  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.

Astrophysics