Pulsars as Axion Laboratories

Pulsar magnetospheres host gap regions, which are thought to be the sites of particle acceleration, plasma production, and electromagnetic emission (radio to gamma rays). In recent work (Prabhu, 2021), I suggested that these regions are also efficient axion factories. Once produced, a fraction of the emitted axions convert to photons, leading to anomalous radio emission. In (Noordhuis et al., 2023), we developed a pipeline to model axion production using both kinetic plasma simulations and semi-analytic modeling, model axion-to-photon conversion in the magnetosphere, and compare the predicted outgoing radio flux to real radio data. Our work sets the leading constraints on axions over a range of masses. 

Most axions emitted by a pulsar either convert to photons or escape the magnetosphere (more on this later). However, a fraction of the axions are may be emitted at sufficiently low velocity that they cannot escape the gravitational pull of the neutron star. This bound population can grow over astrophysical timescales to form a dense axion cloud around the neutron star. Near the surface of the star, the cloud can reach densities over twenty orders of magnitude higher than the local dark matter density.  The presence of this cloud leads to rich observational phenomena, including persistent narrowband radio emission and bright radio transients. We explore the formation and observational signatures of these clouds in (Noordhuis et al., 2023). [ANIMATION CREDIT: Sam Witte]

In 2020, The STARE2 and CHIME collaborations saw fast radio bursts associated with galactic magnetar 1935+2154, marking the first galactic FRB observed and the first source identification. This observation lent support to the hypothesis that at least some FRBs are sourced by magnetars. Even in magnetar models of FRBs, there is ongoing debate as to the exact emission mechanism. One question that has been debated is where the FRB is produced relative to the magnetar surface (near or far). Near-surface emission models face the obstacle of explaining how the FRB, once produced, escapes the magnetosphere. In (Prabhu, 2023), I showed that FRBs produced near the NS convert efficiently into axion bursts due to the immense magnetic fields. The axion burst is easily able to escape the magnetosphere, and reconverts to radio waves in the magnetar wind region. The efficiency of this process is within energetics constraints for a wide range of parameter space.

There are a number of existing and planned experiments to detect dark matter axions. Dark matter has the property that it is cold (low velocity), meaning that dark matter axion signals should be narrow lines with frequency set by the axion mass. Unfortunately, the axion mass is unknown, requiring multiple experiments and intensive scanning. In contrast, astrophysical sources of axions are at known frequencies, set by fundamental astrophysical scales such as temperature and plasma frequency. However, these signals are often broadband, making them difficult to distinguish from backgrounds. An exception is axion signals emitted at the rotation frequency of the pulsar. Such signals occur at known frequencies AND are highly monochromatic, providing  robust experimental targets. In recent work (Khelashvili et al., 2024), we propose to look for these pulsar-sourced axion signals using proposed axion dark matter detection experiments.