LMC self lensing for OGLE-II microlensing observations

TL;DR

This paper analyzes OGLE-II microlensing data towards the LMC, evaluating lens populations and setting upper limits on dark matter halo fractions based on observed events and expected signals.

Contribution

It provides a detailed assessment of LMC self-lensing and dark halo lensing, offering new constraints on the fraction of dark matter in compact objects.

Findings

OGLE-II observed 2 microlensing candidates consistent with luminous lens predictions.

Upper limit of 15% for halo dark matter in the mass range (10^{-2}-10^{-1}) M_sun.

Upper limit of 26% for halo dark matter at 0.5 M_sun.

Abstract

In the framework of microlensing searches towards the Large Magellanic Cloud (LMC), we discuss the results presented by the OGLE collaboration for their OGLE-II campaign \citep{lukas09}. We evaluate the optical depth, the duration and the expected rate of events for the different possible lens populations: both luminous, dominated by the LMC self lensing, and "dark", the would be compact halo objects (MACHOs) belonging to either the Galactic or to the LMC halo. The OGLE-II observational results, 2 microlensing candidate events located in the LMC bar region with duration of 24.2 and 57.2 days, compare well with the expected signal from the luminous lens populations: $n_\mathrm{exp}=1.5$, with typical duration, for LMC self lensing, of about 50 days. Because of the small statistics at disposal, however, the conclusions that can be drawn as for the halo mass fraction, $f$, in form of compact halo objects are not too severe. By means of a likelihood analysis we find an \emph{upper} limit for $f$, at 95% confidence level, of about 15% in the mass range $(10^{-2}-10^{-1}) \mathrm{M}_\odot$ and 26% for $0.5 \mathrm{M}_\odot$.