Transfer observables of rotating acoustic black holes from ray tracing: shadow centroid, redshift asymmetry and flux imbalance

TL;DR

This paper develops a transfer framework for null acoustic rays in rotating black hole analogs, identifying key observables like shadow centroid and flux imbalance that reveal rotation effects.

Contribution

It introduces a novel impact-parameter-resolved transfer formalism that isolates geometric and source effects, providing new diagnostics for rotation in acoustic black holes.

Findings

Shadow centroid shifts linearly with circulation.

Rotation causes a left-right redshift tilt and flux imbalance.

Total flux alone cannot diagnose circulation; differential observables are more effective.

Abstract

We construct an impact-parameter-resolved transfer framework for null acoustic rays in the rotating draining-bathtub spacetime. The formalism separates the source-independent ray geometry from the source and detector model by keeping explicit the acoustic redshift, transfer convention, emissivity, emitter velocity field, and source-to-screen mapping. The geometric capture interval provides two clean observables: a shadow centroid that shifts linearly with circulation and a shadow width that grows monotonically with circulation. Observable profiles are obtained from direct ray-source intersections, finite source width or extended-disk integration, detector convolution, and convergence checks, rather than from an approximate semi-analytic ring map. The transfer calculation shows that rotation produces a left-right redshift tilt and a branch-dependent flux imbalance, while the total flux alone remains a degenerate circulation diagnostic. The most useful diagnostics are differential quantities: the shadow centroid, branch-integrated flux asymmetry, peak asymmetry, left-right redshift asymmetry, and global redshift contrast. We also discuss how these observables respond to the transfer convention, intrinsic azimuthal emissivity, the choice of left-right split, finite resolution, and physical limitations such as dispersion, viscosity, and finite-depth corrections.