Comparison of the two follow-up observation strategies for gravitational microlensing planet searches

There are two different strategies of follow-up observations for the detection of planets by using microlensing. One is detecting the light-curve anomalies affected by the planetary caustic from continuous monitoring of all events detected by microlensing survey programs (type I strategy), and the other is detecting anomalies near the peak amplification affected by the central caustic from intensive monitoring of high-amplification events (type II strategy). It was shown by Griest & Safizadeh that the type II strategy yields high planet detection efficiency per event. However, it is not known whether the planet detection rate by this strategy can make up a substantial fraction of the total rate. In this paper, we estimate the relative planet detection rates expected under the two follow-up observation strategies. From this estimation, we find that the rate under the type II strategy is substantial and will comprise similar to1/4-1/2 of the total rate. We also find that compared to the type I strategy the type II strategy is more efficient in detecting planets located outside of the lensing zone. We determine the optimal monitoring frequency of the type II strategy to be similar to 20 times per night, which can be easily achieved by the current microlensing follow-up programs even with a single telescope.