Microlensing optical depth as a function of source apparent magnitude

TL;DR

This paper investigates how the microlensing optical depth varies with source apparent magnitude, revealing oscillations linked to the Galactic bar's inclination and constraining its angle using simulations and survey data.

Contribution

It demonstrates that optical depth oscillations depend on the Galactic bar angle and provides new constraints on the bar's inclination based on microlensing data.

Findings

Optical depth oscillates with source magnitude, especially between I_0 13.5 and 15.5.

Constraints on the Galactic bar angle exclude large angles recently reported.

EROS-2 data shows the predicted tau oscillation, supporting the model.

Abstract

Measurements of the microlensing optical depth, tau, 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 dependance 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 I_0 >~ 13.0, except in the range 13.5 <~ I_0 <~ 15.5. Here many RCGs are detected, causing a significant oscillation in tau. The amplitude of this oscillation is a function of the inclination angle of the Galactic bar, theta_bar, which may thus be constrained. A further constraint comes from a similar dependance of tau with theta_bar: combining the predicted trends with the measured values provides 1-sigma 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 tau 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.