X-ray Observations of the Zwicky 3146 Galaxy Cluster Reveal a 3.5 Kev Excess

In this note, we present spectral fits of the well-documented sloshingcool-core cluster Zwicky 3146 (z=0.291), to test the existence of the highlyspeculated 3.5 keV line. We report excesses at >3σ significance atE=3.575 keV, yielding a flux F = 8.73_-2.22^+2.17 × 10^-6photons cm^-2 s^-1, in XMM-Newton, and E=3.55 keV, with aflux F = 10.0_-2.96^+3.05 × 10^-6 photons cm^-2 s^-1 inChandra. We explore the possibility that the 3.5 keV excess iscorrelated to the presence of cold gas within the cluster, based on optical andsub-mm literature analyses. Following the launch of the X-ray Imaging andSpectroscopy Mission (XRISM), high resolution spectroscopy (≤ 7 eV) willreveal in unprecedented detail, the origin of this unidentified feature, forwhich Zwicky 3146 should be considered a viable target, due to the strength ofthe feature in two independent X-ray telescopes, opening a new window intoplasma or charge exchange studies in galaxy clusters.