Probing MACHOs Toward the Galactic Bulge(2)

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

roughly25timeshigher.Here,vrot=220kms 1istheMilkyWayrotationspeed.Hence,whentheexperimentsbegan,itseemedasthoughevenacrudemeasurementofτwouldunambiguouslydeterminewhetherthehalowascomposedofMACHOs.

Adecadelater,thesituationisfarlessclearthanwasanticipated.MACHO(Alcocketal.2000)foundτ～1×10 7,roughlytheroot-mean-squareoftheresultsexpectedfromMACHOsandstars.Theyinterpretedthistomeanthatthehalowas20%composedofMACHOsandestimatedthetypicalmasstobeM～0.4M⊙.Ontheotherhand,theEROScollaboration(Afonsoetal.2003a;Tisserand&Milsztajn2005)foundanupperlimitfortheopticaldepthduetoMACHOsof5%ofthefull-halovalue.

Oneoptionforresolvingthiscon ictistoexploreotherlinesofsight.Crotts(1992)andBaillonetal.(1993)advocatedM31,andseveralcollaborations,includingAGAPE(Ansarietal.1999),Columbia-VATT(Uglesichetal.2004),MEGA(deJongetal.2004),NainiTal(Joshietal.2005)POINT-AGAPE(Auri`ereetal.2001),SLOTT-AGAPE(CalchiNovati2003),andWeCAPP(Ri eser2003),havepursuedthissuggestion.InmanywaysthisissubstantiallymorechallengingthantheobservationstowardtheLMC,simplybecauseM31is15timesfartherawayandhencethesourcesaresubstantiallyfainter.Eventsarenowbeingreportedfromtheseexperiments,andtheirimplicationsfordarkmattershouldbeavailablesoon.

Themicrolensingtarget eldthathasbeenmonitoredthemostintensivelyistheGalac-ticbulge.OriginallyproposedbyPaczy´nski(1991)andGriestetal.(1991),majorsurveyshavebeencarriedoutbytheOGLE(Udalskietal.1993;Udalski2003),DUO(Alardetal.1995),MACHO(Popowski2005),EROS(Afonsoetal.2003b),andMOA(Abeetal.2004)collaborations.TheprimarymotivationofbothproposalswastoprobefordiskdarkmatterandotherexoticobjectssuchasalargepopulationofJupiters.Griestetal.(1991)doesmentionthatifthehaloiscomposedofMACHOs,thenthesewillgiverisetoanoptical

bulgedepthτhalo=1.3×10 7,butsincethisis4timessmallerthanthepredictedopticaldepth

bulgeduetodiskstarsτdisk=5.1×10 7,theredidnotappeartobeanywaytoisolatethe

MACHOevents.

Bulgemicrolensingobservationshavebeenenormouslyfruitful.Kiraga&Paczy´nski(1994)showedthattheopticaldepthduetobulgeself-lensingwasevengreaterthanthatduetodiskstars.Thehigheventrateencouragedsearchesforlensinganomaliesduetoplanetarycompanionsofthelenses(Mao&Paczy´nski1991;Gould&Loeb1992;Rhieetal.2000;Albrowetal.2001b;Gaudietal.2002;Abeetal.2004),whichhasnowyieldedthe rst rmmicrolensingplanetdetection(Bond2004).Bulgemicrolensinghasenabledthe rstmicrolensmassmeasurement(Anetal.2003)andtheprobingofbulge-staratmosphereswithµasresolutionbothphotometrically(Alcocketal.1997;Albrowetal.1999,2000;Fieldset

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