SuperRad: A black hole superradiance gravitational waveform model

Nils Siemonsen, Taillte May, William E. East

Submitted on 7 November 2022


Gravitational signatures of black hole superradiance are a unique probe of ultralight particles that are weakly-coupled to ordinary matter. The existence of an ultralight boson would lead spinning black holes with size comparable to the Compton wavelength of the boson to become superradiantly unstable to forming an oscillating cloud, spinning down the black hole, and radiating gravitational waves in the process. However, maximizing the chance of observing such signals or, in their absence, placing the strongest constraints on the existence of such particles, requires accurate theoretical predictions. In this work, we introduce a new gravitational waveform model, SuperRad, that models the dynamics, oscillation frequency, and gravitational wave signals of these clouds by combining numerical results in the relativistic regime with fits calibrated to analytical estimates, covering the entire parameter space of ultralight scalar and vector clouds with the lowest two azimuthal numbers (m=1 and 2). We present new calculations of the gravitational wave frequency evolution as the boson cloud dissipates, including using fully general-relativistic methods to quantify the error in more approximate treatments. Finally, as a first application, we assess the viability of conducting follow-up gravitational wave searches for ultralight vector clouds around massive black hole binary merger remnants. We show that LISA may be able to probe vector masses in the range from 1×1016 eV to 6×1016 eV using follow-up gravitational wave searches.


Comment: 22 pages, 15 figures, code repository:

Subjects: General Relativity and Quantum Cosmology; Astrophysics - High Energy Astrophysical Phenomena; High Energy Physics - Phenomenology


The GW strain $h$ and frequency $f_{\rm GW}$ as a function of time for a BH with $M=62\ M_{\odot}$ and $a_*=0.67$ at a distance of 410 Mpc subject to the superradiant instability of a boson with mass $3.6\times10^{-13}$ eV. The top set of panels shows the scalar boson case, while the bottom set shows the vector case. Note the difference in timescales shown, since in the scalar (vector) case the cloud grows on timescales of $\sim 5$ years (9 hours) and decays through GW radiation on timescales of $\sim 9000$ years (1 day). Time is measured since the BH was formed, assuming the cloud started as a single boson.