Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters (1402.2301v2)

Abstract: 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 significantly detected in all three independent MOS spectra and the PN "all others" spectrum. It is also detected in the Chandra spectra of Perseus with the flux consistent with XMM (though it is not seen in Virgo). However, it is very weak and located within 50-110eV of several known faint lines, and so is 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(2theta)=7e-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. This appears to be because of an anomalously bright line at E=3.62 keV in Perseus, possibly an Ar XVII dielectronic recombination line, although its flux would 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 factor 10-20. Confirmation with Chandra and Suzaku, and eventually Astro-H, are required to determine the nature of this new line.(ABRIDGED)

• The paper reports a robust detection (>3σ) of a 3.57 keV emission line in the stacked X-ray spectra of 73 galaxy clusters.
• The paper employs rigorous spectral modeling to rule out known atomic transitions and supports a dark matter decay hypothesis implying a sterile neutrino mass of about 7.1 keV.
• The paper notes anomalously high line intensity in the Perseus cluster, prompting further high-resolution X-ray studies to confirm its astrophysical origin.

Overview of "Detection of An Unidentified Emission Line in the Stacked X-ray Spectrum of Galaxy Clusters"

The paper reports on the detection of an unexplained emission line in the stacked X-ray spectra of 73 galaxy clusters. The line, detected at a rest energy of approximately 3.57 keV, appears consistently in both MOS and PN observations from XMM-Newton's EPIC instrument, albeit at slightly different energies. This detection remains robust over three independent subsamples and shows statistical significance greater than 3σ in most of these scenarios.

The paper's authors provide thorough technical analysis and modeling to ensure the line is not a result of known atomic transitions or metallic lines normally expected in such spectra. Interestingly, the line's properties fuels speculation about non-thermal processes; particularly, it is suggested that it might signal the decay of sterile neutrinos, a hypothetical dark-matter candidate. Specifically, the paper estimates that if the detected line was indeed caused by sterile neutrino decay, this would imply a neutrino mass of about 7.1 keV and a mixing angle of approximately $\sin^2(2\theta)\approx 7 \times 10^{-11}$.

The measurement contradicts expectations for some of the clusters, especially Perseus. The line's intensity there exceeds expectations significantly, raising the possibility it might be linked to anomalously strong processes or unidentified atomic lines. Notably, the Ar xvii dielectronic recombination line at 3.62 keV could be a contributing anomaly. However, its measured intensity exceeds theoretical predictions by about 30-fold, complicating this simple explanation.

A separate but related examination of the Perseus Cluster with Chandra supports the detection of this spectral feature, but not in the Virgo Cluster. Various fitting methodologies substantiate the energy and flux findings and underscore the need for an additional line at around 3.57 keV in the model fits.

Implications and Future Directions

The detection of this line, implying a dark matter-related interpretation, should propel future studies into confirming its existence using other spectral datasets and missions. Theoretical models for sterile neutrino production, especially non-standard mechanisms like inflaton decay, could be critical in the event the mixing angle and mass constraints remain consistent. Additional observations across a spectrum of galactic environments, leveraging next-generation X-ray observatories equipped with higher resolution capabilities, could help discern any linkage with dark matter and confirm or refute any particle decay hypotheses.

The potential significance of this work in further elucidating the composition of dark matter cannot be overstated. However, the authors wisely caution against premature conclusions and emphasize the importance of further scrutiny to eliminate instrumental artifacts, modeling inaccuracies, or unconsidered astrophysical processes. This paper lays a solid foundation for both corroborative and exploratory research efforts to unravel the nature of this emission line and its broad cosmological significance.