Plunge spectra as discriminators of black hole mimickers
Sreejith Nair
TL;DR
This paper proposes plunge radiation from extreme mass ratio events as a direct probe of near-horizon physics to distinguish black hole mimickers from true black holes. By analyzing the Schwarzschild-like exterior perturbations with a reflecting surface, it identifies two robust spectral features: a low-frequency resonance comb at the real parts of the mimicker's QNMs and a high-frequency tail that grows beyond a threshold $M\omega_{\rm th}\approx0.39$, with the tail scaling as $A_{m}e^{-k_{m}\omega}$ and $A_{m}\propto|R|^{2}$. The authors show, through Green's-function-based calculations of the plunge energy spectrum $\frac{dE}{d\omega}$, that these features persist across different surface locations $\epsilon$ and angular momenta, and argue that stacking multiple EMRI plunge signals could boost detectability with LISA. The results offer a complementary observable to standard QNM spectroscopy, linking near-horizon boundary conditions to observable gravitational-wave spectra and suggesting practical strategies for horizon-scale physics in future GW observations.
Abstract
This work explores the prospect of using the plunge to identify potential black hole mimickers. We show that the plunge excites two generic spectral features. (i) At low frequencies, there is a comb of sharp resonances at the real parts of the mimicker quasi-normal modes. (ii) Above a threshold $Mω_{\rm th}\!\approx\!0.39$ (for the dominant mode), the spectrum undergoes a qualitative break: with the black hole mimicker displaying significant deviations from the black hole. Though individual plunge SNRs in extreme mass ratio events are low and detecting them in a sea of noise is difficult, the coherent spectral features identified here may allow for enhancing the SNR by using multiple events.