The OGLE View of Microlensing towards the Magellanic Clouds. I. A Trickle of Events in the OGLE-II LMC data

TL;DR

The OGLE-II survey towards the LMC detected a few microlensing events, suggesting that dark matter compact objects (Machos) contribute minimally to the Galactic halo mass, with self-lensing explaining the observations.

Contribution

This study provides new constraints on Macho dark matter in the Galactic halo using OGLE-II data and detailed Monte Carlo simulations.

Findings

Detected two microlensing candidates in the LMC.

Estimated optical depth of 0.43±0.33×10⁻⁷.

Set upper limits on Macho contribution to dark matter halo.

Abstract

We present the results from the OGLE-II survey (1996-2000) towards the Large Magellanic Cloud (LMC), which has the aim of detecting the microlensing phenomena caused by dark matter compact objects in the Galactic Halo (Machos). We use high resolution HST images of the OGLE fields and derive the correction for the number of monitored stars in each field. This also yield blending distributions which we use in 'catalogue level' Monte Carlo simulations of the microlensing events in order to calculate the detection efficiency of the events. We detect two candidates for microlensing events in the All Stars Sample, which translates into an optical depth of 0.43+-0.33x 10e-7. If both events were due to Macho the fraction of mass of compact dark matter objects in the Galactic halo would be 8+-6 per cent. This optical depth, however, along with the characteristics of the events, seems to be consistent with the self-lensing scenario, i.e., self-lensing alone is sufficient to explain the observed microlensing signal. Our results indicate a non-detection of Machos lensing towards the LMC with an upper limit on their abundance in the Galactic halo of 19 per cent for M=0.4 Msun and 10 per cent for masses between 0.01 and 0.2 Msun.