| Abstract: | M87 is one of the closest (z=0.00436) extragalactic sources emitting at
very-high-energies (VHE, E > 100 GeV). The aim of this work is to locate the
region of the VHE gamma-ray emission and to describe the observed broadband
spectral energy distribution (SED) during the low VHE gamma-ray state. The data
from M87 collected between 2012 and 2015 as part of a MAGIC monitoring
programme are analysed and combined with multi-wavelength data from Fermi-LAT,
Chandra, HST, EVN, VLBA and the Liverpool Telescope. The averaged VHE gamma-ray
spectrum can be fitted from 100GeV to 10TeV with a simple power law with a
photon index of (-2.41 $pm$ 0.07), while the integral flux above 300GeV is
$(1.44 pm 0.13) imes 10^{-12} cm^{-2} s^{-1}$. During the campaign between
2012 and 2015, M87 is generally found in a low emission state at all observed
wavelengths. The VHE gamma-ray flux from the present 2012-2015 M87 campaign is
consistent with a constant flux with some hint of variability ($sim3sigma$)
on a daily timescale in 2013. The low-state gamma-ray emission likely
originates from the same region as the flare-state emission. Given the
broadband SED, both a leptonic synchrotron self Compton and a hybrid
photo-hadronic model reproduce the available data well, even if the latter is
preferred. We note, however, that the energy stored in the magnetic field in
the leptonic scenario is very low suggesting a matter dominated emission
region. |
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