A team of scientists at the University of California, San Diego (UCSD)
has used data from X-ray telescopes to set a limit on the
characteristics of a possible dark matter candidate — sterile
neutrinos. They predict that future observations could confirm or
disprove the theory that most of the matter in the universe is in the
form of sterile neutrinos produced in the first nanosecond of the Big

In a paper published in the December 1 issue of the Astrophysical
Journal, the scientists show that sterile neutrinos could betray their
presence to sensitive X-ray telescopes because on extremely rare
occasions they will disintegrate into a light neutrino and an X-ray.

“On average a sterile neutrino will decay only once every septillion
years or so,” said Kevork Abazajian, lead author of the paper. “The
chance of catching one would appear to be impossibly small. However, if
sterile neutrinos make up the dark matter that permeates gigantic galaxy
clusters, the odds actually become reasonable.”

A number of astrophysicists have suggested that sterile neutrinos with a
mass between 0.2 and 2 per cent of the mass of the electron could
explain many observations relating to dark matter and the formation of
galaxies that more massive and stable dark matter particles cannot. The
decay of sterile neutrinos would produce a sharp peak in the X-ray
spectrum from the hot gas clouds present in galaxy clusters. The
predicted height of the peak in the spectrum increases with the assumed
mass of the sterile neutrino.

Data already collected from the Chandra and XMM-Newton X-ray
observatories show that the X-ray spectra from the Virgo cluster of
galaxies sets a limit on the mass of the hypothetical sterile
neutrinos. In particular, assuming that neutrinos and antineutrinos
were produced in equal numbers in the Big Bang, the observations rule
out the possibility that the dark matter is in the form of sterile
neutrinos with masses greater than about 1 percent of the mass of the

Much longer observations with these telescopes could lower the mass
limit dramatically. The next generation of X-ray telescopes could either
directly detect the decay of sterile neutrino dark matter, or
effectively rule it out as a dark matter candidate.

“In any event,” says George Fuller a coauthor of the paper, “it is
remarkable and unexpected that we can use modern X-ray observatories to
probe very early epochs of the universe, and explore a new regime of
particle physics inaccessible in a laboratory.”

This research was supported by the National Science Foundation and NASA.


Science contacts:

Kevork Abazajian, NASA/Fermilab Theoretical Astrophysics Group
(630)840-8195 aba@fnal.gov

George Fuller, Center for Astrophysics & Space Science, UCSD
gfuller@physics.ucsd.edu, 858-534-6324

Wallace Tucker, Center for Astrophysics & Space Science, and
Harvard-Smithsonian Center for Astrophysics, wtucker@ucsd.edu