Constraints on 3.55 keV line emission from stacked observations of dwarf spheroidal galaxies

TL;DR

This study analyzes XMM-Newton data of dwarf spheroidal galaxies to test the dark matter decay origin of the 3.55 keV X-ray line, finding no evidence for it and setting new constraints on sterile neutrino dark matter parameters.

Contribution

It provides the first comprehensive analysis of dwarf spheroidal galaxies for the 3.55 keV line and updates upper limits on sterile neutrino dark matter mixing angles.

Findings

No evidence for the 3.55 keV line in stacked dwarf galaxy spectra.

Excludes sterile neutrino decay origin at 4.1sigma level under standard assumptions.

Provides updated upper limits on sterile neutrino mixing angles between 2 and 20 keV.

Abstract

Several recent works have reported the detection of an unidentified X-ray line at 3.55~keV, which could possibly be attributed to the decay of dark matter (DM) particles in the halos of galaxy clusters and in the M31 galaxy. We analyze all publicly-available XMM-Newton data of dwarf spheroidal galaxies to test the possible DM origin of the line. Dwarf spheroidal galaxies have high mass-to-light ratios and their interstellar medium is not a source of diffuse X-ray emission, thus they are expected to provide the cleanest DM decay line signal. Our analysis shows no evidence for the presence of the line in the stacked spectra of the dwarf galaxies. It excludes the sterile neutrino DM decay origin of the 3.5 keV line reported by Bulbul et al. (2014) at the level of 4.1sigma under standard assumptions about the Galactic DM column density in the direction of selected dwarf galaxies and at the level of 3.2sigma assuming minimal Galactic DM column density. Our analysis is still consistent with the estimate of sterile neutrino DM parameters by Boyarsky et al. (2014), because of its larger uncertainty. However, the central value of their estimate of the mixing angle is inconsistent with our dwarf spheroidals data at 3.4sigma (2.5sigma) level assuming the mean (minimal) Galactic DM column density. As a by-product of our analysis, we provide updated upper limits to the mixing angle of sterile neutrino DM in the mass range between 2 and 20 keV.