OGLE-2016-BLG-1227L: A Wide-separation Planet from a Very Short-timescale Microlensing Event

We present the analysis of the microlensing event OGLE-2016-BLG-1227. The light curve of this short-duration event appears to be a single-lens event affected by severe finite-source effects. Analysis of the light curve based on single-lens single-source (1L1S) modeling yields very small values of the event timescale, $t_{\rm E}\sim 3.5$ days, and the angular Einstein radius, $\theta_{\rm E}\sim 0.009$ mas, making the lens a candidate of a free-floating planet. Close inspection reveals that the 1L1S solution leaves small residuals with amplitude $\Delta I\lesssim 0.03$ mag. We find that the residuals are explained by the existence of an additional widely-separated heavier lens component, indicating that the lens is a wide-separation planetary system rather than a free-floating planet. From Bayesian analysis, it is estimated that the planet has a mass of $M_{\rm p} = 0.79^{+1.30}_{-0.39} M_{\rm J}$ and it is orbiting a low-mass host star with a mass of $M_{\rm host}=0.10^{+0.17}_{-0.05} M_\odot$ located with a projected separation of $a_\perp=3.4^{+2.1}_{-1.0}$ au. The planetary system is located in the Galactic bulge with a line-of-sight separation from the source star of $D_{\rm LS}=1.21^{+0.96}_{-0.63}$ kpc. The event shows that there are a range of deviations in the signatures of host stars for apparently isolated planetary lensing events and that it is possible to identify a host even when a deviation is subtle.