The complex gamma-ray behaviour of the Radio Galaxy M87
  Context.In recent years, non-blazar Active Galactic Nuclei (AGN) such as
Radio Galaxies have emerged as a highly instructive source class providing
unique insights into high energy acceleration and radiation mechanisms.
Aims.Here we aim at a detailed characterization of the high-energy (HE;>100
MeV) gamma-ray emission from the prominent radio galaxy M87. Methods.We analyze
8 years of Fermi-LAT data and derive the spectral energy distribution between
100 MeV and 300 GeV. We extract lightcurves and investigate the variability
behaviour for the entire energy range as well as below and above 10 GeV.
Results.Our analysis provides (i)evidence for HE gamma-ray flux variability and
(ii)indications for a possible excess over the standard power-law model above
Eb~10 GeV, similar to the earlier indications in the case of Cen A. When viewed
in HE-VHE context, this is most naturally explained by an additional component
dominating the highest-energy part of the spectrum. Investigation of the
gamma-ray lightcurves suggests that the lower-energy (<10 GeV) component is
variable on timescales of (at least) a few months. The statistics of the high
energy component (>10 GeV) does not allow significant constraints on
variability. We find indications, however, for spectral changes with time that
support variability of the putative additional component and seem to favor
jet-related scenarios for its origin capable of accommodating month-type
variability. Conclusions.The current findings suggest that both the high-energy
(> Eb) and the very high energy (VHE;>100 GeV) emission in M87 are compatible
with originating from the same physical component. The variability behaviour at
VHE then allows further constraints on the location and the nature of the
second component. In particular, these considerations suggest that the VHE
emission during the quiescent state originates in a similar region as during
the flare.
