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 (≲0.9 mag) and a short timescale (≲3 days). The analysis yields an Einstein timescale of t E = 0.54 ± 0.02 days and an angular Einstein radius of θ E = 6.30 ± 0.48 μ as. The measurements make it possible to estimate the lens mass as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>M</mml:mi> <mml:mspace width="0.25em"/> <mml:mo>≃</mml:mo> <mml:mn>102</mml:mn> <mml:mspace width="0.25em"/> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊕</mml:mo> </mml:mrow> </mml:msub> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>π</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">rel</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:mn>16</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>μ</mml:mi> <mml:mi mathvariant="normal">as</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> , where π rel is the relative lens-source parallax. Depending on the unknown π rel , the lens could be a Neptune-mass planet ( π rel ≃ 0.1 mas) or a Saturn-mass planet ( π rel ≃ 16 μ as). A Bayesian analysis yields the lens mass <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>M</mml:mi> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>67.3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>42.5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>103.2</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊕</mml:mo> </mml:mrow> </mml:msub> </mml:math> and the lens distance <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">L</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>7.34</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2.11</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.96</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">kpc</mml:mi> </mml:math> . This lens is thirteenth isolated microlens with a measurement of θ E &lt; 10 μ as. We find that additional searches for possible signatures of a lens host do not show significant evidence for the host.