The Einstein Gap: an unrecognised strong lensing feature in weak lensing mass density profiles
Magnification bias cross-correlations between galaxy clusters and high-redshift submillimetre galaxies have recently revealed a characteristic signal deficit at intermediate angular scales, termed the Einstein Gap, that appears consistently across all lens types and cannot be explained by any single theoretical mass density profile. In this work we identify for the first time the most plausible physical origin of this feature through a combination of observational tests and improved lensing simulations. Using galaxy clusters to characterise the satellite population and as the lens sample, we show that the gap is detected at high statistical significance, accounting for the full covariance of the stacked profile. Then, we demonstrate that the Einstein Gap is robust across three independent estimators of the angular cross-correlation function, strengthens as the positional smoothing is reduced, and scales systematically with the bright central galaxy stellar mass. Finally, we show that the projected radial distribution of satellite galaxies is smooth and continuous at all angular scales with no depression corresponding to the gap, disfavouring both a statistical artifact and a genuine absence of mass as its origin. An improved magnification bias simulator incorporating full ray-tracing via the lens equation and a NFW+Sersic mass density profile reproduces the observed signal deficit, strongly supporting that the Einstein Gap is a strong lensing feature: background sources within the Einstein radius are displaced outward, creating a characteristic empty annulus whose angular size scales. This feature is consistent with features visible in previously published weak-lensing profiles that were not, however, interpreted as a strong-lensing signature. Its identification opens new possibilities for constraining halo masses and concentrations from magnification bias measurements alone.