Superradiant axion clouds and their interaction with astrophysical plasma

Sammanfattning: Axions are one of the best-motivated particles beyond the standard model of particle physics and a promising candidate for dark matter. Through the superradiant instability, axions can extract a significant amount of rotational energy from spinning black holes resulting in dense axion clouds. These axion clouds can imprint themselves on the spin of the black hole and even emit detectable gravitational waves, making them very potent tools in the search for axions. The considerable number of axions present in these clouds can also compensate for the weak coupling between the axion and the standard model particles. However, the interaction between the cloud and the astrophysical plasma that the black hole accretes is often assumed to be negligible. In this thesis, we examine this assumption by studying the interaction between the astrophysical plasma and the axion cloud to determine if it can cause any significant effects.  We find no new gravitational signatures and can conclude that the interaction is not effective enough to halt the evolution of the cloud. Therefore, the main focus of this work is the emitted low-frequency photons that the axions convert into through the interaction. We find that the emission from systems with typical accretion rates can reach up to 10^14 W and is most efficient around fast-spinning stellar black holes that accrete spherically without an accretion disk. However, we conclude that most of this emission will quickly be reabsorbed into the plasma and not cause any detectable signals.  We also study resonant conversion of axions, which can occur when the plasma frequency is comparable to the axion mass. We find that the low accretion rates that enable this are reachable around isolated stellar-mass black holes that travel rapidly through low-density regions of space. In these systems, the luminosity can reach 10^25 W, and possibly even higher if we include stimulation effects. We can, therefore, conclude that a population of fast-traveling isolated black holes can pose a new tool in the search for axions.