A deep search for decaying dark matter with XMM-Newton blank-sky observations

Abstract: Sterile neutrinos with masses in the keV range are well-motivated extensions to the Standard Model that could explain the observed neutrino masses while also making up the dark matter (DM) of the Universe. If sterile neutrinos are DM then they may slowly decay into active neutrinos and photons, giving rise to the possibility of their detection through narrow spectral features in astrophysical X-ray data sets. In this work, we perform the most sensitive search to date for this and other decaying DM scenarios across the mass range from 5 to 16 keV using archival XMM-Newton data. We reduce 547 Ms of data from both the MOS and PN instruments using observations taken across the full sky and then use this data to search for evidence of DM decay in the ambient halo of the Milky Way. We determine the instrumental and astrophysical baselines with data taken far away from the Galactic Center, and use Gaussian Process modeling to capture additional continuum background contributions. No evidence is found for unassociated X-ray lines, leading us to produce the strongest constraints to date on decaying DM in this mass range.

An Analysis of Decaying Dark Matter Using {\it XMM-Newton} Observations

This paper presents a comprehensive investigation into potential X-ray signals resulting from the decay of dark matter (DM), focusing primarily on sterile neutrinos with sub-GeV masses, using extensive archival data gathered by the {\it XMM-Newton} telescope. The research, authored by Foster et al., reveals significant and robust insights into the presence—or more accurately—absence of X-ray emissions in the specified energy range, providing valuable constraints on the properties and viability of certain DM candidates.

Research Framework and Methodology

The authors anchored their study on sterile neutrinos, which are postulated as an extension of the Standard Model to potentially constitute the observable DM that pervades the universe. These neutrinos, if existing, would have masses in the keV range and could decay into detectable X-ray emissions. Utilizing the {\it XMM-Newton} telescope's data post-2018, the research meticulously scrutinizes signals across the energy range of 2.5 to 8 keV, corresponding to DM masses between 5 to 16 keV.

A key methodological advancement of this paper is the strategic leveraging of Gaussian Process (GP) modeling for the continuum emissions, coupled with the implementation of spurious-signal parameters that allow the background modeling to be non-parametric and more refined. This flexible modeling approach markedly enhances the sensitivity of the search while minimizing mismodeling that conventional parametric methods might incur.

The analysis uses data from hundreds of millions of seconds worth of observations which significantly boosts the capability to detect or rule out the narrow spectral emissions indicative of DM decay. These comprehensive databases are carefully parsed and organized across concentric annuli from the Galactic Center, enabling a geographical sensitivity to potential emissions.

Numerical Results and Interpretations

The study yields the strongest constraints yet on the DM lifetime and mixing angles within the tested mass range, effectively challenging certain DM constitutional theories including the $\nu$MSM—theoretical frameworks requiring that $\sin^2(2 \theta)$ values remain relatively large for stability. Its numerical limits are juxtaposed against prior constraints from other X-ray investigations, broadly contesting sterility in the keV domain as a plausible strategy for DM production without violating observed density limits.

In the region near 3.5 keV, known for its historical claims of anomalous emissions, the methodology refines prior detection techniques. Through enhanced modeling and energy side-band analysis, the results are found consistent with null detection, thus firmly dismissing decaying DM theories attached to this previously contentious signal.

Theoretical and Practical Implications

The implications of this study are multifaceted. Theoretically, the paper constrains the parameter space pertinent to sterile neutrinos as DM candidates, delimiting their feasibility based on decay dynamics and emission likelihoods under the assumed astrophysical models. Practically, it questions the validity of DM theories reliant on sterile neutrinos, potentially guiding future studies to account for alternative models or adjust existing ones to comply with these stringent constraints.

Further developments cite anticipated missions such as {\it Athena} and {\it XRISM}, suggesting an outlook of refined detection modalities with potential to probe deeper into underexplored areas of astrophysical X-ray spectrum data. Such advancements might unlock novel dimensions of DM properties potentially masked by current instrument sensitivities.

In conclusion, this paper advances potent technical methodology in dark matter research and utilizes expansive astronomical data to yield finely honed constraints on a class of DM candidates. As part of the broader scientific endeavor to unravel the mysteries of dark matter, its findings constitute a meticulous progress in understanding and challenging existing theoretical models, speeding up the scientific journey to potentially identify these elusive cosmic constituents.