7.1 keV sterile neutrino constraints from X-ray observations of 33 clusters of galaxies with Chandra ACIS

TL;DR

This study uses Chandra X-ray observations of 33 galaxy clusters to search for the 3.55 keV line potentially caused by 7.1 keV sterile neutrino decay, setting new constraints on their mixing angle and dark matter contribution.

Contribution

It provides the first comprehensive constraints on the 3.55 keV line in a large cluster sample, improving limits on sterile neutrino dark matter parameters.

Findings

No evidence for the 3.55 keV line was found.

Upper limits on the mixing angle were established, constraining sterile neutrino models.

The results guide future X-ray dark matter searches with upcoming observatories.

Abstract

Recently an unidentified emission line at 3.55 keV has been detected in X-ray spectra of clusters of galaxies. The line has been discussed as a possible decay signature of 7.1 keV sterile neutrinos, which have been proposed as a dark matter candidate. We aim at putting constraints on the proposed line emission in a large sample of Chandra-observed clusters and obtain limits on the mixing-angle in a 7.1 keV sterile neutrino dark matter scenario. For a sample of 33 high-mass clusters of galaxies we merge all observations from the Chandra data archive. Each cluster has more than 100 ks of combined exposure. The resulting high signal-to-noise spectra are used to constrain the flux of an unidentified line emission at 3.55 keV in the individual spectra and a merged spectrum of all clusters. We obtained very detailed spectra around the 3.55 keV range and limits on an unidentified emission line. Assuming all dark matter were made of 7.1 keV sterile neutrinos the upper limits on the mixing angle are $\rm{sin^2(2\Theta)}$ $\rm{<10.1\times10^{-11}}$ from ACIS-I, and $\rm{<40.3\times10^{-11}}$ from ACIS-S data at 99.7 per cent confidence level. We do not find evidence for an unidentified emission line at 3.55 keV. The sample extends the list of objects searched for an emission line at 3.55 keV and will help to identify the best targets for future studies of the potential dark matter decay line with upcoming X-ray observatories like Hitomi (Astro-H), eROSITA, and Athena.