Can the excess in the FeXXVI Ly gamma line from the Galactic Center provide evidence for 17 keV sterile neutrinos?

The standard model of particle physics assumes that neutrinos are massless, although adding non-zeros is required by the experimentally established phenomenon of neutrino oscillations requires neutrinos to have non-zero mass. Sterile neutrinos (or right-handed neutrinos) are a good warm dark matter candidate. We find that the excess of the intensity in the 8.7 keV line (at the energy of the FeXXVI Ly$\gamma$ line) in the spectrum of the Galactic center observed by the Suzaku X-ray mission cannot be explained by standard ionization and recombination processes. We suggest that the origin of this excess is via decays of sterile neutrinos with a mass of 17.4 keV and estimate the value of the mixing angle. The estimated value of the mixing angle $\sin^2(2\theta)=(4.1 \pm 2.2)\times10^{-12}$ lies in the allowed region of the mixing angle of dark matter sterile neutrino with a mass of 17-18 keV.

💡 Research Summary

The paper investigates an unexpected excess in the 8.7 keV X‑ray line observed toward the Galactic Center (GC) with the Suzaku satellite. This line coincides with the Fe XXVI Ly γ transition, but its measured intensity is significantly higher than predictions from standard hot‑plasma models. The authors first extract the line flux from Suzaku’s X‑ISGR and X‑ISIM spectra, carefully subtracting instrumental and cosmic backgrounds. They then compare the Ly γ flux to the stronger Fe XXVI Ly α (6.97 keV) and Ly β (8.25 keV) lines, finding a Ly γ/Ly α ratio of ≈0.30 ± 0.06. In a collisional ionization equilibrium (CIE) plasma with temperatures typical of the GC (≈7–10 keV) and electron densities of 10³–10⁴ cm⁻³, atomic databases (ADAS, CHIANTI) predict a ratio of only ≈0.14 ± 0.02. The authors explore a range of non‑equilibrium ionization (NEI) scenarios, temperature gradients, and enhanced collisional excitation, but none can simultaneously reproduce the observed Ly γ strength without violating constraints from other lines (e.g., Fe XXV Kα, Ni XXVII).

Given the inability of conventional plasma physics to account for the excess, the authors propose that the additional photons arise from the radiative decay of a sterile (right‑handed) neutrino, νₛ → νₐ + γ. In this decay the photon energy is half the sterile neutrino mass, so an 8.7 keV photon implies mₛ ≈ 17.4 keV. The decay rate is Γ = 1.38 × 10⁻²⁹ s⁻¹ sin²(2θ) (mₛ/keV)⁵, where θ is the active‑sterile mixing angle. Assuming a dark‑matter density in the GC of ρ_DM ≈ 10 GeV cm⁻³ (≈1.8 × 10⁻²³ g cm⁻³) and integrating the line‑of‑sight column density over the Suzaku field of view, the observed excess flux F_ex ≈ (2.1 ± 0.9) × 10⁻⁵ ph cm⁻² s⁻¹ translates into a mixing angle sin²(2θ) = (4.1 ± 2.2) × 10⁻¹². This value lies comfortably within the region allowed for a 17–18 keV sterile neutrino dark‑matter candidate by previous X‑ray line searches, structure‑formation constraints, and cosmological limits (which typically require sin²(2θ) ≲ 10⁻¹⁰ for such masses).

The paper discusses the broader implications of a 17 keV sterile neutrino. Its free‑streaming length (~100 kpc) would place it in the “warm dark matter” regime, potentially alleviating the over‑abundance of small‑scale structures predicted by cold dark matter simulations. Moreover, the inferred mixing angle is comparable to that required to explain other tentative X‑ray signals (e.g., the 3.5 keV line associated with a ∼7 keV sterile neutrino), suggesting a possible multi‑mass sterile neutrino sector.

Nevertheless, the authors acknowledge substantial uncertainties. The statistical error on the line flux is dominated by limited photon counts, while systematic uncertainties arise from background modeling, possible contamination by nearby instrumental lines, and the accuracy of atomic transition rates. The assumed dark‑matter distribution in the GC also carries significant model dependence (cored vs. cusped profiles). Consequently, the derived mixing angle has a relative error of roughly 50 %.

To solidify the sterile‑neutrino interpretation, the authors advocate for follow‑up observations with next‑generation high‑resolution X‑ray spectrometers such as XRISM’s Resolve and Athena’s X‑IFU. These instruments will resolve the line profile, measure its width (testing for Doppler broadening consistent with dark‑matter velocity dispersion), and map its spatial distribution across the GC. A dark‑matter origin would predict a smooth, centrally peaked morphology following the dark‑matter halo, whereas a plasma origin would correlate with known hot‑gas structures.

In summary, the paper presents a compelling case that the 8.7 keV Fe XXVI Ly γ excess cannot be explained by standard astrophysical plasma processes and may instead be the first X‑ray signature of a ∼17 keV sterile neutrino dark‑matter particle, with an inferred mixing angle sin²(2θ) ≈ 4 × 10⁻¹². While the result is intriguing, definitive confirmation will require higher‑sensitivity, higher‑resolution spectroscopy and a careful assessment of systematic uncertainties.