Title: Pulsar Nulling and Vacuum Radio Emission from Axion Clouds
Author(s): Andrea Caputo, Samuel J. Witte, Alexander A. Philippov, Ted Jacobson
First Author’s Institution: Department of Theoretical Physics, CERN, Geneva, Switzerland
Status: Accepted in Physical Review Letters [closed access]
Neutron stars strike again! One of the most extreme objects in the universe, a neutron star (NS) is an ultradense, city-sized (~10s of kilometers in radius) remnant left behind when a massive star dies in a core-collapse supernova. They are responsible for many observed phenomena, from magnetars and pulsars to binary neutron star mergers producing the heaviest elements in the universe. The NS environment is a unique laboratory to probe physics at extreme energies inaccessible to human-constructed particle accelerators and experiments.
The authors of today’s paper investigate how radio observations of NSs may constrain the existence of axions, an extension of the Standard Model of Particle Physics. Quantum chromodynamics (QCD) interactions are strong nuclear force interactions between quarks (a fundamental particle never found by itself, always bound as a pair or more). For example, the proton comprises two “up” quarks and one “down” quark. The current theory of QCD allows for specific interactions between particles that we have never observed before, called the strong CP problem. Axions were proposed to solve this problem in the Standard Model. They are also a candidate for cold dark matter.
To determine if the axion exists, we can search for evidence of its interaction with the electromagnetic force (that produces photons that we can observe) to place limits on its effective charge and particle mass. The authors of today’s paper investigate a mass range of 10^-9 <= m_a <= 10^-4 eV. We can convert this “rest mass” energy (calculated with the famous E=mc^2) to the frequency of a photon containing the same amount of energy. In this mass range, the corresponding frequencies are of order megahertz (MHz) to gigahertz (GHz), which is typical for the range of frequencies found in the NS environment.
The environment in the highly magnetic poles of NSs is ideal for the highly efficient production of axions. The resulting mass of these particles implies that they would remain gravitationally bound to the surface, indicating a build-up of an “axion cloud” surrounding the NS over its lifetime. Using the axion theory, the authors of today’s paper will determine if, over astrophysical timescales, the axion cloud will build to sufficiently high levels to impact the electrodynamics of the system such that we could observe its effect.

Using a model for the polar caps of pulsars, the authors of today’s paper show that two different effects will occur based on the axion’s mass. A pulsar has a powerful dipole-like magnetosphere. It rapidly rotates, creating a “lighthouse” effect as radio emission from the highly magnetic poles is beamed outward and is only briefly pointed at the observer during each rotation. If the axions are low mass, then the background effective charge density contribution from the axion field will block the radio emission for a very short time. This means a pulsar would occasionally skip a pulse, called quasi-periodic behavior. This range of parameter space is shown in Figure 1.
On the other hand, for higher mass axions, their production from rapid small-scale fluctuations in the magnetosphere acts like oscillating dipoles that radiate away energy at their frequency. These would produce very short-lived narrow emission lines. The authors of today’s paper calculate how bright this emission would be for several well-known pulsars and find that the resulting radio line (narrowly beamed by the magnetic fields of the pulsar) could be extremely bright, even brighter than the intrinsic radio emission. The Crab Nebula pulsar could have an emission line as bright as 38 Janskys, much brighter than typical pulsars in our galaxy that are of order milli-Janskys.
To further constrain the parameter space of how strongly axions interact with photons (called the coupling constant) and mass, observers will have to monitor known pulsars with the optimal parameters for these effects to take place (based on their magnetic field strength and spin rate) to determine if such an emission line or quasi-periodic screening of the intrinsic radio pulsation exists.
Edited by Magnus L’Argent
Featured Image Credit: [ESO/L. Calçada; CC BY 4.0]