MAGIC TeV gamma-ray observations of Markarian 421 during multiwavelength campaigns in 2006

¹ IFAE, Edifici Cn., Campus UAB, 08193 Bellaterra, Spain
² ETH Zurich, 8093, Switzerland
³ INAF National Institute for Astrophysics, 00136 Rome, Italy
⁴ Universidad Complutense, 28040 Madrid, Spain e-mail: oya@gae.ucm.es
⁵ Technische Universität Dortmund, 44221 Dortmund, Germany
⁶ Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
⁷ Università di Padova and INFN, 35131 Padova, Italy
⁸ Inst. de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁹ University of Łódź, 90236 Łódź, Poland
¹⁰ Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland
¹¹ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹² Max-Planck-Institut für Physik, 80805 München, Germany e-mail: robert.wagner@mpp.mpg.de
¹³ Università di Siena, and INFN Pisa, 53100 Siena, Italy
¹⁴ Universitat de Barcelona (ICC/IEEC), 08028 Barcelona, Spain
¹⁵ Universität Würzburg, 97074 Würzburg, Germany e-mail: snruegam@astro.uni-wuerzburg.de
¹⁶ Depto. de Astrofisica, Universidad, 38206 La Laguna, Tenerife, Spain
¹⁷ Università di Udine, and INFN Trieste, 33100 Udine, Italy
¹⁸ Supported by INFN Padova
¹⁹ Institut de Ciències de l'Espai (IEEC-CSIC), 08193 Bellaterra, Spain
²⁰ : Max-Planck-Institut für Kernphysik, 69029 Heidelberg, Germany
²¹ : Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain
²² Inst. de Astrofísica de Andalucía (CSIC), 18080 Granada, Spain
²³ Croatian MAGIC Consortium, Institute R. Boskovic, University of Rijeka and University of Split, 10000 Zagreb, Croatia
²⁴ University of California, Davis, 95616-8677, USA
²⁵ Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
²⁶ INAF/Osservatorio Astronomico and INFN, 34143 Trieste, Italy
²⁷ ICREA, 08010 Barcelona, Spain
²⁸ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy

Received: 22 December 2009
Accepted: 19 May 2010

Abstract

Context. Wide-range spectral coverage of blazar-type active galactic nuclei is of paramount importance for understanding the particle acceleration mechanisms assumed to take place in their jets. The Major Atmospheric Gamma Imaging Cerenkov (MAGIC) telescope participated in three multiwavelength (MWL) campaigns, observing the blazar Markarian (Mkn) 421 during the nights of April 28 and 29, 2006, and June 14, 2006.

Aims. We analyzed the corresponding MAGIC very-high energy observations during 9 nights from April 22 to 30, 2006 and on June 14, 2006. We inferred light curves with sub-day resolution and night-by-night energy spectra.

Methods. MAGIC detects γ-rays by observing extended air showers in the atmosphere. The obtained air-shower images were analyzed using the standard MAGIC analysis chain.

Results. A strong γ-ray signal was detected from Mkn 421 on all observation nights. The flux (E > 250 GeV) varied on night-by-night basis between (0.92±0.11) × 10^-10 cm^-2 s^-1 (0.57 Crab units) and (3.21±0.15) × 10^-10 cm^-2 s^-1 (2.0 Crab units) in April 2006. There is a clear indication for intra-night variability with a doubling time of 36± min on the night of April 29, 2006, establishing once more rapid flux variability for this object. For all individual nights γ-ray spectra could be inferred, with power-law indices ranging from 1.66 to 2.47. We did not find statistically significant correlations between the spectral index and the flux state for individual nights. During the June 2006 campaign, a flux substantially lower than the one measured by the Whipple 10-m telescope four days later was found. Using a log-parabolic power law fit we deduced for some data sets the location of the spectral peak in the very-high energy regime. Our results confirm the indications of rising peak energy with increasing flux, as expected in leptonic acceleration models.