Parallax Effect in Microlensing Events due to Free-Floating Planets

One of most important applications of microlensing observations is detecting free-floating planets (FFPs). The time scale of microlensing due to FFPs (t ₄) is short (a few days). Discerning the annual parallax effect in observations from these short-duration events by one observer is barely possible, though their parallax amplitude is larger than that in common events. In microlensing events due to FFPs, the lens-source relative trajectory alters because of the observer's motion by u. This deviation is a straight line if t ₄ P_, and its size is u ₑ₄₋ (P_ is the observer's orbital period). So, most of observed microlensing events due to close FFPs have simple Paczy\'nsky lightcurves with indiscernible and valuable parallax. To evaluate destructive effects of invisible parallax in such events, we simulate 9650 microlensing events due to FFPs with t ₄0. 1~and~ _>0. 1. Our study reveals the importance of simultaneous and dense observations of microlensing events viewed by \ by other observers rotating the Sun in different orbits.