Black Hole Superradiance
In the presence of an ultralight bosonic field, spinning black holes are unstable to superradiance. The process is illustrated in the figure below. The rotational energy of the black hole is converted into a non-axisymmetric, oscillating boson cloud which dissipates through the emission of nearly monochromatic gravitational radiation. Thus, gravitational wave observations by ground- or space-based detectors can be used to probe the existence of dark particles weakly coupled to the Standard Model. I primarily study massive vector bosons, which grow much faster through superradiance, and produce significantly stronger gravitational waves compared to the scalar case.
In this reference, I used techniques from black hole perturbation theory to compute the relativistically-correct gravitational wave signal across the parameter space of different boson masses and black hole masses and spins, in collaboration with William E. East. This filled in a gap in the literature between flatspace approximations, which underestimate the gravitational wave amplitude in the non-relativistic limit, and overestimate it in the relativistic regime, and time-domain calculations, which have only covered a limited part of the parameter space. We also identified parameter ranges where overtone superradiantly unstable modes will grow faster than the lower frequency fundamental modes. Such cases will produce a distinct gravitational wave signal due to the beating of the simultaneously populated modes, which we computed.
Dark Photon Superradiance
In this reference, we considered a kinetically mixed superradiant vector cloud. The visible field component of the boson cloud efficiently accelerate charges, resulting in a pair production cascade saturating in a tenuous pair plasma around the central black hole. The plasma is characterized by strong magnetic reconnetion inside the bulk of the cloud, powering luminous high energy electromagnetic emissions accompanying the gravitational waves sent out by the oscillating cloud. The figure shows a few representative magnetic field lines.
Fig.: Magnetic field geometry in a dark photon superradiant cloud filled with highly magnetized plasma around the central black hole.
Multi-messenger observations of both solar-mass and supermassive black holes are able to access unexplored dark photon parameter space. We explored different search strategies and isolate two particularly promising avenues: (i) X-/Gamma-ray follow-up searches of binary black hole merger remnants, detected by ground-based gravitational wave detectors, with current instruments are already able improve upon existing constraints, while planned future high-energy electromagnetic observatories can push the accessible kinetic mixing parameters down by another order of magnitude; and (ii), gravitational wave follow-up searches of known periodically pulsating electromagnetic sources are promising detection machines and may be integrated into the LVK machinery easily, enabling probing heavier-mass dark photons more efficiently. The parameter space, as well as the regions accessible by multi-messenger observations of solar-mass isolated black holes are shown in the plot below:
Fig.: The kinetic mixing vs. dark photon mass parameter space. Regions accessible by the two search strategies outlined above are indicated in purple and green. The total electromagnetic luminosity expected from a dark photon cloud is shown as solid and dashed black lines.