Direct X-ray Constraints on Sterile Neutrino Warm Dark Matter

TL;DR

The paper investigates keV-scale sterile neutrino warm dark matter by directly constraining its radiative decay with X-ray observations. Using the diffuse X-ray spectrum of Andromeda (M31) and comparing to Virgo A (M87), the authors derive a strongest direct bound of $m_s \lesssim 3.5$ keV (95% CL) for $\Omega_s=0.24$, outperforming previous cluster-based limits. They implement both a scaling method and a direct-data method to map exclusions in the $m_s$–$\sin^2 2\theta$ plane and demonstrate that, when combined with Ly$-\alpha$ and other indirect limits, the viable mass range tightens to about $1.7$–$3.5$ keV for minimal lepton asymmetry, though larger $L$ can shift this window. The results highlight the complementary nature of direct X-ray searches and structure-formation constraints in testing sterile neutrino WDM, and motivate deeper X-ray studies of nearby galaxies to probe even lower $m_s$ values.

Abstract

Warm dark matter (WDM) might more easily account for small scale clustering measurements than the heavier particles typically invoked in Lambda cold dark matter (LCDM) cosmologies. In this paper, we consider a Lambda WDM cosmology in which sterile neutrinos nu_s, with a mass m_s of roughly 1-100 keV, are the dark matter. We use the diffuse X-ray spectrum (total minus resolved point source emission) of the Andromeda galaxy to constrain the rate of sterile neutrino radiative decay: nu_s -> nu_{e,mu,tau} + gamma. Our findings demand that m_s < 3.5 keV (95% C.L.) which is a significant improvement over the previous (95% C.L.) limits inferred from the X-ray emission of nearby clusters, m_s < 8.2 keV (Virgo A) and m_s < 6.3 keV (Virgo A + Coma).

Figures (2)

• Figure 1: Here we compare the detectability of $\nu_s$ decays in Andromeda M31 and Virgo A Aba05bVirgoX. The first statistically significant ($4\sigma_{\rm f}$) $\nu_s$ decay peak relative to the measured spectrum of Andromeda occurs at $E_{\gamma, \rm s} = m_{\rm s, lim}/2$ = 1.75 keV, which excludes $m_s > 3.5$ keV (95% C.L.). According to the analysis presented in Aba05b, the spectrum of Virgo A excludes $m_s > 8.2$ keV (95% C.L.), which would produce a decay signature like the dashed histogram. Because Andromeda would produce a similar $\nu_s$ decay signal to Virgo A (Eqn. \ref{['DMflux']}), but over a much smaller background, the prospective decay signature of 8.2 keV sterile neutrinos in Andromeda is enormous by comparison. As an intermediate case, we also show what the decay peak associated with a 6.3 keV sterile neutrino, the estimated Virgo A + Coma mass limit Boyarsky2006AbaSav, would look like in Andromeda. The vertical ($1\sigma$) error bars reflect the Poisson statistics of the signal and background count rates measured during each observation.
• Figure 2: Here we present constraints on $m_s$ as a function of mixing angle, sin$^2 2\theta$, assuming that all dark matter is comprised of sterile neutrinos. To facilitate comparisons, we adopt many of the conventions used by Abazajian and Koushiappas AbaSav. For $L = 0$, the thick, solid line corresponds to $\Omega_s= 0.24$ (Eqn. \ref{['Omega_sin2th']}), while the shaded region to the right corresponds to $\Omega_s > 0.24$. Three density-production relationships associated with $\Omega_s= 0.3$ and $L \gg 10^{-10}$ are also shown AbaSav. The two previous direct radiative decay ($\nu_s \rightarrow \nu_{e,\mu,\tau} + \gamma$) upper limits (both 95% C.L.) are based on measurements LumbReadPonXMarshallGruber of the Cosmic X-ray Background Boyarsky:2005us and XMM observations VirgoXBriel2001Neumann2003 of Virgo A (M87) and the Coma cluster Aba05bBoyarsky2006AbaSav. The most stringent direct limits, from the present work (also 95% C.L.), are based on XMM observations of the Andromeda galaxy M31. The region bounded by the dashed line is excluded by the "$\Gamma_{\rm s,tot}-$scaling method", while the region above the solid, slightly jagged line is excluded by the more accurate "direct data method" (see Sec. \ref{['mslimit']}). The indirect lower limits (all 95% C.L.) labeled Ly$\alpha$(1) and Ly$\alpha$(2) were derived in Ref. Aba05b, while Ly$\alpha$(3) was derived in Ref. Seljak. Sterile neutrinos that occupy the horizontally hatched region could explain pulsar kicks Kusenko1998Fuller2003Kusenko2004Barkovich2004Fryer2005.