The OGLE View of Microlensing towards the Magellanic Clouds. III. Ruling out sub-solar MACHOs with the OGLE-III LMC data

TL;DR

This study uses 8 years of OGLE-III data to search for MACHOs in the LMC, finding limited evidence for sub-solar mass MACHOs and suggesting self-lensing as a more probable explanation for observed microlensing events.

Contribution

First comprehensive analysis of OGLE-III LMC data constraining MACHO dark matter in the sub-solar mass range.

Findings

Detected two main microlensing candidates with low optical depth.

Ruled out sub-solar mass MACHOs as major dark matter component.

Provided upper limits on MACHO contribution across various mass ranges.

Abstract

In the third part of the series presenting the Optical Gravitational Lensing Experiment (OGLE) microlensing studies of the dark matter halo compact objects (MACHOs) we describe results of the OGLE-III monitoring of the Large Magellanic Cloud (LMC). This unprecedented data set contains almost continuous photometric coverage over 8 years of about 35 million objects spread over 40 square degrees. We report a detection of two candidate microlensing events found with the automated pipeline and an additional two, less probable, candidate events found manually. The optical depth derived for the two main candidates was calculated following a detailed blending examination and detection efficiency determination and was found to be tau=(0.16+-0.12)10^-7. If the microlensing signal we observe originates from MACHOs it means their masses are around 0.2 M_Sun and they compose only f=3+-2 per cent of the mass of the Galactic Halo. However, the more likely explanation of our detections does not involve dark matter compact objects at all and rely on natural effect of self-lensing of LMC stars by LMC lenses. In such a scenario we can almost completely rule out MACHOs in the sub-solar mass range with an upper limit at f<7 per cent reaching its minimum of f<4 per cent at M=0.1 M_Sun. For masses around M=10 M_Sun the constraints on the MACHOs are more lenient with f ~ 20 per cent. Owing to limitations of the survey there is no reasonable limit found for heavier masses, leaving only a tiny window of mass spectrum still available for dark matter compact objects.