Long-term multi-wavelength variability and correlation study of Markarian 421 from 2007 to 2009

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Aims. We study the multi-band variability and correlations of the TeV blazar Mrk 421 on year time scales, which can bring additional insight on the processes responsible for its broadband emission. Methods. We observed Mrk 421 in the very high energy (VHE) γ-ray range with the Cherenkov telescope MAGIC-I from March 2007 to June 2009 for a total of 96 hours of effective time after quality cuts. The VHE flux variability is quantified with several methods, including the Bayesian Block algorithm, which is applied to data from Cherenkov telescopes for the first time. The 2.3 year long MAGIC light curve is complemented with data from the Swift/BAT and RXTE/ASM satellites and the KVA, GASP-WEBT, OVRO, and Metsähovi telescopes from February 2007 to July 2009, allowing for an excellent characterisation of the multi-band variability and correlations over year time scales. Results. Mrk 421 was found in different γ-ray emission states during the 2.3 year long observation period: The flux above 400 GeV spans from the minimum nightly value of (1.3 ± 0.4)·10^(−11)cm^(−2)s^(−1) to the about 24 times higher maximum flux of (3.1 ± 0.1)·10^(−10)cm^(−2)s^(−1). Flares and different levels of variability in the γ-ray light curve could be identified with the Bayesian Block algorithm. The same behaviour of a quiet and active emission was found in the X-ray light curves measured by Swift/BAT and the RXTE/ASM, with a direct correlation in time. The behaviour of the optical light curve of GASP WEBT and the radio light curves by OVRO and Metsähovi are different as they show no coincident features with the higher energetic light curves and a less variable emission. The fractional variability is overall increasing with energy. The comparable variability in the X-ray and VHE bands and their direct correlation during both high- and low-activity periods spanning many months show that the electron populations radiating the X-ray and γ-ray photons are either the same, as expected in the Synchrotron-Self-Compton mechanism, or at least strongly correlated, as expected in electromagnetic cascades.
© ESO, 2016. Artículo firmado por 77 autores. We would like to thank the Instituto de Astrofísica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The financial support of the German BMBF and MPG, the Italian INFN and INAF, the Swiss National Fund SNF, the ERDF under the Spanish MINECO (FPA2012-39502), and the Japanese JSPS and MEXT is gratefully acknowledged. This work was also supported by the Centro de Excelencia Severo Ochoa SEV-2012-0234, CPAN CSD2007-00042, and MultiDark CSD2009-00064 projects of the Spanish Consolider-Ingenio 2010 programme, by grant 268740 of the Academy of Finland, by the Croatian Science Foundation (HrZZ) Project 09/176 and the University of Rijeka Project, by the DFG Collaborative Research Centers SFB823/C4 and SFB876/C3, and by the Polish MNiSzW grant 745/N-HESS-MAGIC/2010/0. The public data archives of Swift/BAT and RXTE/ASM are acknowledged. We thank the OVRO telescope for making its results available for the public. The OVRO 40 m monitoring program is supported in part by NASA grants NNX08AW31G and NNX11A043G, and NFS grants AST-0808050 and AST- 1109911. We also thank the KVA and Metsähovi telescopes for making their light curves available. M. Villata organized the optical-to-radio observations by GASPWEBT as the president of the collaboration. The Metsähovi team acknowledges the support from the Academy of Finland to our observing projects (numbers 212656, 210338, 121148, and others). St. Petersburg University team acknowledges support from Russian RFBR grant 15- 02-00949 and St. Petersburg University research grant 6.38.335.2015. The Abastumani Observatory team acknowledges financial support by the Shota Rustaveli National Science Foundation under contract FR/577/6-320/13.