Circulation and operational sparsity of analogue Hawking radiation in rotating acoustic horizons

Abstract

Temporal sparsity compares the mean interval between emitted quanta with a characteristic single-quantum timescale. We study this operational quantity for the rotating draining-bathtub acoustic horizon in two spatial dimensions. The Hawking temperature is fixed by the radial drain and does not depend on circulation, while the geometric capture width and the transmission spectrum do. A geometric black-body benchmark therefore predicts reduced temporal separation as circulation increases. We then solve the radial scattering problem and compute converged greybody-corrected number and energy fluxes, including the superradiant sector. At a circulation-to-drain ratio of unity, the thermal-clock sparsity is 0.44 whereas the spectrum-based sparsity is 1.82. The former crosses the temporal-overlap benchmark near a ratio of 0.40, while the latter remains above unity at all sampled circulation ratios. We also show that the binned sparsity definition is logarithmically infrared sensitive in two spatial dimensions. Rotation therefore enhances the number flux while hardening the spectrum, making temporal overlap intrinsically dependent on the operational clock used to resolve the emitted quanta.