KMT-2023-BLG-2669: Ninth Free-floating Planet Candidate with ${\theta }_{\mathrm{E}}$ measurements

We report a free-floating planet (FFP) candidate identified from the analysis of the microlensing event KMT-2023-BLG-2669. The lensing light curve is characterized by a short duration $(\lesssim 3\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s})$ and a small amplitude $(\lesssim 0.7\mathrm{m}\mathrm{a}\mathrm{g})$. From the analysis, we find the Einstein timescale of $t_{\mathrm{E}}\backsimeq 0.33\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s}$ and the Einstein radius of ${\theta }_{\mathrm{E}}\backsimeq 4.41\mu \mathrm{a}\mathrm{s}$. These measurements enable us to infer the lens mass as $M=8M_{\oplus }({\pi }_{\mathrm{r}\mathrm{e}\mathrm{l}}\mathrm{/}0.1\mathrm{m}\mathrm{a}\mathrm{s}{)}^{-1}$, where ${\pi }_{\mathrm{r}\mathrm{e}\mathrm{l}}$ is the relative lens-source parallax. The inference implies that the lens is a sub-Neptune- to Saturn-mass object depending on its unknown distance. This is the ninth isolated planetary-mass microlens with ${\theta }_{\mathrm{E}}<10\mu \mathrm{a}\mathrm{s}$, which (as shown by \citealt{gould22}) is a useful threshold for a FFP candidate. We conduct extensive searches for possible signals of a host star in the light curve, but find no strong evidence for the host. We investigate the possibility of using late-time high-resolution imaging to probe for possible hosts. In particular, we discuss that for the case of finite-source point-lens FFP candidates, it would be possible to search for very wide separation hosts immediately, although such searches are “high-risk, high-reward”.