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 sloshing cool-core cluster Zwicky 3146 (z=0.291), to test the existence of the highly speculated 3.5 keV line. We report excesses at >3σ significance at E=3.575 keV, yielding a flux F = 8.73_-2.22^+2.17 × 10^-6 photons cm^-2 s^-1, in XMM-Newton, and E=3.55 keV, with a flux F = 10.0_-2.96^+3.05 × 10^-6 photons cm^-2 s^-1 in Chandra. We explore the possibility that the 3.5 keV excess is correlated to the presence of cold gas within the cluster, based on optical and sub-mm literature analyses. Following the launch of the X-ray Imaging and Spectroscopy Mission (XRISM), high resolution spectroscopy (≤ 7 eV) will reveal in unprecedented detail, the origin of this unidentified feature, for which Zwicky 3146 should be considered a viable target, due to the strength of the feature in two independent X-ray telescopes, opening a new window into plasma or charge exchange studies in galaxy clusters.