DETECTION OF AN UNIDENTIFIED EMISSION LINE IN THE STACKED X-RAY SPECTRUM OF GALAXY CLUSTERS

By E. Bulbul, M. Markevitch, A. Foster, Randall K. Smith, M. Loewenstein, S. Randall. 2014

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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. 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. It is also detected in the Chandra spectra of the Perseus Cluster. However, it is very weak and located within 50–110 eV of several known lines. The detection is at the limit of the current instrument capabilities. 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 corresponds to a neutrino decay rate consistent with 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. 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 of 10–20. Confirmation with Astro-H will be critical to determine the nature of this new line.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/bd3c1d4eb7e21808588253eba4e809910d83285b

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Citations in the corpus

A new bound on axion-like particles (2017, background): “5 keV line from galaxy clusters [67, 68], which may be explained by dark matter decaying into ALPs, which subsequently convert to photons [18, 69, 70].”
Constraints from Ly-α forests on non-thermal dark matter including resonantly-produced sterile neutrinos (2017, background): “12Curiously, the 7 keV RPSN with the mixing angle within the range reported in [81, 82] produces the suppression of the flux power spectrum at small scales that would fit the XQ + HR data if the intergalactic medium is cold at redshifts z ≥ 5 [24].”
History of dark matter (2018, neutral): “In particular, reports of a 3.55 keV line observed from a collection of galaxy clusters (Boyarsky et al., 2014; Bulbul et al., 2014) have recently received a great deal of attention within the context of a decaying sterile neutrino (Abazajian, 2017).”
The mass–concentration–redshift relation of cold and warm dark matter haloes (2016, background): “Interest in such models has been revitalized by recent claims of detection of an ∼ 3.5 keV X-ray line, which is in principle consistent with the decay of a WDM particle (e.g. Bulbul et al. 2014; Boyarsky et al. 2014, 2015, but see, Malyshev, Neronov & Eckert 2014; Anderson, Churazov & Bregman 2015).”
Dark matter annihilation and decay in dwarf spheroidal galaxies: the classical and ultrafaint dSphs (2015, background): “Dark matter decay is less often considered than annihilation, however recent observations of an unidentified X-ray line at 3.55 keV in galaxy clusters has generated increasing interest in this possibility (e.g., Bulbul et al. 2014a; Boyarsky et al. 2014b).”
A White Paper on keV sterile neutrino Dark Matter (2016, background): “WDM also delays the collapse of dwarf-sized haloes and their associated star formation to later epochs.”
WIMP dark matter candidates and searches—current status and future prospects (2017, methodology): “5 keV was reported after analyzing the XMM-Newton data from observations of the Andromeda galaxy and various galaxy clusters with possible connection to decays of sterile neutrino DM [429, 430] (see also e.”
Dark matter direct-detection experiments (2015, methodology): “5 keV was found in the data recorded by both satellites [89][90].”