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

Abstract

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 $(\lesssim 0.9\mathrm{mag})$ and a short timescale $(\lesssim 3\mathrm{days})$. The analysis yields an Einstein timescale of $t_{\mathrm{E}}=0.54\pm 0.02\mathrm{days}$ and an angular Einstein radius of $\hat{I}{¸}_{\mathrm{E}}=6.30\pm 0.48\hat{I}¼\mathrm{as}$. The measurements make it possible to estimate the lens mass as $M\simeq 102M_{\oplus }(\ddot{I}{€}_{\mathrm{rel}}/16\hat{I}¼\mathrm{as}{)}^{-1}$, where $\ddot{I}{€}_{\mathrm{rel}}$ is the relative lens-source parallax. Depending on the unknown $\ddot{I}{€}_{\mathrm{rel}}$, the lens could be a Neptune-mass planet $(\ddot{I}{€}_{\mathrm{rel}}\simeq 0.1\mathrm{mas})$ or a Saturn-mass planet $(\ddot{I}{€}_{\mathrm{rel}}\simeq 16\hat{I}¼\mathrm{as})$. A Bayesian analysis yields the lens mass $M={67.3}_{-42.5}^{+103.2}{M}_{\oplus }$ and the lens distance $D_{\mathrm{L}}={7.34}_{-2.11}^{+0.96}\mathrm{kpc}$. This lens is the eleventh isolated microlens with a measurement of $\hat{I}{¸}_{\mathrm{E}}<10\hat{I}¼\mathrm{as}$. We find that additional searches for possible signatures of a lens host do not show significant evidence for the host.