2007MNRAS.380..901W

Measurements of the microlensing optical depth τ towards the Galactic bulge appear to depend on the method used to obtain them. Those values based on the lensing of red clump giants (RCGs) appear to be significantly lower than those based on the lensing of all stars along the line of sight. This discrepancy is still not understood. Through Monte Carlo simulations, it is found that the discrepancy cannot be explained by a dependence on the flux limits of the two methods. The optical depth is expected to be generally constant as a function of source apparent magnitude for I₀≳ 13.0, except in the range 13.5 ≲ I₀≲ 15.5. Here many RCGs are detected, causing a significant oscillation in τ. The amplitude of this oscillation is a function of the inclination angle of the Galactic bar, θ_bar, which may thus be constrained. A further constraint comes from a similar dependence of τ with θ_bar: combining the predicted trends with the measured values provides 1 σ upper limits, which exclude the large bar angles recently reported by the GLIMPSE and EROS surveys. The latest survey data from EROS-2 appear to show the predicted τ oscillation, though currently at a low significance. However, a further sign comes from EROS-2 event counts, which show a clear skew towards fainter magnitudes.