Evidence for a sim 43 GeV γ-ray line signal in a stacking analysis of the Virgo, Fornax, and Ophiucus Galaxy clusters

As the largest gravitationally bound objects in the Universe, galaxy clusters have provided the first piece of evidence for the presence of dark matter and may be suitable targets for indirect dark matter searches. Among various signals, the GeV-TeV $\gamma$-ray line has been taken as the smoking-gun signal of the dark matter annihilation/decay since no known astrophysical/physical process(es) could generate such a peculiar spectrum. With 15.5 years of Fermi-LAT P8R3 publicly available data, we search for the $\gamma$-ray line emission in the directions of a group of 13 massive galaxy clusters at redshifts $z \leq 0.028$ with an unbinned likelihood analysis. A $\gamma$-ray line signal at $\sim 43.2$ GeV has a net TS value of $\approx 30$ if we only take into account the data in the directions of Virgo, Fornax and Ophiuchus clusters, three massive clusters with the highest J-factors expected to generate the dark matter annihilation signal. The signal still presents when the data of other 10 nearby massive clusters have also been included, though the TS value decreases to $\approx 21$ likely because of their lower signal-to-noise ratios. The absence of this signal in the inner Galaxy disfavors both the instrumental effect and the canonical dark matter annihilation interpretation, and a more sophisticated dark matter model or very peculiar astrophysical scenario might be needed. This $\gamma$-ray line signal, if intrinsic, could be unambiguously verified by the Very Large Area $\gamma$-ray Space Telescope in its first two years of performance.