Can absorption enhance differences between black-hole images?

Abstract

We investigate whether absorption can enhance morphological differences between images of different spacetimes, particularly when their critical impact parameters are similar. Using a phenomenological thick-accretion-flow model with an effective absorption coefficient $χ_0$, we compare Schwarzschild black-hole images with those of Bardeen, Hayward, and Ayón-Beato--García black holes. In the face-on Schwarzschild--Bardeen comparison with $g/M=0.5$, the dominant brightness shifts from a narrow lensed feature to a broader outer feature at $χ_0M\simeq$0.585 and 0.717, respectively. Near the first transition, the integrated difference between independently normalized images increases by $23.5$% on a common physical grid and by $46.6$% after rescaling by the respective critical impact parameters. Enhancement occurs across all three regular-black-hole families and over a broad range of the sampled flow parameters and viewing inclinations, although its magnitude and occurrence depend on the prescribed flow. Asynchronous brightness-peak transitions alone are insufficient to guarantee enhancement. These results suggest that the opacity dependence of image morphology may provide information complementary to the critical-curve scale, potentially helping distinguish spacetimes with similar characteristic shadow sizes. Assessing this possibility observationally requires physically consistent, frequency-dependent radiative transfer.