MAGIC observations of the February 2014 flare of 1ES 1011+496 and
  ensuing constraint of the EBL density
  In 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>100GeV). The flux reached a level more than 10 times higher than any
previously recorded flaring state of the source. We present the description of
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 at detecting the imprint of the Extragalactic
Background Light (EBL) in the VHE spectrum of the source, in order to constrain
its intensity in the optical band. For this 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, 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. From the data collected differential energy spectra was
produced for all nights of the observed period. 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
(Dominguez et al.2011), is preferred at the 4.6 sigma 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 um,4.25 um], with a peak value at 1.4 um
of F=12.27_{-2.29}^{+2.75} nW m^{-2} sr^{-1}, including systematics.
