Astronomy and Astrophysics – Astrophysics – High Energy Astrophysical Phenomena
Scientific paper
2011-06-08
Astronomy and Astrophysics
Astrophysics
High Energy Astrophysical Phenomena
Submitted A&A
Scientific paper
Context. The HBL-type 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 VHE (E > 100 GeV) band by the single dish Major Atmospheric Gamma-ray Imaging Cherenkov telescope (MAGIC-I). Aims. We aimed to sample the evolution of the source emission at VHE and in other bands, and to model the broad band spectral energy distribution (SED) of selected states, reconstructed by means of sets of multi-wavelength (MWL) data observed simultaneously. Methods. We performed a dense monitoring of the source in VHE with MAGIC-I, collecting also complementary data in soft X-rays and optical-UV bands; then, we modeled the SEDs derived from simultaneous MWL data within the Synchrotron Self-Compton (SSC) framework. Results. The source showed intense and prolonged gamma-ray activity during the whole period, with integral fluxes (E > 200 GeV) sel- dom below the level of 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. Robust constraints could be put to the physical parameters of the jet, interpreting the spectral energy distributions obtained 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 \leq {\delta} \leq 80) are needed to reproduce the observed SED; but this model can not explain the observed short time scale variability, while it can be argued that inho- mogeneous models could allow for less extreme Doppler factors, larger magnetic fields and shorter electron cooling times compatible with hour or sub-hour scale variability.
Aleksić J.
Alvarez E. A.
Angelis Alessando de
Antonelli Lucio Angelo
Antoranz P.
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