Direct Detection of Warm Dark Matter in the X-ray

TL;DR

This paper argues that the 1–10 keV X-ray window, accessible to Chandra, XMM-Newton, and future Constellation X observations, provides a direct probe of warm dark matter candidates that decay radiatively. By relating the decay luminosity of dark matter halos to observable line fluxes via ${\\cal L} \\approx \\frac{E_\\gamma}{m_X} M_{DM} \\Gamma_\\gamma$ and $F={\\cal L}/(4\\pi D_L^2)$, the authors derive bounds on radiative decay rates and translate them into constraints on sterile neutrino masses $m_s$, mixing $\\sin^2 2\\theta$, and lepton-number $L_{\\nu_\alpha}$ for production scenarios. Using Virgo Cluster data, they obtain $m_s \lesssim 2.6$ keV for the zero-lepton-number case, with Constellation X poised to test broader parameter space up to $L_{\\nu_\alpha} \lesssim 0.1$; analogous analyses for field galaxies yield $m_s$ limits of several keV. Diffuse X-ray background limits further constrain decays integrated over cosmic history, depending on the structure formation epoch. Overall, current and planned X-ray observations offer a powerful, indirect means to constrain or potentially detect warm dark matter candidates, linking cosmology with particle physics through observable X-ray lines.

Abstract

We point out a serendipitous link between warm dark matter (WDM) models for structure formation on the one hand and the high sensitivity energy range (1-10 keV) for x-ray photon detection on the Chandra and XMM-Newton observatories on the other. This fortuitous match may provide either a direct detection of the dark matter or exclusion of many candidates. We estimate expected x-ray fluxes from field galaxies and clusters of galaxies if the dark matter halos of these objects are composed of WDM candidate particles with rest masses in the structure formation-preferred range (~1 keV to ~20 keV) and with small radiative decay branches. Existing observations lead us to conclude that for singlet neutrinos (possessing a very small mixing with active neutrinos) to be a viable WDM candidate they must have rest masses < 5 keV in the zero lepton number production mode. Future deeper observations may detect or exclude the entire parameter range for the zero lepton number case, perhaps restricting the viability of singlet neutrino WDM models to those where singlet production is driven by a significant lepton number. The Constellation X project has the capability to detect/exclude singlet neutrino WDM for lepton number values up to 10% of the photon number. We also consider diffuse x-ray background constraints on these scenarios. These same x-ray observations additionally may constrain parameters of active neutrino and gravitino WDM candidates.

Figures (2)

• Figure 5: A synthesized spectrum viewed by Chandra's ACIS modeling the central region of the Virgo cluster, for two different cases of singlet neutrino mass, showing a strong mass dependence of the flux in a line at (a) a 4 keV singlet neutrino halo (producing a 2 keV line), and (b) a 5 keV singlet neutrino halo (producing a 2.5 keV line). Integration time is assummed to be 50 ksec, with a $T_{\rm X} = 2.54\rm keV$ thermal flux ($1.5 \times 10^{-12}{\rm\ erg\ cm^{-2}\ sec^{-1}}$) from the gas in Virgo generated with the MEKAL model, which is shown as the solid line. Residuals from the gas emission model alone are shown at the bottom. The width of the line is nearly entirely due to instrumental energy resolution.
• Figure 6: Limits on the diffuse flux from singlet neutrino decay in the $L_{\nu_\alpha}\approx 0$ case from photons produced at (a) $z\geq 0$ and (b) $z\geq 10$, assuming isotropy of the dark matter at that redshift.