Friday, May 3, 2013

1305.0186 (T. Sumi et al.)

The Microlensing Event Rate and Optical Depth Toward the Galactic Bulge from MOA-II    [PDF]

T. Sumi, D. P. Bennett, I. A. Bond, F. Abe, C. S. Botzler, A. Fukui, K. Furusawa, Y. Itow, C. H. Ling, K. Masuda, Y. Matsubara, Y. Muraki, K. Ohnishi, N. Rattenbury, To. Saito, D. J. Sullivan, D. Suzuki, W. L. Sweatman, P., J. Tristram, K. Wada, P. C. M. Yock
We present measurements of the microlensing optical depth and event rate toward the Galactic Bulge based on two years of the MOA-II survey. In the central fields with |l|<5 degree, we find an event rates of $\Gamma=[36.1\pm1.9]e^{[0.60\pm0.05](3-|b|)}$/star/yr and an optical depth of $\tau_{200}=[3.55\pm0.27]e^{[0.51\pm0.07](3-|b|)}x10^{-6}$ for the 427 events using all sources brighter than I_s=20 mag at (l,b)=(0.38,-3.72). We find that the event rate is maximized at low latitudes and a longitude of l~1 degree. We also consider a Red Clump Giant (RCG) star sample with I_s < 17.5 mag. As with previous measurements, the RCG samples give a somewhat lower optical depth than the all-star sample,but the RCG value for the event rate is within 1-$\sigma$ of the event rate for the all-star sample. Thus, the optical depth difference between the RCG and all-star samples is mostly due to the dearth of long timescale events in the RCG sample, which is at least partly a selection effect. Another possible source of uncertainty is that we may have underestimated the number faint source stars in the central fields that are far from the location of the deep Hubble Space Telescope luminosity function that is used for this estimate. To avoid this potential problem, we report the event rate per square degree for sources brighter than I_s = 20 mag, These event rate measurements towards the central galactic bulge are necessary to predict the microlensing event rate and to optimize the survey fields in the future space mission such as WFIRST.
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