Axions in Pulsars

Has Dark Matter been converted to X-rays in Neutron Star magnetic fields?

Many people have heard of neutrinos (little ‘neutral ones’). They are produced in abundance in the Sun, come in three flavors (electron, muon, and tau) and have no charge, and very small masses. The upper limit of the combined masses of the three flavors is 0.12 eV.

There is a hypothesized particle known as the sterile neutrino that is a candidate for dark matter, but these would be heavier, extremely so, probably much more than a proton mass.

But there is another dark matter candidate, never observed in the laboratory, which would have a mass much less than even the three known neutrinos. And is less well known than neutrinos.

Axion: From the Laundry to the Particle Zoo

This is the axion, which is named after (yes) a (no longer sold) laundry detergent. The mass may be less than one-thousandth of an eV (when we say eV we are using energy units as particle physicists like to do, so divide by c² for mass units).

In other words the axion could have a mass less than 1% of the three neutrinos combined. So no wonder it is so elusive. Axions with masses near to the neutrino mass have been ruled by limits on soft thermal X-ray emission from neutron star surfaces.

What is attractive about the axion as a dark matter candidate is that it does not require any extensions to the Standard Model of particle physics. They were postulated to solve the charge and parity…