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Recently, the Fermi-LAT collaboration reported upper limits on the GeVgamma-ray flux from nearby clusters of galaxies. Motivated by these limits, westudy corresponding constraints on gamma-ray emissions from two specificdecaying dark matter models, one via grand unification scale suppressedoperators and the other via R-parity violating operators. Both can account forthe PAMELA and Fermi-LAT excesses of e^\pm. For GUT decaying dark matter, thegamma-rays from the M49 and Fornax clusters, with energy in the range of 1 to10 GeV, lead to the most stringent constraints to date. As a result, this darkmatter is disfavored with conventional model of e^\pm background. In addition,it is likely that some tension exists between the Fermi-LAT e^\pm excess andthe gamma ray constraints for any decaying dark matter model, providedconventional model of e^\pm background is adopted. Nevertheless, the GUTdecaying dark matter can still solely account for the PAMELA positron fractionexcess without violating the gamma-ray constraints. For the gravitino darkmatter model with R-parity violation, cluster observations do not give tightconstraints. This is because a different e^\pm background has been adoptedwhich leads to relatively light dark matter mass around 200 GeV.

Gamma-rays from Nearby Clusters: Constraints on Selected Decaying Dark Matter Models