Galaxy-cluster-stacked Fermi-LAT, part IV: $\sim 70$ GeV WIMP annihilation lines

Abstract

The strongest constraints on the velocity-dependent ($p$-wave) annihilation of weakly interacting massive particle (WIMP) dark matter were derived from the deep potential wells of galaxy clusters. Even weaker signals can be extracted from sufficient aggregated clusters, by cross-correlating $\gamma$-rays with large-scale structure tracers or stacking over extensive cluster catalogs. Three independent such analyses show a similar triad of emission lines in Fermi-LAT data, around 70, 40, and 13 GeV, emerging from featureless spectra in each hemisphere upon cross-correlation with eROSITA maps, and in stacked MCXC, eROSITA, and DESI catalog clusters only once boosted to the cluster frame. These lines fit the anticipated $\chi \chi \to \gamma \gamma$, $\gamma Z$, and $\gamma h$ annihilation channels of a $\sim 70$ GeV WIMP $\chi$, detected by composite matched filters at trial-corrected global $Z$-scores reaching $5.6\sigma$ (cross-correlations) and $2.3\sigma$ (stacking), with intrinsic $\sim 10^{[-20,-19]}$ cm${}^3$ s${}^{-1}$ channel cross-sections. High-resolution spectra establish six lines and an unresolved ($2$--$3$ line) feature in total, naturally aligned with the anticipated nine channels of two cross-annihilating WIMPs of masses $67.3_{-0.1}^{+0.1}$ and $71.4_{-0.1}^{+0.2}$ GeV (profile-likelihood bounds; ${}_{-5\%}^{+3\%}$ systematic; $5.3\sigma$). The Galactic-center GeV excess is broadly consistent with the corresponding broad annihilation continuum.

Comments: Additional robustness tests