KMT-2019-BLG-1339L: An M Dwarf with a Giant Planet or a Companion near the Planet/Brown Dwarf Boundary

We analyze KMT-2019-BLG-1339, a microlensing event with an obvious but incompletely resolved brief anomaly feature around the peak of the light curve. Although the origin of the anomaly is identified to be a companion to the lens with a low mass ratio q, the interpretation is subject to two different degeneracy types. The first type is the ambiguity in ρ, representing the angular source radius scaled to the angular radius of the Einstein ring, θ_E, and the other is the s ↔ s⁻¹ degeneracy. The former type, "finite-source degeneracy," causes ambiguities in both s and q, while the latter induces an ambiguity only in s. Here, s denotes the separation (in units of θ_E) in projection between the lens components. We estimate that the lens components have masses (M₁,M₂)∼(0.27^(+0.36)_(−0.15) M⊙,11⁺¹⁶₋₇ M_J) and ~(0.48^(+0.40)_(−0.28) M⊙, 1.3^(+1.1)_(−0.7) M_J) according to the two solutions subject to the finite-source degeneracy, indicating that the lens comprises an M dwarf and a companion with a mass around the planet/brown dwarf boundary or a Jovian-mass planet. It is possible to lift the finite-source degeneracy by conducting future observations utilizing a high-resolution instrument because the relative lens–source proper motion predicted by the solutions is widely different.