Abstract
Context. An unidentified emission line at 3.55 keV was recently detected in X-ray spectra of clusters of galaxies. The line has been discussed as a possible decay signature of 7.1 keV sterile neutrinos, which have been proposed as a dark matter (DM) candidate.Aims. We aim to further constrain the line strength and its implied mixing angle under the assumption that all DM is made of sterile neutrinos.Methods. The X-ray observations of the Limiting Window (LW) towards the Galactic bulge (GB) offer a unique dataset for exploring DM lines. We characterise the systematic uncertainties of the observation and the fitted models with simulated X-ray spectra. In addition, we discuss uncertainties of indirect DM column density constraints towards the GB to understand systematic uncertainties in the assumed DM mass in the field of view of the observation.Results. We find tight constraints on the allowed flux for an additional line at 3.55 keV with a positive (∼1.5σ) best fit valueFX3.55 keV ≈ (4.5 ± 3.5) × 10−7 cts cm−2 s−1. This would translate into a mixing angle of sin2(2Θ) ≈ (2.3 ± 1.8) × 10−11which, while consistent with some recent results, is in tension with earlier detections.Conclusions. We used a very deep dataset with well understood systematic uncertainties to derive tight constraints on the mixing angle of a 7.1 keV sterile neutrino DM candidate. The results highlight that the inner Milky Way will be a good target for DM searches with upcoming missions like eROSITA, XRISM, and ATHENA.
Highlights
The Limiting Window (LW) is located 1.5 deg south of Sgr A*
The line has been proposed as a candidate dark matter (DM) decay line and could be explained by the decay of sterile neutrinos with a mass of ms ≈ 7.1 keV
Sterile neutrinos with masses in the kiloelectronvolt range have long been discussed as a possible component of DM (e.g. Dodelson & Widrow 1994; Abazajian et al 2001; Boyarsky et al 2009), but until recently only upper limits could be derived
Summary
The Limiting Window (LW) is located 1.5 deg south of Sgr A* (in Galactic coordinates). In the case of sterile neutrino decay the measured additional flux at ∼3.55 keV would be related to two defining properties of the particles: the particle mass ms and the mixing-angle sin2(2Θ), which describes interaction of the sterile neutrinos with their active neutrino counterparts and the likelihood of decay in the γ/ν channel. These properties are related through the following equation (adapted from Bulbul et al 2014).
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