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 (FFPs) 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 one to resolve 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 (∼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 (tE = 0.346 ± 0.008 d) indicates an FFP as the explanation. Based on the finite-source effects in the light curve, we measure an angular Einstein radius value of θE = 4.78 ± 0.23 μas, suggesting a super-Earth in the Galactic disk or a sub-Saturn-mass planet in the Galactic bulge. We have not detected any potential companion to the lens with the HST data, which is consistent with the FFP origin of the event. Though we detect no potential companion to the lens in the HST imaging, we find that the HST imaging is insufficient to constrain beyond 25-48% of potential companions (depending on whether or not the occurrence rate of wide-orbit planets is correlated with the host mass); hence, we are unable to confidently confirm this event as an FFP. Despite this, our results show that archival high-resolution images should be available for many microlensing events, providing a potential new avenue to confirm FFP candidates in future observations.