Astrometric Signatures due to Dark Matter Subhalos on Caustic Crossing Events

The small-scale structure of dark matter remains one of the key frontiers in cosmology. While the Cold Dark Matter (CDM) model successfully explains large-scale observations, alternative scenarios such as warm or self-interacting dark matter predict a deficit of low-mass subhalos. Strong gravitational lensing provides a uniquely direct probe of these small scales, particularly in massive cluster lenses where highly magnified arcs trace the vicinity of fold caustics. In these regions, background stars occasionally cross the caustic, producing brief and extreme “caustic-crossing” events now routinely observed with HST and JWST. The exceptional magnification in these environments amplifies the influence of small deflectors: even subhalos with 1 million solar masses—otherwise too diffuse to act as a relevant lens on its own—can produce measurable astrometric perturbations on casutic crossing events.

We present a new statistical framework that connects the covariance of these astrometric shifts to the convergence power spectrum of dark matter substructure. This approach treats the field of deflection perturbations as a statistical tracer of small-scale structure, enabling constraints on the subhalo population. Using tailored numerical simulations of the Abell 370 “Dragon” arc, we quantify the expected correlations in image positions induced by realistic subhalo populations.

Our results show that dark-matter–induced astrometric correlations of order 10–100 mas should be detectable with current and forthcoming JWST observations of caustic-crossing stars. This work establishes a new pathway for probing the nature of dark matter through ensemble astrometric measurements in highly magnified cluster lenses.