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GravitationallensingOlafWucknitz†JointInstituteforVLBIinEurope,Dwingeloo,NetherlandsE-mail:wucknitz@astro.uni-bonn.deThisisashortandbiasedreviewofgravitationallensingwithemphasisontheradioandespeciallyVLBIaspects.Webrieflyexplainthebasicideaandgiveashortsketchofthediscoveryofthefirstlensbeforewemoresystematicallydiscussthegeneralfieldsthatcanbestudiedwithlensing.Weintentionallyomitthedetailstheaveragelensingexpertwouldliketoseeandinsteadtrytogiveaverygeneraloverviewaddressedtotheradioastronomerworkinginsomeotherfield.ThelensB0218+357ispresentedasanexampletoshowmanyaspectsoflensinginacasethatalreadyhasledtointerestingresultsbutstillhasadditionalpotentialforthefuture.8thEuropeanVLBINetworkSymposiumSeptember26-29,2006Toru´n,PolandSpeaker.†Currentaddress:Argelander-InstitutfürAstronomie,UniversitätBonn,GermanycCopyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence.:0709.4005v1[astro-ph]25Sep2007GravitationallensingOlafWucknitz1.TheideaTheideathatgravitymightinfluencethepropagationoflightismucholderthantheEVNorevenradioastronomy.EvenNewton(1704)mentionedthepossibility1,butwithoutgoingintoanydetails.HenryCavendishwasthefirsttocalculatedeflectionsusingthesimplifiedmodelthatlightconsistsofclassicalparticlesmovingwiththespeedoflight,thusbeingweaklydeflectedinagravitationalfield(seeWill,1988).Withoutknowingofthisunpublishedwork,Soldner(1801)usedasimilarconceptandwrotethefirstarticleaboutgravitationallensing.Hefoundthatthedeflectionangle(seeFig.1)betweentheasymptoticlightpathsbeforeandafteranencounterwithamassMatadistancerisa=2GM=(c2r),whereGandcarethegravitationalconstantandthespeedoflight,respectively.Beforedevelopingthetheoryofgeneralrelativity,Einstein(1911)usedanotherapproachthatdoesnotrelyontheincorrectclassicalmodeloflightbut,instead,usedtheprincipleofequivalencetofindthesameresultasSoldner.Afewyearslater,ourstillbesttheoryofgravitationwasfinishedandEinstein(1915)couldreconsiderthedeflectionoflight,findingthatitshouldbetwiceaslargeasinnon-relativistictheory:a=4GMc2r(1.1)Thisresultwasconfirmed(withlimitedaccuracy)byEddington(1919),whodeterminedthede-flectionoflightcausedbytheSunbymeasuringpositionsofstarsclosetoitslimbduringasolareclipse.Thisservedasafirstindependenttestofthenewtheory.aFigure1:LightdeflectionAlmostallaspectsofgravitationallensingtheorycanbederivedfromEq.(1.1),togetherwiththeastrophysicalbackgroundthatisrequiredtorelateittoobservablequantities.Itiseasytoseethatinthepresenceofsufficientlylargeandcompactmasses,situationsarepossiblewherelightfromabackgroundsourcereachestheobserveralongdifferentlightpaths,whicharetheninterpretedasmultipleimagesofthissource.Typicallensesshowtwoorfourimages,whichinverysymmetricalsystemsmergetoformringsaroundthelens,theso-calledEinsteinrings.2.Thefirstlenseddouble:0957+561Thisradiosource(withoutknowingaboutitslensednatureatthattime)wasdiscoveredina966-MHzsurveymadewiththeJodrellBankMkIAtelescope.Oneofthesourcesinthatsurvey,0958+56,wasablendofthespiralgalaxyNGC3079withthenewsource0957+561.Thelatterhadanopticalidentificationastwopoint-likeimageswithaseparationofabout600(Porcasetal.,1980,andFig.2,left).1“Donotbodiesactuponlightatadistance,andbytheiractionbenditsrays;andisnotthisactionstrongestattheleastdistance?”2GravitationallensingOlafWucknitzFigure2:Left:OpticalPOSSimageandidentificationof0957+561(fromPorcasetal.,1980).Inset:HSTIimage(fromKochaneketal.,2006)showingthetwoimagesAandBandthelensinggalaxyGclosetoB.Right:Spectraofbothimages(fromWalshetal.,1979)OpticalspectraofthetwoimagesappearedlikealmostidenticalcopiesofaQSOspectrumwitharedshiftofz=1:4.Sinceachancealignmentofsosimilarobjectswithsuchaclosesepara-tionisextremelyunlikely,thefavouredinterpretationwas(andstillis)thatweseetwogravitation-allylensedimagesofoneandthesamesource(Walshetal.,1979).Manyinteresting(andamusing)detailsofthefirstdiscovery,includingadescriptionofallcoincidencesthathadtoconspiretoallowthesuccess,aredescribedbyWalsh(1989).Figure3:Interferometricobservationsof0957+561at6cm.Left:earlyVLA(Robertsetal.,1979).Centre:Cambridge5km.Right:fullVLA(Harvaneketal.,1997).DoubtswereexpressedbyRobertsetal.(1979)whenfirstinterferometricobservationsshowedthecomplicatedstructureofthedoubleradiosource(seeFig.3).TheAimageseemstoshowajetandradiolobesandonecouldnaivelyexpectthatthesameshouldbetrueforBifbotharereallylensedimagesofonesource.Inthemaps,ontheotherhand,imageBlooksperfectlypoint-like.However,Fig.4illustrateshowdifferentpartsofthesourcemaywellbelensedwithdifferentmultiplicity.Thespecificsituationfor0957+561isshowninFig.5.3GravitationallensingOlafWucknitzzszFigure4:Left:Illustrationofatypicallensmappinginonedimension,showingthetruesourcepositionzsasafunctionoftheapparentimagepositionz.Forsourcepositionsfarawayfromtheorigin,weseeonlyoneimage.Whenthesourcecrossestheextremaofthecurve,twoadditionalimagesappearordisappear.Centreandright:Illustrationofthiseffectintwodimensions.Thecentralpanelshowsthesourceplanewiththecausticcurves,therightonetheimageplanewiththecorr