Limits on the Radiative Decay of Sterile Neutrino Dark Matter from the Unresolved Cosmic and Soft X-ray Backgrounds

TL;DR

This work constrains the radiative decay of sterile neutrino dark matter by analyzing unresolved cosmic X-ray background data and a soft X-ray calorimeter measurement. Using Milky Way halo models, it translates decay-rate predictions into flux limits and derives 2$\sigma$ bounds on the sterile neutrino mass in the Dodelson-Widrow production framework, finding $m_s < 2.87$ keV (high-mass halo) or $m_s < 5.66$ keV (low-mass halo). The primary CXB analysis yields a robust upper limit of $m_s \lesssim 5.7$ keV (95% CL), with halo-model uncertainties dominating the constraint, while the soft X-ray calorimeter data illustrate potential improvements with future wide-field, high-resolution observations. Overall, the results are competitive with other X-ray bounds and highlight the importance of MW halo modeling and future missions to fully probe the sterile neutrino dark matter parameter space.

Abstract

We present upper limits on line emission in the Cosmic X-ray background (CXB) that would be produced by decay of sterile neutrino dark matter. We employ the spectra of the unresolved component of the CXB in the Chandra Deep Fields North and South obtained with the Chandra CCD detector in the E=0.8-9 keV band. The expected decay flux comes from the dark matter on the lines of sight through the Milky Way galactic halo. Our constraints on the sterile neutrino decay rate are sensitive to the modeling of the Milky Way halo. The highest halo mass estimates provide a limit on the sterile neutrino mass of m_s<2.9 keV in the Dodelson-Widrow production model, while the lowest halo mass estimates provide the conservative limit of m_s<5.7 keV (2-sigma). We also discuss constraints from a short observation of the softer (E<1 keV) X-ray background with a rocket-borne calorimeter by McCammon and collaborators.

Figures (4)

• Figure 1: The three unresolved CXB spectra [CDFN-VF (black), CDFN-F (red), and CDFS (blue)] in the 0.4--10 keV band. They are fit well by a simple model consisting of a power law absorbed by the Galactic hydrogen column representing the extragalactic component, plus unabsorbed local warm thermal emission. All three spectra are fit well, with a total reduced $\chi^2$ of 0.87, and no residual line-like features are seen. Only energies $E> 0.8\rm\ keV$ are used for the line flux limits. See text for details.
• Figure 2: Shown are the limits on the flux in a line as a function of energy from the CDFN-VF, CDFN-F, and CDFS. For each line energy in the range 0.8--9 keV, all model parameters were allowed to vary while deriving upper limits on the sterile neutrino line normalization. The upper line and (cyan) band are the $3\sigma$ limit, with the band representing uncertainty in the detector background modeling. The lower band and line correspond to that from the $2\sigma$ limits. The diagonal band is the range of expected line flux from the MW halo as a function of energy for the sterile neutrino in a DW production model.
• Figure 3: Full parameter space constraints for the sterile neutrino production models, assuming sterile neutrinos constitute the dark matter. Contours labeled with lepton number $L=0$, $L=0.003$, $L=0.01$, $L=0.1$ are production predictions for constant comoving density of $\Omega_s=0.24$ for $L=0$, and $\Omega_s=0.3$ for non-zero $L$Abazajian:2002yz. Constraints from the CXB with the minimal MW halo model are in the solid (blue) region, while the maximal MW halo model excludes the adjacent diagonally hatched region. Also shown are exclusion regions from the diffuse X-ray background (green) Boyarsky:2005us, from XMM-Newton observations of the Coma and Virgo clusters (light blue) Boyarsky:2006zi, observations of Andromeda (M31) in wide hatching Watson:2006qb, and limits from the MW by Boyarsky et al. Boyarsky:2006ag (BMW). The region at $m_s<0.4\rm\ keV$ is ruled out by a conservative application of the Tremaine-Gunn bound Bode:2000gq. The grey region to the right of the $L=0$ case is where sterile neutrino dark matter is over-produced. The constraint from the MW calorimeter soft X-ray background observation is the star and arrow, marked "Calor." Also shown is the horizontal band of the mass scale consistent with producing a 100--300 pc core in the Fornax dwarf galaxy Strigari:2006ue. The non-resonant and resonant production curves come from Refs. AbazajianProduction05 and Abazajian:2002yz, respectively.
• Figure 4: The upper panel is the spectrum soft X-ray background as measured by McCammon et al. McCammon:2002gb. The lower panel shows the atomic line model, detector response function (blue) and expected contribution ($\times$60) due to sterile neutrino decay of the DW production model with the minimal (lower, green) and maximal (upper, red) MW halo models.