Preprint typeset using LATEX style emulateapj v. 04/17/13 DETECTION OF AN UNIDENTIFIED EMISSION LINE IN THE STACKED X-RAY SPECTRUM OF GALAXY CLUSTERS

We detect a weak unidentified emission line at E = (3.55 − 3.57) ± 0.03 keV in a stacked XMM spectrum of 73 galaxy clusters spanning a redshift range 0.01 − 0.35. MOS and PN observations independently show the presence of the line at consistent energies. When the full sample is divided into three subsamples (Perseus, Centaurus+Ophiuchus+Coma, and all others), the line is seen at> 3σ statistical significance in all three independent MOS spectra and the PN “all others ” spectrum. The line is also detected at the same energy in the Chandra ACIS-S and ACIS-I spectra of the Perseus cluster, with a flux consistent with XMM-Newton (however, it is not seen in the ACIS-I spectrum of Virgo). The line is present even if we allow maximum freedom for all the known thermal emission lines. However, it is very weak (with an equivalent width in the full sample of only ∼ 1 eV) and located within 50–110 eV of several known faint lines; the detection is at the limit of the current instrument capabilities and subject to significant modeling uncertainties. On the origin of this line, we argue that there should be no atomic transitions in thermal plasma at this energy. An intriguing possibility is the decay of sterile neutrino, a long-sought dark matter particle candidate. Assuming that all dark matter is in sterile neutrinos with ms = 2E = 7.1 keV, our detection in the full sample corresponds to a neutrino decay mixing angle sin2(2θ) ≈ 7 × 10−11, below the previous upper limits. However, base