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 $(\lesssim 0.9\,{\rm mag})$ and a short timescale $(\lesssim 3\,{\rm days})$. The analysis yields an Einstein timescale of $t_{\rm E} = 0.54\pm0.02\,{\rm days}$ and an angular Einstein radius of $\theta_{\rm E} = 6.30\pm0.48\,\mu{\rm as}$. 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 $\pi_{\rm rel}$ is the relative lens-source parallax. Depending on the unknown $\pi_{\rm rel}$, the lens could be a Neptune-mass planet $(\pi_{\rm rel} \simeq 0.1\,{\rm mas})$ or a Saturn-mass planet $(\pi_{\rm rel} \simeq 16\,\mu{\rm as})$. 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 the eleventh isolated microlens with a measurement of $\theta_{\rm E} < 10\,\mu{\rm as}$. We find that additional searches for possible signatures of a lens host do not show significant evidence for the host.