Search for a gamma-ray line feature from a group of nearby Galaxy
  clusters with Fermi LAT Pass 8 data
  Galaxy clusters are the largest gravitationally bound objects in the universe
and may be suitable targets for indirect dark matter searches. With 85 months
of Fermi-LAT Pass 8 publicly available data, we analyze the gamma-ray emission
in the directions of 16 nearby Galaxy Clusters with an unbinned likelihood
analysis. No globally statistically-significant $\gamma-$ray line feature is
identified and a tentative line signal may be present at $\sim 43$ GeV. The
95\% confidence level upper limits on the velocity-averaged cross section of
dark matter particles annihilating into double $\gamma-$rays (i.e., $\langle
\sigma v \rangle_{\chi\chi\rightarrow \gamma\gamma}$) are derived. Unless very
optimistic boost factors of dark matter annihilation in these Galaxy Clusters
have been assumed, such constraints are much weaker than the bounds set by the
Galactic $\gamma-$ray data.
