favor the synchrotron self-Compton process as the underlying radiative mechanism.