Microlensing Zone of Planets Detectable through the Channel of High-Magnification Events

TL;DR

This paper redefines the microlensing zone for high-magnification events, showing it depends on planet-star mass ratio and extends the range for detecting giant planets, enhancing the method's effectiveness.

Contribution

It introduces a new definition of the microlensing zone specific to high-magnification events, accounting for planet mass ratio effects.

Findings

Lensing zone varies with planet/star mass ratio.

For Jupiter-mass planets, the zone spans 0.25 to 3.9 Einstein radii.

Wider zones improve detection prospects for giant planets.

Abstract

A microlensing lensing zone refers to the range of planet-star separations where the probability of detecting a planetary signal is high. Its conventional definition as the range between $\sim 0.6$ and 1.6 Einstein radii of the primary lens is based on the criterion that a major caustic induced by a planet should be located within the Einstein ring of the primary. However, current planetary lensing searches focus on high-magnification events to detect perturbations induced by another caustic located always within the Einstein ring very close to the primary lens (stellar caustic) and thus a new definition of a lensing zone is needed. In this paper, we determine this lensing zone. By applying a criterion that detectable planets should produce signals $\geq 5%$, we find that the new lensing zone varies depending on the planet/star mass ratio unlike the fixed range of the classical zone regardless of the mass ratio. The lensing zone is equivalent to the classical zone for a planet with a planet/star mass ratio $q\sim 3\times 10^{-4}$ and it becomes wider for heavier planets. For a Jupiter-mass planet, the lensing zone ranges from 0.25 to 3.9 Einstein radii, corresponding to a physical range between $\sim 0.5$ AU and 7.4 AU for a typical Galactic event. The wider lensing zone of central perturbations for giant planets implies that the microlensing method provides an important tool to detect planetary systems composed of multiple ice-giant planets.