MAGIC observations of the February 2014 flare of 1ES 1011+496 and ensuing constraint of the EBL density

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, 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, Tenerife, Spain; 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 IFAE, Campus UAB, 08193 Bellaterra, Spain
¹³ Universität Würzburg, 97074 Würzburg, Germany
¹⁴ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
¹⁵ Università di Padova and INFN, 35131 Padova, Italy
¹⁶ Institute for Space Sciences (CSIC\IEEC), 08193 Barcelona, Spain
¹⁷ Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁸ Unitat de Física de les Radiacions, Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
¹⁹ Universitat de Barcelona, ICC, IEEC-UB, 08028 Barcelona, Spain
²⁰ Japanese MAGIC Consortium, ICRR, The University of Tokyo, Department of Physics and Hakubi Center, Kyoto University, Tokai University, The University of Tokushima, KEK, Japan
²¹ Finnish MAGIC Consortium, Tuorla Observatory, University of Turku and Department of Physics, University of Oulu, 90014 Oulu, Finland
²² Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
²³ Università di Pisa and INFN Pisa, 56126 Pisa, Italy
²⁴ ICREA and Institute for Space Sciences (CSIC\IEEC), 08193 Barcelona, Spain
²⁵ Università dell’Insubria and INFN Milano Bicocca, Como, 22100 Como, Italy
²⁶ now at Centro Brasileiro de Pesquisas Físicas (CBPF\MCTI), R. Dr. Xavier Sigaud, 150 – Urca, Rio de Janeiro 22290-180 – RJ, Brazil
²⁷ now at: NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA and Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
²⁸ Humboldt University of Berlin, Istitut für Physik Newtonstr. 15, 12489 Berlin, Germany
²⁹ now at: École polytechnique fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
³⁰ also at: INFN, 35131 Padova, Italy
³¹ now at: Laboratoire AIM, Service d’Astrophysique, DSM\IRFU, CEA\Saclay 91191 Gif-sur-Yvette Cedex, France
³² now at: Finnish Centre for Astronomy with ESO (FINCA), 20014 Turku, Finland
³³ also at: INAF-Trieste  
³⁴ also at: ISDC – Science Data Center for Astrophysics, 1290 Versoix ( Geneva )
³⁵ now at: Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, 01000 México D. F., Mexico

Received: 25 August 2015
Accepted: 16 February 2016

Abstract

Context. During February–March 2014, the MAGIC telescopes observed the high-frequency peaked BL Lac 1ES 1011+496 (z = 0.212) in flaring state at very-high energy (VHE, E> 100 GeV). The flux reached a level of more than ten times higher than any previously recorded flaring state of the source.

Aims. To describe the characteristics of the flare presenting the light curve and the spectral parameters of the night-wise spectra and the average spectrum of the whole period. From these data we aim to detect the imprint of the extragalactic background light (EBL) in the VHE spectrum of the source, to constrain its intensity in the optical band.

Methods. We analyzed the gamma-ray data from the MAGIC telescopes using the standard MAGIC software for the production of the light curve and the spectra. To constrain the EBL, we implement the method developed by the H.E.S.S. collaboration, in which the intrinsic energy spectrum of the source is modeled with a simple function (≤4 parameters), and the EBL-induced optical depth is calculated using a template EBL model. The likelihood of the observed spectrum is then maximized, including a normalization factor for the EBL opacity among the free parameters.

Results. The collected data allowed us to describe the night-wise flux changes and also to produce differential energy spectra for all nights in the observed period. The estimated intrinsic spectra of all the nights could be fitted by power-law functions. Evaluating the changes in the fit parameters, we conclude that the spectral shape for most of the nights were compatible, regardless of the flux level, which enabled us to produce an average spectrum from which the EBL imprint could be constrained. The likelihood ratio test shows that the model with an EBL density 1.07 (–0.20, +0.24)_stat+sys, relative to the one in the tested EBL template, is preferred at the 4.6σ level to the no-EBL hypothesis, with the assumption that the intrinsic source spectrum can be modeled as a log-parabola. This would translate into a constraint of the EBL density in the wavelength range [0.24 μm, 4.25 μm], with a peak value at 1.4 μm of λF_λ = 12.27_-2.29^+2.75 nW m^-2 sr^-1, including systematics.