Caustic crossings in giant arcs with extended dark matter objects

TL;DR

This work extends caustic-crossing stellar lensing from point-like lenses to extended dark objects (EDOs) embedded in cluster macrolenses, introducing an analytic framework based on a normalized mass profile $m( au)$ with $ au= heta/ heta_E$ to compute the magnification and caustic structure. It shows that EDOs can generate additional, narrow caustics or suppress standard caustics depending on the relative size to the effective Einstein radius, summarized by the criterion $ au_m \\lesssim \,\sqrt{\mu_t}$ for point-like behavior and by an extended-lens efficiency $ar{R}_{E, m EDO} = \epsilon_{ m EDO} \sqrt{\mu_t} \theta_E D_L$ with $\epsilon_{ m EDO}^2 - m(\epsilon_{ m EDO}\sqrt{\mu_t})=0$. The paper quantifies how EDOs modify light curves and provides a method to constrain their abundance from the Icarus event, deriving $R_{ m lens}$ limits up to $10^7 R_\odot$ and relating the local object fraction to cosmological dark matter fractions via $f_{\rm co}$ and $f_{\rm DM}$. It discusses broader observational prospects with future high-cadence surveys and more accurate lens models, showing that cluster caustic crossings can probe DM substructure in a distinct size–mass regime compared with traditional microlensing.

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.

Figures (3)

• Figure 1: Boson star caustics compared to point-like lens caustics for a macrolens with $\mu_{\rm{t}} \sim 100$ and $\mu_{\rm{r}}\sim3$, and light curves resulting from the indicated source trajectory. Left:$\tau_m=2$ gives rise to extra caustics in the light curve; Right: for $\tau_m=22$ ($\gtrsim \sqrt{\mu_t}$) the caustics are spaced so closely together that they appear as one in the light curve.
• Figure 2: Extended lens efficiency for different examples of EDOs (see Croon:2020wpr for further details). As expected, for $R_{90}\leq \sqrt{\mu_T} R_{\rm E}$, EDOs are effectively point-like, whereas they become less efficient when they are more dilute.
• Figure 3: Constraints on the compact object fraction $f_{\rm co}$ from the MACS J1149 LS1 Icarus event. Colored lines show the constraints for extended dark objects (EDOs) of different radii while the dotted line shows the constraints for point-like objects such as primordial black holes. As shown in text, we expect $f_{\rm DM} \simeq [0.92-0.97] f_{\rm co} < f_{\rm co}$ for this event.