Particle Physics

Ultralight bosons can induce superradiant instabilities in spinning black holes, tapping their rotational energy to trigger the growth of a bosonic condensate. In this talk I will give an overview on superradiance and its applications focusing on the observational imprints of this process around spinning black holes which include: (i) the emission of monochromatic gravitational waves emitted by bosonic condensate formed through superradiant instabilities, potentially observable by current and future gravitational-wave detectors, and (ii) the formation of “gaps” in the spin versus mass plane of astrophysical black holes.

Traditionally, searches for continuous gravitational waves have looked for neutron stars with varying mass or current quadrupoles. If information is known about the source — such as a sky position or even a full ephemeris — this information can be used to run a more sensitive search around the known parameters of that source. These directed and targeted searches have set new and ever-improving constraints on the properties of individual neutron stars. The searches can also be applied to other astrophysical sources, such as continuous waves from boson clouds that have built up around black holes due to superradiance. I will discuss the current status of directed and targeted searches for continuous gravitational waves with the advanced LIGO and Virgo detectors, and sketch out some considerations for running directed searches for boson clouds.

The Continuous Waves (CW) Search Group of the LIGO Scientific Collaboration and Virgo Collaboration carries out a diverse suite of searches for a diverse set of possible CW sources. Assumptions underlying these searches will be discussed, along with strategies used so far to keep our eyes wide open while also giving due attention to the most promising sources. One important assumption to date has been that fast-spinning, non-axisymmetric neutron stars are the most promising class of CW sources. If the most promising source class is, instead, black hole super-radiance from axion clouds, then current searches may not be optimal. Potential modifications to current all-sky searches to address super-radiance will be discussed.