Detection of M31 Binaries via High-Cadence Pixel-Lensing Surveys

The Angstrom Project is using a distributed network of two-meter class telescopes to conduct a high cadence pixel-lensing survey of the bulge of the Andromeda Galaxy (M31). With the expansion of global telescope network, the detection efficiency of pixel-l

DRAFTVERSIONFEBRUARY4,2008

APreprinttypesetusingLTEXstyleemulateapjv.10/09/06

DETECTIONOFM31BINARIESVIAHIGH-CADENCEPIXEL-LENSINGSURVEYS

D.KIM1,S.-J.CHUNG1

M.J.DARNLEY2,J.P.DUKE2,A.GOULD5,C.HAN1,8,M.A.IBRAHIMOV7,M.IM6,Y.-B.JEON4,R.G.KARIMOV7,E.KERINS2,3,

C.-U.LEE4,A.NEWSAM2,ANDB.-G.PARK4

(THEANGSTROMCOLLABORATION)

DraftversionFebruary4,2008

AND

arXiv:astro-ph/0701588v1 20 Jan 2007

ABSTRACT

TheAngstromProjectisusingadistributednetworkoftwo-meterclasstelescopestoconductahighcadencepixel-lensingsurveyofthebulgeoftheAndromedaGalaxy(M31).Withtheexpansionofglobaltelescopenetwork,thedetectionef ciencyofpixel-lensingsurveysisrapidlyimproving.Inthispaper,weestimatethedetectionrateofbinarylenseventsexpectedfromhigh-cadencepixel-lensingsurveystowardM31suchastheAngstromProjectbasedondetailedsimulationofeventsandapplicationofrealisticobservationalcondi-tions.Undertheconservativedetectioncriteriathatonlyhighsignal-to-noisecaustic-crossingeventswithlongenoughdurationsbetweencausticcrossingscanbe rmlyidenti edasbinarylensevents,weestimatethattheratewouldbeΓb~(7 15)fb(N/50)perseason,wherefbisthefractionofbinarieswithprojectedseparations

<103AUoutofalllensesandNistherateofstellarpixel-lensingevents.We ndthatdetectedof10 3AU<d

binarieswouldhavemassratiosdistributedoverawiderangeofq 0.1butwithseparationspopulatedwithin

5AU.Implementationofanalertsystemandsubsequentfollow-upobservationsanarrowrangeof1AU d

wouldbeimportantnotonlyfortheincreaseofthebinarylenseventratebutalsoforthecharacterizationoflensmatter.

Subjectheadings:gravitationallensing–galaxies:individual(M31)

1.INTRODUCTION

Surveystodetecttransientvariationsofstellarbrightnesscausedbygravitationmicrolensinghavebeenandarebeingconductedtowardsvariousstar elds.These eldsincludetheMagellanicClouds(MACHO:etEROS:Afonsoetal.Galacticbulge(MACHO:Alcocketal.(2001),EROS:Hamadacheetal.OGLE:et(2006),MOA:(2001)),andM31(AGAPE:Ansarietal.POINT-AGAPE:CalchiNovatietal.(2005),VATT-Colombia:Uglesichetal.(2004),MEGA:deJongetal.(2004),WeCAPP:Thenumberoflensingeventsdetectedsofarfromthesesurveysisabout3000.MostoftheseeventsweredetectedtowardtheGalacticbulge eldandthenumberofeventsdetectedtowardtheMagellanicCloudandM31 elds,whicharesev-eraldozenstowardtheindividual elds,havemeagercontri-butionstothetotalnumberofevents.TowardstheMagellanicCloud eld,thelineofsightpassesmainlythroughthehaloofourGalaxyandthusthelowdetectionrateisattributedtothescarcityofmassivecompacthaloobjectsthatcanworkaslenses(Alcocketal.2000;Onthe

DepartmentofPhysics,InstituteforBasicScienceResearch,ChungbukNationalUniversity,Chongju361-763,Korea

2AstrophysicsResearchInstitute,LiverpoolJohnMooresUniversity,TwelveQuaysHouse,Birkenhead,MerseysideCH411LD,UnitedKingdom3JordellBankObservatory,UniversityofManchester,Maccles eld,CheshireSK119DL,UnitedKingdom

4KoreaAstronomyandSpaceScienceInstitute,Hwaam-Dong,Yuseong-Gu,Daejeon305-348,Korea

5DepartmentofAstronomy,TheOhioStateUniversity,140West18thAvenue,Columbus,OH43210

6DepartmentofPhysicsandAstronomy,SeoulNationalUniversity,Seoul151-742,Korea

7UlughBegAstronomicalInstitute,UzbekAcademyofSciences,Tashkent,Uzbekistan8Correspondingauthor

1

otherhand,thelineofsighttowardM31passesthroughthedensestellarregionofM31andthusthesmallnumberofde-tectedstellarlensingeventsismainlyduetothelowdetectionef ciencyofthesurveys.

Withtheexpansionofglobaltelescopenetwork,however,thedetectionef ciencyofM31lensingsurveysisexpectedtogreatlyimprove.Forexample,anewM31pixel-lensingsurvey,theAndromedaGalaxyStellarRoboticMicrolensing(Angstrom)Project9willbeabletoachieveamonitoringfre-quencyof~5observationsper24-hourperiodbyusinganet-workoftelescopes,includingtherobotic2mLiverpoolTele-scopeatLaPalma,FaulkesTelescopeNorthinHawaii,1.8mtelescopeattheBohyunsanObservatoryinKorea,the2.4mHiltnerTelescopeattheMDMObservatoryinArizona,and1.5mattheMaidanakObservatoryinUzbekistan.IntensivemonitoringprogramssuchasAngstromareexpectedtodetecteventswitharateof~100perseason(Kerinsetal.2006).Amongthem,aconsiderablefractionwouldbecausedbybi-naries.Baltz&Gondolo(2001)pointedoutthattherateofbinaryeventsrelativetosinglelenseventsishigherinpixel-lensingbecausemanyofbinaryeventsinvolvecausticcross-ingsduringwhichthemagni cationishighandthusmoredetectable.Inthiswork,weestimatethedetectionrateofbi-narylenseventsexpectedfromahighcadencepixel-lensingsurveytowardM31basedondetailedsimulationofeventsandapplicationofrealisticobservationalconditions.

Thepaperisorganizedasfollows.In§2,wedescribeba-sicsofbinarylensing.In§3,weinvestigatethedependencyofthedetectionef ciencyonthebinarylensparameters.In§4,weestimatetheeventratebasedonthesimulationofM31pixel-lensingevents.Wedescribethedetailsofthesimulationandthecriteriaappliedfortheselectionofbinaryevents.Wealsoinvestigatethecharacteristicsofthebinariesdetectable

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http://www.astro.livjm.ac.uk/angstrom/

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