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

¹ ETH Zurich, 8093 Zurich, Switzerland
² Università di Udine, and INFN Trieste, 33100 Udine, Italy
³ INAF National Institute for Astrophysics, 00136 Rome, Italy
⁴ Università di Siena, and INFN Pisa, 53100 Siena, Italy
⁵ Croatian MAGIC Consortium, Rudjer Boskovic Institute, University of Rijeka, University of Split and University of Zagreb, Croatia
⁶ Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Salt Lake, Sector-1, 700064 Kolkata, India
⁷ Max-Planck-Institut für Physik, 80805 München, Germany
⁸ Universidad Complutense, 28040 Madrid, Spain
⁹ Inst. de Astrofísica de Canarias, 38200 La Laguna; Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
¹⁰ University of Łódź, 90236 Lodz, Poland
¹¹ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹² Institut de Fisica d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra (Barcelona), Spain
¹³ Universität Würzburg, 97074 Würzburg, Germany
¹⁴ Università di Padova and INFN, 35131 Padova, Italy
¹⁵ Institute for Space Sciences (CSIC/IEEC), 08193 Barcelona, Spain
¹⁶ Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁷ Unitat de Física de les Radiacions, Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
¹⁸ Universitat de Barcelona, ICC, IEEC-UB, 08028 Barcelona, Spain
¹⁹ Japanese MAGIC Consortium, ICRR, The University of Tokyo, Department of Physics and Hakubi Center, Kyoto University, Tokai University, The University of Tokushima, KEK, Japan
²⁰ Finnish MAGIC Consortium, Tuorla Observatory, University of Turku and Astronomy Division, University of Oulu, 90014 Oulu, Finland
²¹ Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
²² Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²³ ICREA and Institute for Space Sciences (CSIC/IEEC), 08193 Barcelona, Spain
²⁴ Centro Brasileiro de Pesquisas Físicas (CBPF/MCTI), R. Dr. Xavier Sigaud, 150 – Urca, 22290-180 Rio de Janeiro, Brazil
²⁵ NASA Goddard Space Flight Center, Greenbelt, MD 20771; and Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
²⁶ Humboldt University of Berlin, Institut für Physik Newtonstr. 15, 12489 Berlin, Germany
²⁷ École polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland
²⁸ Japanese MAGIC Consortium, Kyoto, Japan
²⁹ Finnish Centre for Astronomy with ESO (FINCA), Turku, Finland
³⁰ INAF-Trieste, 34143 Trieste, Italy
³¹ ISDC – Science Data Center for Astrophysics, 1290 Versoix, (Geneva), Switzerland
³² Astron. Inst., St.-Petersburg State Univ., 198504 St. Petersbourg, Russia
³³ University of Crete, Heraklion, Greece
³⁴ Graduate Institute of Astronomy, National Central University, 300 Zhongda Rd, 32001 Zhongli, Taoyuan, Taiwan
³⁵ Pulkovo Observatory, 196140 St.-Petersburg, Russia
³⁶ Agrupació Astronòmica de Sabadell, 08206 Sabadell, Barcelona, Spain
³⁷ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
³⁸ School of Cosmic Physics, Dublin Institute For Advanced Studies, Dublin 2, Ireland
³⁹ Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
⁴⁰ Engelhardt Astronomical Observatory, Kazan Federal University, Tatarstan, Russia
⁴¹ Aalto University Department of Radio Science and Engineering, PO BOX 13000, 00076 Aalto, Finland
⁴² Circolo Astrofili Talmassons, via Cadorna, 57, 33030 Talmassons, Italy
⁴³ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
⁴⁴ INAF–Osservatorio Astrofisico di Torino, 10025 Pino Torinese (TO), Italy
⁴⁵ Department of Physics, University of Colorado, Denver, CO 80217-3364, USA

Received: 7 March 2016
Accepted: 23 May 2016

Abstract

Aims. We study the multi-band variability and correlations of the TeV blazar Mrk 421 on year timescales, 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 h of effective time after quality cuts. The VHE flux variability is quantified using several methods, including the Bayesian Block algorithm, which is applied to data from Cherenkov telescopes here for the first time. The 2.3 yr 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 timescales.

Results. Mrk 421 was found in different γ-ray emission states during the 2.3 yr 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 maximum flux, that is about 24 times higher, at (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. Overall, the fractional variability increases 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.