Constraints on parameters of radiatively decaying dark matter from the galaxy cluster 1E0657-56

TL;DR

This work constrains radiatively decaying sterile neutrino dark matter in the keV mass range by searching for a decay line in Chandra X-ray spectra of the bullet cluster 1E 0657--56. By exploiting a merger-induced separation between dark matter and hot gas and using lensing-based measurements of the dark-matter mass within the X-ray field of view, the authors translate nondetections into limits on the sterile neutrino mass $M_s$ and mixing $\sin^2(2θ)$ through the decay flux relation $F_{\mathrm{dm}}$. They fit deep X-ray spectra with multi-temperature plasma models, add narrow Gaussian lines to assess possible decay lines across 0.8–9 keV, and account for statistical and systematic uncertainties, including background normalization, calibration residuals, and lensing-mass disagreements. The resulting constraints are competitive with existing bounds and provide a robust cross-check using a higher-redshift system with an independent mass determination; in the simplest Dodelson–Wilczek production scenario, the intersection yields $M_s < 6.3$ keV, though the limit is model-dependent if DM production deviates from that framework.$

Abstract

We derived constraints on parameters of a radiatively decaying warm dark matter particle, e.g., the mass and mixing angle for a sterile neutrino, using Chandra X-ray spectra of a galaxy cluster 1E0657-56 (the ``bullet'' cluster). The constraints are based on nondetection of the sterile neutrino decay emission line. This cluster exhibits spatial separation between the hot intergalactic gas and the dark matter, helping to disentangle their X-ray signals. It also has a very long X-ray observation and a total mass measured via gravitational lensing. This makes the resulting constraints on sterile neutrino complementary to earlier results that used different cluster mass estimates. Our limits are comparable to the best existing constraints.

Figures (7)

• Figure 1: Regions used for extracting the X-ray spectra overlaid on a Chandra X-ray 0.8--4 keV image of 1E 0657--56 ( left panel) and its weak lensing mass map Clowe:06a ( right panel). The panel size is $12'\times 12'$. All point sources seen in the X-ray image are spatially excluded from the spectral analysis. The region that we refer to as sub is the green circle centered on the western subcluster's mass peak, excluding the smaller black circle encompassing most of the X-ray gas bullet. The region peaks is a combination of sub and the green circle centered on the eastern mass peak, again excluding the corresponding X-ray peak (the bigger black circle). The region whole is the big ($r=6'$) green circle, excluding only the gas bullet (the smaller black circle).
• Figure 2: Spectrum for the sub region with the best-fit apec model showin in red. For illustration, we show an additional narrow line at $E=1.3{\:\mathrm{keV}}$ (blue model line and residuals), which worsens the fit at a $3\sigma$ level.
• Figure 3: Statistical upper limits ($3\sigma$) for the flux in a nonthermal, narrow emission line as a function of line energy, for our fitting regions.
• Figure 4: The effect of a $\pm3$% systematic uncertainty in the ACIS background normalization for the region whole on our line limits. It is significant for large regions and at high energies; for smaller regions such as sub and peaks, it is negligible (not shown).
• Figure 5: Chandra ACIS spectrum of the Perseus cluster from a 880 ks exposure, extracted from the $8'\times 8'$ central region, excluding the very center ($r<1'$) with complex gas structure. The fit residuals illustrate the current calibration uncertainties. The residuals around 2--4 keV are 2--3%.