Caustic crossing microlensing events provide us a unique opportunity to measure the relative proper motion of the lens to the source , and so those caused by binary MACHOs are of great importance for understanding the structure of the Galactic halo and the nature of MACHOs .
The microlensing event 98-SMC-01 , occurred in June 1998 , is the first event for which the proper motion is ever measured through the caustic crossing , and this event may be caused by binary MACHOs as we argue in this Letter .
Motivated by the possible existence of binary MACHOs , we have performed the Monte Carlo simulations of caustic crossing events by binary MACHOs and investigated the properties and detectability of the events .
Our calculation shows that typical caustic crossing events have the interval between two caustic crossings ( t _ { cc } ) of about 5 days .
We argue that with the current strategy of binary event search the proper motions of these typical events are not measurable because of the short time scale .
Therefore the proper motion distribution measured from caustic crossing events suffers significantly from ‘ time scale bias ’ , which is a bias toward finding long time scale events and hence slowly moving lenses .
We predict there are two times more short time scale events ( t _ { cc } \leq 10 days ) than long time scale events ( t _ { cc } \geq 10 days ) , and propose an hourly monitoring observation instead of the nightly monitoring currently undertaken to detect caustic crossing events by binary MACHOs more efficiently .