Galaxy clusters are predicted to produce \gamma -rays through cosmic ray interactions and/or dark matter annihilation , potentially detectable by the Fermi Large Area Telescope ( Fermi -LAT ) .
We present a new , independent stacking analysis of Fermi -LAT photon count maps using the 78 richest nearby clusters ( z < 0.12 ) from the Two Micron All-Sky Survey ( 2MASS ) cluster catalog .
We obtain the lowest limit on the photon flux to date , 2.3 \times 10 ^ { -11 } ~ { } \hbox { phot~ { } cm$ { } ^ { -2 } $~ { } s$ { } ^ { -1 } $ } ( 95 % confidence ) per cluster in the 0.8–100 GeV band , which corresponds to a luminosity limit of 3.5 \times 10 ^ { 44 } ~ { } \hbox { phot~ { } s$ { } ^ { -1 } $ } .
We also constrain the emission limits in a range of narrower energy bands .
Scaling to recent cosmic ray acceleration and \gamma -ray emission models , we find that cosmic rays represent a negligible contribution to the intra-cluster energy density and gas pressure .