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 direction of 16 nearby galaxy clusters with an unbinned likelihood analysis. No statistically or globally significant $\gamma \text{−}\mathrm{ray}$ line feature is identified and a tentative line signal may present at $\sim 43\mathrm{GeV}$. The 95% confidence level upper limits on the velocity-averaged cross section of dark matter particles annihilating into double $\gamma$ rays (i.e., $⟨\sigma v{⟩}_{\chi \chi \to \gamma \gamma }$) 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 $\gamma \text{−}\mathrm{ray}$ data.

• Received 22 February 2016

DOI:https://doi.org/10.1103/PhysRevD.93.103525