Plasma modified black hole shadows and weak gravitational lensing of Einstein–Maxwell-dilaton spacetimes: constraints from the EHT

Abstract We study observational signatures of Einstein–Maxwell-dilaton (EMD) black holes embedded in a cold, isotropic, dispersive plasma. Within geometric optics, we incorporate homogeneous and stratified plasma profiles to assess their impact on photon sphere location, shadow size, and weak gravitational lensing. The shadow radius shrinks monotonically with increasing plasma strength despite an outward photon sphere shift; at fixed dilaton coupling, larger charge suppresses the shadow, whereas stronger dilaton coupling enlarges it. Comparing predictions with Event Horizon Telescope measurements of M87* and Sgr A*, we infer contiguous, model dependent ranges of plasma strength consistent with current bounds on ring size and asymmetry. In the weak field limit, dispersion enhances light deflection and increases the effective Einstein angle, total magnification, and image parity fractions. These shadow lensing trends delineate EMD parameter space compatible with present data and motivate targeted, multi frequency tests.