Microlensing towards the SMC: a new analysis of OGLE and EROS results

TL;DR

This paper reanalyzes microlensing data towards the SMC from EROS and OGLE campaigns, assessing whether events are due to known stellar populations or dark matter MACHOs, and sets upper limits on MACHO contribution.

Contribution

It provides a new statistical analysis distinguishing self-lensing from MACHO lensing and constrains dark matter halo composition using recent microlensing data.

Findings

Self-lensing can explain the number of observed events.

Some events' characteristics suggest non-self lensing origins.

Upper limits on MACHO halo fraction are about 10% for 0.01 M_sun MACHOs.

Abstract

We present a new analysis of the results of the EROS-2, OGLE-II, and OGLE-III microlensing campaigns towards the Small Magellanic Cloud (SMC). Through a statistical analysis we address the issue of the \emph{nature} of the reported microlensing candidate events, whether to be attributed to lenses belonging to known population (the SMC luminous components or the Milky Way disc, to which we broadly refer to as "self lensing") or to the would be population of dark matter compact halo objects (MACHOs). To this purpose, we present profiles of the optical depth and, comparing to the observed quantities, we carry out analyses of the events position and duration. Finally, we evaluate and study the microlensing rate. Overall, we consider five reported microlensing events towards the SMC (one by EROS and four by OGLE). The analysis shows that in terms of number of events the expected self lensing signal may indeed explain the observed rate. However, the characteristics of the events, spatial distribution and duration (and for one event, the projected velocity) rather suggest a non-self lensing origin for a few of them. In particular we evaluate, through a likelihood analysis, the resulting upper limit for the halo mass fraction in form of MACHOs given the expected self-lensing and MACHO lensing signal. At 95% CL, the tighter upper limit, about 10%, is found for MACHO mass of $10^{-2} \mathrm{M}_\odot$, upper limit that reduces to above 20% for $0.5 \mathrm{M}_\odot$ MACHOs.