Searching for gamma-ray emission from galaxy clusters at low redshift
  We report the identification of a positive cross-correlation signal between
the unresolved $\gamma$-ray emission, measured by the \emph{Fermi} Large Area
Telescope, and four different galaxy cluster catalogues. The selected
catalogues peak at low-redshift and span different frequency bands, including
infrared, optical and X-rays. The signal-to-noise ratio of the detected
cross-correlation amounts to 3.5 in the most significant case. We investigate
and comment about its possible origin, in terms of compact $\gamma$-ray
emission from AGNs inside clusters or diffuse emission from the intracluster
medium. The analysis has been performed by introducing an accurate estimation
of the cross-correlation power-spectrum covariance matrix, built with mock
realisations of the gamma and galaxy cluster maps. Different methods to produce
the mock realizations starting from the data maps have been investigated and
compared, identifying suitable techniques which can be generalized to other
cross-correlation studies.
