Geometric Capture and Optical Critical Curves of Purely Electric, Magnetic and Dyonic Black Holes in Einstein-Euler-Heisenberg Theory
We examine background null capture in the electric, magnetic, and dyonic solutions of Einstein gravity coupled to a quartic Euler-Heisenberg-type electromagnetic Lagrangian. The calculation uses the exact hypergeometric electric solution, the closed-form magnetic solution, and a numerical dyonic solution. Relative to the Reissner-Nordström value at fixed total charge, the EEH correction increases the geometric critical impact parameter in the parameter ranges considered. For the purely magnetic case, the disappearance of the two outer horizons can expose an additional capture barrier and thus forms an unstable circular orbit with a very small radius. In the dyonic case, for a fixed magnetic-to-electric charge ratio, the behavior is consistent with that of the pure electric solution while preserving the phase transition between single- and multi-horizon black hole phases. Moreover, when the coupling parameter is fixed, the effect of the magnetic-to-electric charge ratio depends on the electric charge. The present findings establish that the multi-horizon phase can impart pronounced signatures to geometric photon observables, whereas, in the off-transition regime, the purely electric, purely magnetic, and dyonic solutions remain characterized by clearly distinguishable trends.