Mrk 421 active state in 2008: the MAGIC view, simultaneous multi-wavelength observations and SSC model constrained

¹ IFAE, Edifici Cn., Campus UAB, 08193 Bellaterra, Spain
² Universidad Complutense, 28040 Madrid, Spain
³ INAF National Institute for Astrophysics, 00136 Rome, Italy
e-mail: giacomo.bonnoli@brera.inaf.it; fabrizio.tavecchio@brera.inaf.it
⁴ Università di Siena, and INFN Pisa, 53100 Siena, Italy
e-mail: antonio.stamerra@pi.infn.it
⁵ Technische Universität Dortmund, 44221 Dortmund, Germany
⁶ Università di Padova and INFN, 35131 Padova, Italy
⁷ Inst. de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁸ Depto. de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Spain
⁹ University of Łódź, 90236 Lodz, Poland
¹⁰ Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland
¹¹ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹² ETH Zurich, 8093, Switzerland
¹³ Max-Planck-Institut für Physik, 80805 München, Germany
¹⁴ Universität Würzburg, 97074 Würzburg, Germany
¹⁵ Universitat de Barcelona (ICC/IEEC), 08028 Barcelona, Spain
¹⁶ Università di Udine, and INFN Trieste, 33100 Udine, Italy
¹⁷ Institut de Ciències de l’Espai (IEEC-CSIC), 08193 Bellaterra, Spain
¹⁸ Inst. de Astrofísica de Andalucía (CSIC), 18080 Granada, Spain
¹⁹ Croatian MAGIC Consortium, Rudjer Boskovic Institute, University of Rijeka and University of Split, 10000 Zagreb, Croatia
²⁰ Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
²¹ Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
²² INAF/Osservatorio Astronomico and INFN, 34143 Trieste, Italy
²³ Università dell’Insubria, Como, 22100 Como, Italy
²⁴ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²⁵ ICREA, 08010 Barcelona, Spain
²⁶ Now at École polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland
²⁷ Supported by INFN Padova  
²⁸ Now at: Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain
²⁹ Now at: Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Finland

Received: 8 June 2011
Accepted: 7 January 2012

Abstract

Context. The blazar Markarian 421 is one of the brightest TeV gamma-ray sources of the northern sky. From December 2007 until June 2008 it was intensively observed in the very high energy (VHE, E > 100 GeV) band by the single-dish Major Atmospheric Gamma-ray Imaging Cherenkov telescope (MAGIC-I).

Aims. We aimed to measure the physical parameters of the emitting region of the blazar jet during active states.

Methods. We performed a dense monitoring of the source in VHE with MAGIC-I, and also collected complementary data in soft X-rays and optical-UV bands; then, we modeled the spectral energy distributions (SED) derived from simultaneous multi-wavelength data within the synchrotron self-Compton (SSC) framework.

Results. The source showed intense and prolonged γ-ray activity during the whole period, with integral fluxes (E > 200 GeV) seldom below the level of the Crab Nebula, and up to 3.6 times this value. Eight datasets of simultaneous optical-UV (KVA, Swift/UVOT), soft X-ray (Swift/XRT) and MAGIC-I VHE data were obtained during different outburst phases. The data constrain the physical parameters of the jet, once the spectral energy distributions obtained are interpreted within the framework of a single-zone SSC leptonic model.

Conclusions. The main outcome of the study is that within the homogeneous model high Doppler factors (40 ≤ δ ≤ 80) are needed to reproduce the observed SED; but this model cannot explain the observed short time-scale variability, while it can be argued that inhomogeneous models could allow for less extreme Doppler factors, more intense magnetic fields and shorter electron cooling times compatible with hour or sub-hour scale variability.