New Limits on Sterile Neutrinos from Suzaku Observations of the Ursa Minor Dwarf Spheroidal Galaxy

TL;DR

This study uses Suzaku X-ray observations of the Ursa Minor dwarf galaxy to search for sterile neutrino decay signals, setting new constraints on their properties and viability as dark matter candidates.

Contribution

First deep X-ray observation of Ursa Minor targeting sterile neutrino decay, providing new constraints on sterile neutrino parameters and dark matter composition.

Findings

No positive detection of decay line.

Constraints on sterile neutrino mass and mixing angle.

Limits approach best previous results in 1-20 keV range.

Abstract

We present results of our search for X-ray line emission associated with the radiative decay of the sterile neutrino, a well-motivated dark matter candidate, in Suzaku Observatory spectra of the Ursa Minor dwarf spheroidal galaxy. These data represent the first deep observation of one of these extreme mass-to-light systems and the first dedicated dark matter search using an X-ray telescope. No such emission line is positively detected, and we place new constraints on the combination of the sterile neutrino mass and the active-sterile neutrino oscillation mixing angle. Line flux upper limits are derived using a maximum-likelihood-based approach that, along with the lack of intrinsic X-ray emission, enables us to minimize systematics and account for those that remain. The limits we derive match or approach the best previous results over the entire 1--20 keV mass range from a single Suzaku observation. These are used to place constraints on the existence of sterile neutrinos with given parameters in the general case and in the case where they are assumed to constitute all of the dark matter. The range allowed implies that sterile neutrinos remain a viable candidate to make up some -- or all -- of the dark matter and also explain pulsar kicks and various other astrophysical phenomena.