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.
Categories: astro-ph.EP astro-ph.GA