KMT-2024-BLG-3237: Another Free-floating Planet Candidate with Angular Einstein Radius Measurement

Planet-formation theories suggest the presence of free-floating planets (FFPs) that are ejected from their formation sites. While these planets emit very little light, they can be identified through gravitational microlensing. Here, we report the discovery of a FFP candidate in the microlensing event KMT-2024-BLG-3237. The observed light curve exhibits strong finite-source effects characterized by a small amplitude (≲0.9 mag) and a short timescale (≲3 days). The analysis yields an Einstein timescale of t _E  = 0.54 ± 0.02 days and an angular Einstein radius of θ _E  = 6.30 ± 0.48 μ as. The measurements make it possible to estimate the lens mass as $M\,\simeq 102\,{M}_{\oplus }\,{({\pi }_{{\rm{rel}}}/16\,\mu {\rm{as}})}^{-1}$ , where π _rel is the relative lens-source parallax. Depending on the unknown π _rel , the lens could be a Neptune-mass planet ( π _rel  ≃ 0.1 mas) or a Saturn-mass planet ( π _rel  ≃ 16 μ as). A Bayesian analysis yields the lens mass $M={67.3}_{-42.5}^{+103.2}\,{M}_{\oplus }$ and the lens distance ${D}_{{\rm{L}}}={7.34}_{-2.11}^{+0.96}\,{\rm{kpc}}$ . This lens is thirteenth isolated microlens with a measurement of θ _E  < 10 μ as. We find that additional searches for possible signatures of a lens host do not show significant evidence for the host.