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 $(\lesssim 0.9\,{\rm mag})$ 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 $θ_{\rm E} = 6.30\pm0.48\,μ{\rm as}$. The measurements make it possible to estimate the lens mass as $M \simeq 102\,M_{\oplus}\,(π_{\rm rel}/16\,μ{\rm as})^{-1}$, where $π_{\rm rel}$ is the relative lens-source parallax. Depending on the unknown $π_{\rm rel}$, the lens could be a Neptune-mass planet $(π_{\rm rel} \simeq 0.1\,{\rm mas})$ or a Saturn-mass planet $(π_{\rm rel} \simeq 16\,μ{\rm as})$. 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 $θ_{\rm E} < 10\,μ{\rm as}$. We find that additional searches for possible signatures of a lens host do not show significant evidence for the host.

Figures (5)

• Figure 1: Light curve of KMT-2024-BLG-3237. The upper panel displays the full light curve from the 2024 season, while the lower panel provides the zoom-in of the event. The black and orange curves indicate the FSPL $(f_{\rm B} = 0)$ and the PSPL model, respectively.
• Figure 2: KMTS41 and VVV CMDs for stars in the vicinity (within $4^{\prime}$) of KMT-2024-BLG-3237. In each panel, the giant clump and baseline object (source) positions are marked by the red and blue dots, respectively.
• Figure 3: Gaia proper motions of stars in the vicinity (within $6^{\prime}$) of KMT-2024-BLG-3237. The red dots are red giant (bulge population) stars, while the blue dots are main sequence (disk population) stars. In each population, the inner and outer contours envelope 68$\%$ and 95$\%$ of the stars, respectively. The red square corresponds to the mean proper motion of the bulge population, $<\hbox{\boldmath $\mu$}_{\rm bulge}> (l,b) = (-6.22, -0.29) \pm (3.28, 2.65) \,{\rm mas}\,{\rm yr^{-1}}$. The blue square is the mean proper motion of the disk population, $<\hbox{\boldmath $\mu$}_{\rm disk}> (l,b) = (-4.06, -0.74) \pm (4.29, 3.06) \,{\rm mas}\,{\rm yr^{-1}}$. The baseline object (source) is located at the black dot. The black circle represents the locus of the relative lens-source proper motions, $\mu_{\rm rel} = 4.29\,{\rm mas}\,{\rm yr^{-1}}$.
• Figure 4: Lens host detection probability in the $(q, s)$ plane.
• Figure 5: Posterior distributions of $M$ (left) and $D_{\rm L}$ (right) for KMT-2024-BLG-3237. In each panel, the blue and red distributions correspond to the disk- and bulge-lens contributions, respectively. The black distribution shows the total contribution.