Caustic Crossing Microlensing Event by Binary MACHOs and Time Scale Bias

Abstract

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 ($t_{\mathrm{cc}}$) 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 ($t_{\mathrm{cc}}\le 10$ days) than long time scale events ($t_{\mathrm{cc}}\ge 10$ days), and propose an hourly monitoring observation instead of the nightly monitoring currently undertaken to detect caustic crossing events by binary MACHOs more efficiently.

8 pages and 3 figures, accepted for publication in ApJ Letters

References in corpus (5)

• Refining the Oort and Galactic constants
• Evidence for an Intervening Stellar Population Toward the Large Magellanic Cloud
• The LMC Microlensing Events: Evidence for a Warped and Flaring Milky Way Disk?
• Spectroscopy of MACHO 97-SMC-1: self-lensing within the SMC
• Declining Rotation Curve and Brown Dwarf MACHOs

Cited by in corpus (11)

• Binary Microlensing Events from the MACHO Project
• Combined Analysis of the Binary-Lens Caustic-Crossing Event MACHO 98-SMC-1
• Observations of the Binary Microlens Event MACHO-98-SMC-1 by the Microlensing Planet Search Collaboration
• A Caustic View of Halo Microlensing
• Kepler Rotation Effects on the Binary-Lens Microlensing Events
• MACHO Mass Determination Based on Space Telescope Observation
• Are there MACHOs in the Milky Way halo?
• Density Distribution and Shape of Galactic Dark Halo Can be Determined by Low Frequency Gravitational Waves ?
• Distribution of Caustic-Crossing Intervals for Galactic Binary-Lens Microlensing Events
• Probing the mass function of halo dark matter via microlensing
• Constraining the Location of Microlensing Objects towards the LMC through Parallax Measurement in EAGLE Observations