Constraints on sterile neutrino as a dark matter candidate from the diffuse X-ray background

TL;DR

This work derives model-independent constraints on keV-scale sterile-neutrino warm dark matter by analyzing the diffuse extragalactic X-ray background. It models the DM radiative decay signal, its cosmological dependence, and shows the decay signature is dominated by low-redshift contributions, favoring all-sky measurements. By fitting HEAO-1 and XMM-Newton XRB data with a two-component model (continuum plus DM decay line) and enforcing a chi-squared threshold, the authors obtain a stringent exclusion $\Omega_s \sin^2(2\theta) < 3\times10^{-5}(m_s/\mathrm{keV})^{-5}$, applicable in a model-independent way. The results highlight the importance of all-sky, wide-field instruments for DM-decay searches and provide context against prior bounds and DW scenarios.

Abstract

Sterile neutrinos with masses in the keV range are viable candidates for the warm dark matter. We analyze existing data for the extragalactic diffuse X-ray background for signatures of sterile neutrino decay. The absence of detectable signal within current uncertainties of background measurements puts model-independent constraints on allowed values of sterile neutrino mass and mixing angle, which we present in this work.

Figures (2)

• Figure 1: Diffuse X-ray background spectrum from HEAO-1 mission. The black solid curve shows empirical fit Eq. (\ref{['eq:5']}) by Gr99. The reduced $\chi^2$ of this fit is $1.2$. Dashed (green and red) lines represent the result of fit of the same data to the model of the form (\ref{['eq:5']}) with added DM component. The DM decay component (dot-dashed curve) is calculated for the "concordance" model $\Omega_\Lambda=0.7,\Omega_{matter}=0.3$ and for $m_s = 36.5$ keV. The green (long dashed) line represents a fit of DM with the mixing angle $\sin^2 2\theta = 1.9\cdot 10^{-12}$ and the best achievable fit has the reduced $\chi^2 = 2$. For the red (short-dashed) line the mixing angle is $\sin^2 2\theta = 2.4\cdot 10^{-13}$ and reduced $\chi^2 = 3$, which we choose as a border line of allowed quality of fit.
• Figure 2: Exclusion plot on parameters $m_s$ and $\sin^2 2\theta$ using HEAO-1 and XMM data. The values in the non-shaded region are allowed. In the region where both HEAO-1 and XMM-Newton data is available, HEAO-1 provides a more stringent constraint (as discussed in Section \ref{['sec:xmm']}). We supposed that sterile neutrino constitute 100% of all the DM. (i.e. $\Omega_s = \Omega_{DM}$). To remove dependence on the value of $\Omega_{DM}$, we choose to plot $\Omega_s \sin^2 2\theta$ rather than $\sin^2 2\theta$. The results can be described by a simple empirical formula Eq.(\ref{['eq:6']}) (thin blue dashed line on the Figure). The dash-dotted green line represent exclusion region, obtained if one attributes all 100% of the XRB flux to DM at energies $E\lesssim m_s/2$.