Speaker
Description
In the phenomenon of black hole superradiance a rotating black hole loses its angular momentum to a growing cloud of particles. It has been successfully used to place limits on ultralight particles, such as axions and dark photons, with masses $\mu\sim 10^{-20}-10^{-18}$ eV and $\mu\sim 10^{-12}-10^{-11}$ eV. On the other hand, it has been shown that superradiance occurs also in neutron stars due to an instability in the magnetosphere. A proper treatment has however been lacking, and no bounds analogous to those of black holes have been placed yet. In our work we calculate, for the first time, the superradiance rate in a neutron star dipole magnetic field and use it to place new constraints on axions of masses $\mu\sim 10^{-12}$ eV. This is done by requiring that the superradiance-induced spindown not be faster than that observed by pulsar timing arrays. We show current bounds obtained from millisecond pulsars, and projections for future pulsars which may be found by next generation radio telescopes. Finally, we suggest the possibility of other signals from superradiant axion clouds around neutron stars.
