Finite-source magnification residuals from a cNFW-inspired perturbation of an effective spinning black hole binary lens

Abstract

We ask whether a cored-NFW-inspired radial perturbation leaves a finite-source magnification residual after refitting the best effective spinning vacuum binary. We compute the source flux by direct image-plane integration and vary the lens scale, separation, mass ratio, two spin magnitudes, two projected spin directions, and one phase shift. After refitting, the caustics and magnification curves are almost identical and the phase shift is nearly zero. Still, a structured residual remains. At the highest resolution, the full-orbit rms residual is $1.61\times 10^{-2}$ and the maximum local residual is $4.41\times 10^{-2}$. Increasing the grid from $800\times 522$ to $1000\times 653$ changes the full-orbit rms by only $3.39\times 10^{-5}$. The residual also survives nine tests with Gaussian, uniform, and tapered sources at three RMS-size factors. Thus, a finite-source residual survives strong geometric degeneracy in this effective model. We do not claim a unique dark-matter detection or a prediction for a physically scaled binary.

arXiv commentAccepted for publication in EPL. 4 pages, 2 figures. Supplementary Material and Movie S1 are included as ancillary files