HST pre-imaging of a free-floating planet candidate microlensing event

High-cadence microlensing observations uncovered a population of very short-timescale microlensing events, which are believed to be caused by the population of free-floating planets (FFP) roaming the Milky Way. Unfortunately, the light curves of such events are indistinguishable from those caused by wide-orbit planets. To properly differentiate both cases, one needs high-resolution observations that would allow resolving a putative luminous companion to the lens long before or after the event. Usually, the baseline between the event and high-resolution observations needs to be quite long ($\sim 10$ yr), hindering potential follow-up efforts. However, there is a chance to use archival data if they exist. Here, we present an analysis of the microlensing event OGLE-2023-BLG-0524, the site of which was captured in 1997 with the Hubble Space Telescope (HST). Hence, we achieve a record-breaking baseline length of 25 years. A very short duration of the event ($t_E = 0.346 \pm 0.008$ d) indicates an FFP as the explanation. We have not detected any potential companion to the lens with the HST data, which is consistent with the FFP origin of the event. Thanks to the available HST data, we are able to reject from 25% to 48% of potential stellar companions depending on the assumed population model. Based on the finite-source effects in the light curve we measure the angular Einstein radius value $θ_E = 4.78 \pm 0.23 μas$, suggesting a super-Earth in the Galactic disk or a sub-Saturn-mass planet in the Galactic bulge. We show that the archival high-resolution images should be available for several microlensing events, providing us with the unprecedented possibility of seeing the lensing system as it was many years before the event.

💡 Research Summary

The paper presents a comprehensive study of the microlensing event OGLE‑2023‑BLG‑0524, which exhibits an exceptionally short Einstein timescale (tE = 0.346 ± 0.008 days). Such brief events are the hallmark of planetary‑mass lenses, but they can also be produced by a wide‑orbit planet bound to a distant host star. The authors exploit a unique archival resource: the field containing the source star was imaged by the Hubble Space Telescope (HST) with the WFPC2 camera on 1997‑11‑01, providing a 25.55‑year “pre‑imaging” baseline. This unprecedented time gap allows a direct test for any luminous companion to the lens without waiting for post‑event proper‑motion separation.

Ground‑based high‑cadence photometry from OGLE (I‑band) and KMTNet (I‑band, with a few V‑band points) captured the event. The light curve shows pronounced finite‑source effects, suppressing the peak magnification to only ~2 mag. The authors model the data with a finite‑source point‑lens (FSPL) model, employing the emcee MCMC sampler to explore the posterior distribution of the parameters. The best‑fit values are t0 = HJD − 2460000 = 86.69579, t∗ = tE ρ = 0.0504 d, ρ = 0.119, u0 ≈ 0.025, and tE ≈ 0.422 d. The source star’s calibrated I‑band magnitude is I = 19.72 ± 0.22 mag, with significant blending (fs ranging from 0.30 to 0.63 across datasets).

Finite‑source measurements combined with an estimate of the source angular radius (derived from its color and extinction) yield an angular Einstein radius θE = 4.78 ± 0.23 μas. Assuming a typical relative proper motion µrel ≈ 7 mas yr⁻¹, the lens mass is inferred to be either a super‑Earth (≈1–10 M⊕) if it resides in the Galactic disk, or a sub‑Saturn (≈30–80 M⊕) if it lies in the bulge.

The archival HST data consist of three F555W and two F814W exposures (40 s each) taken with the second WFPC2 detector (pixel scale ≈0.0996″). To locate the source in these images, the authors construct a four‑parameter linear transformation (rotation, shift, scale) between the OGLE reference image and the HST frame, using bright stars (I < 18.5 mag) cross‑matched with Gaia DR3 to correct for proper motions. The transformation achieves a root‑mean‑square error of ~0.8 HST pixels (≈0.08″). Additional HST/WFC3 observations obtained in 2025 (F606W and F814W, 16 × ~70 s exposures) confirm the source position and provide a post‑event reference.

No additional point source is detected at the expected lens position in either the 1997 or 2025 images. By injecting artificial stars of varying brightness, the authors quantify detection limits and translate them into constraints on possible stellar companions. Depending on the assumed companion mass function and spatial distribution (disk versus bulge models), they can rule out 25 %–48 % of potential stellar hosts. This represents a substantial improvement over traditional post‑event high‑resolution follow‑up, which typically requires a decade or more for the lens and source to separate sufficiently.

The study demonstrates that archival high‑resolution imaging can serve as a powerful “pre‑imaging” tool for microlensing events, dramatically shortening the timescale for testing the free‑floating planet hypothesis. The authors argue that many microlensing fields have been observed by HST, JWST, or future missions such as Euclid, implying that a systematic search could provide pre‑event baselines for hundreds of short‑timescale events. This approach not only strengthens the statistical case for a population of free‑floating planets but also offers a practical pathway to identify or exclude bound stellar hosts, thereby refining mass estimates and Galactic distribution.

In conclusion, OGLE‑2023‑BLG‑0524 is a compelling free‑floating planet candidate, supported by its ultra‑short timescale, measured angular Einstein radius, and the absence of any luminous companion in high‑resolution pre‑event HST images. The methodology pioneered here—leveraging archival space‑based imaging to probe lens environments decades before the event—opens a new avenue for rapid, robust characterization of the elusive free‑floating planet population.