Probing Exotic Astrophysical Dark objects through Astrometric Microlensing from Gaia

Abstract

We present the first comprehensive study of astrometric microlensing by exotic astrophysical dark objects, focusing on two theoretically motivated models -- Q-ball and boson star. We demonstrate that these extended objects generate distinctive signatures that depart markedly from point-mass lenses like primordial black holes. The smoking-gun signature for these exotic objects is the emergence of caustics, which form when the lens radius is below a critical threshold. Crossing these caustics induces discontinuous jumps in the images-centroid trajectory, a distinctive feature of these extended dark objects. We show these patterns are sensitive to the internal mass profile, with boson stars generating larger, more prominent caustic structures than Q-balls -- enabling the models to be distinguished. Using the Gaia DR3 stellar catalogue, we forecast a high-yield discovery potential, up to $\sim 6000$ detectable astrometric microlensing events for a 10-year mission, peaking for $M \sim 1-10~M_\odot$ and $R \lesssim 10~\text{AU}$. In the absence of anomalous detections, Gaia can set powerful 90% confidence level constraints on the fractional abundance of these exotic objects, reaching $f_{\mathrm{DM}} \le 10^{-3}$ in the peak region which covers masses from $10^{-1}-10^{7}~M_{\odot}$ and radii $R<10^{6}~\text{AU}$. Crucially, these projected astrometric microlensing constraints are significantly stronger than existing photometric microlensing limits in the $1-10~M_\odot$ mass range. This work establishes astrometric microlensing with Gaia as a powerful, complementary, and near-future probe with the potential to discover exotic astrophysical dark objects.