The Sound of an Orbit: A Quantum Spectrum at the ISCO

TL;DR

This paper uncovers a unique quantum spectral signature at the ISCO of a black hole, revealing a discrete frequency comb that differs from thermal spectra, offering new insights into quantum gravity effects near black holes.

Contribution

It introduces a novel quantum detector model at the ISCO, demonstrating a non-thermal, discrete spectral signature that distinguishes quantum effects on different trajectories in strong gravity.

Findings

Discovery of a discrete frequency comb in the excitation spectrum at the ISCO

Peak locations are determined by the orbital frequency

Spectral intensity increases near the stability boundary

Abstract

We investigate the quantum signature of the innermost stable circular orbit (ISCO), a region of profound importance in black hole astrophysics. By modeling an atom as an Unruh-DeWitt detector coupled to a massless scalar field in the Boulware vacuum, we calculate the excitation rate for a detector following a circular geodesic at the ISCO of a Schwarzschild black hole. In stark contrast to the continuous thermal spectra associated with static or infalling observers, our analysis reveals a unique, non-thermal excitation spectrum characterized by a discrete "frequency comb" of sharp, resonant peaks. We show that the locations of these peaks are determined by the orbital frequency at the ISCO, while their intensity increases dramatically as the orbit approaches this final stability boundary. This distinct spectral signature offers a novel theoretical probe of the quantum vacuum in a strong-field gravitational regime and provides a clear distinction between the quantum phenomena experienced by observers on different trajectories. Our findings have potential implications for interpreting the emission spectra from accretion disks and open new avenues for exploring the connection between quantum mechanics and gravity.