Probing MACHOs Toward the Galactic Bulge(9)

If the massive compact halo object (MACHO) fraction of the Galactic dark halo is f ~ 20% as suggested by some microlensing experiments, then about 1.2% of lensing events toward the Galactic bulge are due to MACHOs. For the 40% of these that lie nearby (D_l

wellbeforepeakinordertomeasureπE(Gould&Salim1999).Hence,afairlyaggressivepostureisrequiredtokeepthehaloeventsinthesample.

However,thefactthatthesehaloeventsaresomewhatshorterthantypicalbulgeeventsmeansthattheyarealsomorefrequentthanwouldbeindicatedbytheiropticaldepthalone.Thatis,theeventrateΓ∝τ/tE,sotherateisinverselyproportionaltothetimescale.Hence,theshortertimescalesenhancestheviabilityofagivenexperimentrelativetowhatwasdiscussedin§3,providedthatnottoomanyhaloeventsarelostbecausetheyaretooshort.

4.2.Signal-to-NoiseRatios

Thetwomicrolensingparametersbeingmeasuredarerelatedtotheunderlyingphysicalparametersby, πE=,θE=κM

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