Detection of an Unidentified Emission Line in the Stacked X-Ray Spectrum of Galaxy Clusters

Abstract

We detect a weak unidentified emission line at E = (3.55−3.57)±0.03 keV in a stacked XMM-Newton 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 m s = 2E = 7.1 keV, our detection in the full sample corresponds to a neutrino decay mixing angle sin 2 (2θ) ≈ 7 × 10 −11 , below the previous upper limits. However, based on the cluster masses and distances, the line in Perseus is much brighter than expected in this model, significantly deviating from other subsamples. This appears to be because of an anomalously bright line at E = 3.62 keV in Perseus, which could be an Ar xvii dielectronic recombination line, although its emissivity would have to be 30 times the expected value and physically difficult to understand. In principle, such an anomaly might explain our line detection in other subsamples as well, though it would stretch the line energy uncertainties. Another alternative is the above anomaly in the Ar line combined with the nearby 3.51 keV K line also exceeding expectation by a factor 10-20. Confirmation with Chandra and Suzaku, and eventually Astro-H, are required to determine the nature of this new line. (APJ HAS THE ABRIDGED ABSTRACT) Γ γ (m s , θ) = 1.38 × 10 −29 s −1 sin 2 2θ 10 −7 m s 1 keV 5 ,(1) where the particle mass m s and the "mixing angle" θ

7.1 keV sterile neutrino constraints from X-ray observations of 33 clusters of galaxies withChandraACIS

“…value is mainly driven by outliers like Hydra A. The average of the calculated mixing angle is much lower than the value of Bulbul et al (2014), but is consistent within the 3σ upper limit (see Fig. 2).…”

“…All 3σ (99.7 per cent) upper limits are compatible with the value of sin 2 (2Θ) ∼ 7 × 10 −11 found for the full sample of Bulbul et al (2014). A simultaneous fit of the merged ACIS-I and ACIS-S spectrum did not improve the limits.…”

“…Bulbul et al 2014). This procedure provided a goodness of fit comparable to the fitting process used in this study.…”

“…Modelling the emission lines separately (e.g. as Bulbul et al 2014) does not significantly improve the fit, except for the two strongest features at 3.85 and 4.7 keV (perhaps due to incomplete modelling of the temperature structure, as discussed in Appendix C). If these two features are included in our model, there is no significant change to our limits.…”

“…To turn this flux into a limit on the expected mixing angle in the 7.1 keV sterile neutrino scenario, we estimated the average mass and luminosity distance of the cluster sample. We weighted the contributions to the average with their expected contribution ω i,DM to the DM line flux with the formula used by Bulbul et al (2014),…”