A Descriptor Surrogate for Kerr Shadow Contours

Abstract

We consider the Kerr black hole shadow contour as a geometric object whose essential shape can be captured by a small set of physically transparent descriptors. Starting from exact critical-curve calculations, we map each contour to a centered radial profile and retain five quantities: the mean radius, the horizontal centroid shift, and three low-order harmonic coefficients. In this way, the aim is to obtain a compact contour-level representation suitable for repeated evaluation and systematic comparison. We then build a boundary-aware surrogate over spin and inclination that recovers the correct circular limits near the Schwarzschild and polar boundaries and includes a simple positivity safeguard for the reconstructed radial profile. On a held-out bulk domain with inclinations i >= 5{\deg}, the production surrogate yields median errors of 0.522% in contour area, 0.261\% in equivalent diameter, and 0.954% in the 95th-percentile radial contour mismatch; the corresponding 95th-percentile errors are 1.100%, 0.551%, and 3.436%, with no negative-radius samples. The exact descriptor reconstruction is already accurate at the sub-percent level for area and equivalent diameter, which shows that the descriptor layer is a meaningful compression of the contour geometry. We also show that contours with nearly identical equivalent diameter can remain visibly distinct, so the retained descriptors encode geometric information beyond a single size observable. The resulting model provides a compact and interpretable contour-level surrogate for parameter scans and repeated contour comparisons in the Kerr problem.