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 >3 σ 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, 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.