Parallax Effect in Microlensing Events Due to Free-floating Planets

Abstract

One of the most important applications of microlensing observations is the detection of free-floating planets (FFPs). The timescale of microlensing due to FFPs (t E) is short (a few days). Discerning the annual parallax effect in observations of these short-duration events due to FFPs 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 δ u . This deviation is a straight line as long as t E ≪ P ⊕, and its size is δ u ∝ π rel (P ⊕ is the observer’s orbital period). So, most observed microlensing events due to close FFPs have simple Paczyńsky light curves with indiscernible but important parallax. To evaluate the destructive effects of invisible parallax in such events, we simulate ∼9650 microlensing events due to FFPs with t E < 10 days that are observed only by the Nancy Grace Roman Space Telescope (Roman). We conclude that in half of these microlensing events the missing parallax alters the real light curves, changing their shape and derived properties (by Δχ 2 ≳ 100). By fitting Paczyński light curves to these affected events we evaluate the relative and dimensionless deviations in the lensing parameters from their real values (δ t E, δ ρ ⋆, ...). We conclude that around 46 FFPs that are discovered by Roman have light curves highly affected by invisible parallax with δ t E > 0.1 and δ ρ ⋆ > 0.1. Our study reveals the importance of simultaneous and dense observations of the same microlensing events viewed by Roman by other observers circling the Sun in different orbits.