摘要：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
λFλ =
12.27-2.29+2.75
nW m-2 sr-1, including systematics.
