Constraining Lens Masses in Moderately to Highly Magnified Microlensing Events from Gaia

TL;DR

This paper develops a photometry-only approach to constrain lens masses in moderately to highly magnified microlensing events that show no clear finite source (FS) effect. By combining Gaia photometry with ground-based follow-up and applying a finite-source/parallax model, the authors derive lower limits on the angular Einstein radius $\theta_E$ and, via DarkLensCode (DLC) with Galactic priors, infer lens masses and distances. Across four Gaia events, the results favor stellar remnants (white dwarfs or neutron stars) with dark-lens probabilities $>80\%$, illustrating a practical pathway to identify compact lenses using photometry alone. The findings have implications for the Galactic remnant census and motivate future astrometric confirmations with Gaia and surveys like Rubin/LSST and Roman.

Abstract

Microlensing events provide a unique way to detect and measure the masses of isolated, non-luminous objects, particularly dark stellar remnants. Under certain conditions, it is possible to measure the mass of these objects using photometry alone, specifically when a microlensing light curve displays a finite source (FS) effect. This effect generally occurs in highly magnified light curves, i.e. when the source and the lens are very well aligned. In this study, we analyse Gaia Alerts and Gaia Data Release 3 datasets, identifying four moderate-to-high-magnification microlensing events without a discernible FS effect. The absence of this effect suggests a large Einstein radius, implying substantial lens masses. In each event, we constrained the FS effect, and therefore established lower limits for the angular Einstein radius and lens mass. Additionally, we used the DarkLensCode software to obtain the mass, distance, and brightness distribution for the lens based on the Galactic model. Our analysis established lower mass limits of $\sim 0.7$ $M_{\odot}$ for one lens and $\sim 0.3-0.5$ $M_{\odot}$ for two others. A DarkLensCode analysis supports these findings, estimating lens masses in the range of $\sim 0.42-1.70$ $M_{\odot}$ and dark lens probabilities exceeding 80%. These results strongly indicate that the lenses are stellar remnants, such as white dwarfs or neutron stars. While further investigations are required to confirm the nature of these lenses, we demonstrate a straightforward yet effective approach to identifying stellar remnant candidates.

Figures (10)

• Figure 1: Spectrum of the Gaia21efs obtained with LBT/PEPSI on November 3, 2021 (blue) and the best-matching fit (red) synthesised for the specific parameters. The Fe I regions in blue (top) and red spectral arms (bottom) are presented.
• Figure 2: Light curve of Gaia21efs with the data from different surveys as well as the follow-up data collected with BHTOM. The solid line shows the parallax FS model, while the dashed line shows a parallax model for the point source. The right figure shows the zoom-in on the peak of the light curve.
• Figure 3: DLC results for Gaia21efs. Top: Mass-distance plot. Bottom: Blend-lens plot.
• Figure 4: Light curve of Gaia21azb with the data from different surveys as well as the follow-up data collected with BHTOM. The solid line shows the parallax FS model, while the dashed line shows a parallax model for the point source. The right figure shows the zoom-in on the peak of the light curve.
• Figure 5: DLC results for Gaia21azb. Top: Mass-distance plot. Bottom: Blend-lens plot.