KMT-2023-BLG-2669: Ninth Free-floating Planet Candidate with $\theta_{\rm E}$ measurements

TL;DR

This paper reports the detection of a ninth free-floating planet candidate through microlensing, measuring its Einstein radius and timescale, and discusses the potential for future high-resolution imaging to find possible host stars.

Contribution

It presents the identification and characterization of a new free-floating planet candidate with precise measurements of its Einstein radius, and explores methods to detect potential host stars.

Findings

First measurement of $ heta_{E}$ for this candidate.

No strong evidence of a host star in the light curve.

Potential for high-resolution imaging to detect wide-separation hosts.

Abstract

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 $(\lesssim 3\,{\rm days})$ and a small amplitude $(\lesssim 0.7\,{\rm mag})$. From the analysis, we find the Einstein timescale of $t_{\rm E} \backsimeq 0.33\,{\rm days}$ and the Einstein radius of $\theta_{\rm E} \backsimeq 4.41\,{\mu}{\rm as}$. These measurements enable us to infer the lens mass as $M = 8\,M_{\oplus} (\pi_{\rm rel} / 0.1\,{\rm mas})^{-1}$, where $\pi_{\rm rel}$ 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 $\theta_{\rm E} < 10\,{\mu}{\rm as}$, 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".