Caustic crossings in giant arcs with extended dark matter objects

TL;DR

This paper extends the microlensing framework to include extended dark matter objects, analyzing their caustic effects on giant arcs and applying the model to constrain the size of ultracompact minihalos using observed events.

Contribution

It develops an analytic model for microlensing by extended dark objects within galaxy clusters and applies it to observational data to constrain their physical size.

Findings

Extended dark objects can create additional caustics, affecting light curves.

Constraints on ultracompact minihalos with radii up to 10^7 R_sun.

Caustic-crossing events can probe larger dark matter structures than point lenses.

Abstract

Caustic-crossing stars observed in giant arcs behind galaxy clusters provide a powerful probe of dark matter substructure. While previous work has focused on point-like lenses such as primordial black holes, we extend this framework to extended dark objects (EDOs), including ultracompact minihalos formed from the collapse of primordial overdensities. We develop an analytic model of microlensing by EDOs embedded in a macrolensing cluster potential and derive the resulting caustics and light curves. Depending on the EDO size relative to the effective Einstein radius, we show that they may generate additional narrow caustics, leading to novel features in the light curve. Applying our framework to the MACS J1149 LS1 ``Icarus'' event, we constrain EDOs with radii up to $10^7 R_\odot$. Our results demonstrate that caustic-crossing events complement galactic microlensing searches, as they can probe EDOs with larger physical size. We discuss the implications for current and future observations, which promise to deliver a statistical sample of caustic transients and correspondingly sharper constraints on dark objects.