Near-Field Microlensing from Wide-Field Surveys

Abstract

We estimate the rate of near-field microlensing events expected from all-sky surveys and investigate the properties of these events. Under the assumption that all lenses are composed of stars, our estimation of the event rate ranges from \Gamma_{tot}~0.2 yr^{-1}$ for a survey with a magnitude limit of V_{lim}=12 to \Gamma_{tot}~20 yr^{-1} for a survey with V_{lim}=18. We find that the average distances to source stars and lenses vary considerably depending on the magnitude limit, while the dependencies of the average event time scale and lens-source transverse speed are weak and nearly negligible, respectively. We also find that the the average lens-source proper motion of events expected even from a survey with V_{lim}=18 would be <\mu> >~ 40 mas yr^{-1}, implying that the source and lens of a significant fraction of near-field events could be resolved from high-resolution follow-up observations. From the investigation of the variation of the event characteristics depending on the position of the sky, we find that the average distances to source stars and lenses become shorter, the lens-source transverse speed increases, and the time scale becomes shorter as the the galactic latitude of the field increases. Due to the concentration of events near the galactic plane, we find that >~ 50 % of events would be detected in the field with b <= 20^\circ.