The OGLE View of Microlensing towards the Magellanic Clouds. II. OGLE-II SMC data

TL;DR

This study searches for microlensing events towards the SMC using OGLE-II data to test if dark matter consists of stellar-mass compact objects, finding results consistent with self-lensing and placing limits on MACHO contributions.

Contribution

First analysis of OGLE-II SMC data providing constraints on dark matter in the form of MACHOs based on microlensing event detection.

Findings

Only one microlensing candidate found, with limited data quality.

Microlensing optical depth consistent with SMC self-lensing.

Upper limits on MACHO dark matter fraction: <20% for 0.4 Msun, <11% for 0.003-0.2 Msun.

Abstract

The primary goal of this paper is to provide the evidence that can either prove or falsify the hypothesis that dark matter in the Galactic halo can clump into stellar-mass compact objects. If such objects existed, they would act as lenses to external sources in the Magellanic Clouds, giving rise to an observable effect of microlensing. We present the results of our search for such events, based on the data from the second phase of the OGLE survey (1996-2000) towards the SMC. The data set we used is comprised of 2.1 million monitored sources distributed over an area of 2.4 square degrees. We found only one microlensing event candidate, however its poor quality light curve limited our discussion on the exact distance to the lensing object. Given a single event, taking the blending (crowding of stars) into account for the detection efficiency simulations, and deriving the HST-corrected number of monitored stars, the microlensing optical depth is tau=(1.55+-1.55)10e-7. This result is consistent with the expected SMC self-lensing signal, with no need of introducing dark matter microlenses. Rejecting the unconvincing event leads to the upper limit on the fraction of dark matter in the form of MACHOs to f<20 per cent for deflectors' masses around 0.4 Msun and f<11 per cent for masses between 0.003 and 0.2 Msun (95 per cent confidence limit). Our result indicates that the Milky Way's dark matter is unlikely to be clumpy and form compact objects in the sub-solar-mass range.