Neutron stars as axion laboratories Harnessing the power of the magnetosphere

This thesis explores the potential of neutron stars as laboratories for studying axions, a hypothetical particle widely regarded as a leading candidate for dark matter. The research centers on two key mechanisms occurring within the region surrounding neutron stars, known as the magnetosphere: resonant axion-photon mixing and axion production via vacuum gaps. The former refers to the conversion of axions into photons, and vice versa, at locations where their four-momenta align; the latter refers to the generation of axions in specific regions of the magnetosphere, where this process is facilitated by the presence of oscillating electromagnetic fields.
The thesis first presents a framework for calculating the photon flux produced by the resonant mixing of dark matter axions in neutron star magnetospheres, accounting for the complex nature of the magnetospheric plasma. The resulting signals are more complex than previously anticipated, being characterized by strong anisotropy and time dependence. Next, the thesis examines axions produced locally within neutron star vacuum gaps. It is shown that the relativistic part of this local axion population produces a broadband radio flux through resonant mixing. By comparing these results to observations of nearby pulsars, strong observational constraints on the axion-photon coupling are established. The non-relativistic part of the population instead leads to the formation of an axion cloud around the parent neutron star, a phenomenon studied here for the first time. It is demonstrated that axion clouds have the potential to reach enormous densities and furthermore generate observable effects through resonant mixing. The thesis concludes with a numerical analysis of axion-photon mixing, addressing current discrepancies between theoretical models and paving the way for more accurate future investigations of this phenomenon in complex astrophysical environments.