13.5www.sss17.com

NEUTRONSTARMERGERLightcurvesoftheneutronstarmergerGW170817/SSS17a:Implicationsforr-processnucleosynthesisM.
R.
Drout,1*A.
L.
Piro,1B.
J.
Shappee,1,2C.
D.
Kilpatrick,3J.
D.
Simon,1C.
Contreras,4D.
A.
Coulter,3R.
J.
Foley,3M.
R.
Siebert,3N.
Morrell,4K.
Boutsia,4F.
DiMille,4T.
W.
-S.
Holoien,1D.
Kasen,5,6J.
A.
Kollmeier,1B.
F.
Madore,1A.
J.
Monson,1,7A.
Murguia-Berthier,3Y.
-C.
Pan,3J.
X.
Prochaska,3E.
Ramirez-Ruiz,3,8A.
Rest,9,10C.
Adams,11K.
Alatalo,1,9E.
Baados,1J.
Baughman,12,13T.
C.
Beers,14,15R.
A.
Bernstein,1T.
Bitsakis,16A.
Campillay,17T.
T.
Hansen,1C.
R.
Higgs,18,19A.
P.
Ji,1G.
Maravelias,20J.
L.
Marshall,21C.
MoniBidin,22J.
L.
Prieto,13,23K.
C.
Rasmussen,14,15C.
Rojas-Bravo,3A.
L.
Strom,1N.
Ulloa,17J.
Vargas-González,4Z.
Wan,24D.
D.
Whitten14,15On17August2017,gravitationalwaves(GWs)weredetectedfromabinaryneutronstarmerger,GW170817,alongwithacoincidentshortgamma-rayburst,GRB170817A.
Anopticaltransientsource,SwopeSupernovaSurvey17a(SSS17a),wassubsequentlyidentifiedasthecounterpartofthisevent.
Wepresentultraviolet,optical,andinfraredlightcurvesofSSS17aextendingfrom10.
9hoursto18dayspostmerger.
Weconstraintheradioactivelypoweredtransientresultingfromtheejectionofneutron-richmaterial.
Thefastriseofthelightcurves,subsequentdecay,andrapidcolorevolutionareconsistentwithmultipleejectacomponentsofdifferinglanthanideabundance.
Thelate-timelightcurveindicatesthatSSS17aproducedatleast~0.
05solarmassesofheavyelements,demonstratingthatneutronstarmergersplayaroleinrapidneutroncapture(r-process)nucleosynthesisintheuniverse.
Thediscoveryofgravitationalwaves(GWs)fromcoalescingbinaryblackholesbytheLaserInterferometerGravitationalWaveObservatory(LIGO)hastransformedthestudyofcompactobjectsintheuniverse(1,2).
Unlikeblackholes,mergingneutronstarsareexpectedtoproduceelectromagneticradia-tion.
TheelectromagneticsignatureofsuchaneventcanprovidemoreinformationthantheGWsignalalone:constraininglocationofthesource,reducingthedegeneraciesinGWparameteresti-mation(3),probingtheexpansionrateoftheuniverse(4,5),andproducingamorecompletepictureofthemergerprocess(6,7).
Shortgamma-raybursts(GRBs)havelongbeenexpectedtoresultfromneutronstarmergers(8,9)andthereforewouldbeanaturalelectro-magneticcounterparttoGWs(10).
Unfortu-nately,theiremissionisbeamed,sothatitmaynotintersectourlineofsight(11).
Thepossibil-itythatonlyasmallfractionofGRBsmaybedetectablehasmotivatedtheoreticalandob-servationalsearchesformore-isotropicelectro-magneticsignatures,suchasanastronomicaltransientpoweredbytheradioactivedecayofneutron-richejectafromthemerger(12–17).
Thedetectionoftheseevents,referredtoasmacro-novaeorkilonovae,wouldprovideinformationontheoriginofmanyoftheheaviestelementsintheperiodictable(18).
Ithaslongbeenrealizedthatabouthalfoftheelementsheavierthanironarecreatedviarapidneutroncapture(r-process)nucleosynthesis—thecaptureofneutronsontolighterseednucleionatimescalemorerapidthanb-decaypathways(19,20).
However,itislessclearwherether-processpredominantlyoccurs,namelywhetherthepri-marysourcesoftheseelementsarecore-collapsesupernovaeorcompactbinarymergers(blackhole–neutronstarorneutronstar–neutronstar)(21–23).
Forsupernovae,directdetectionoftheelectromagneticsignaturesfromr-processnucleo-synthesisisobscuredbythemuchlargerluminos-ityoriginatingfromhydrogenrecombination(forhydrogen-richsupernovae)ornickel-56andcobalt-56decay(forhydrogen-poorsupernovae).
Bycon-trast,itmaybepossibletomeasurether-processnucleosynthesisafteracompactobjectmergerfromtheassociatedtransient,basedonitsradio-activedecay.
Suchameasurementwoulddemon-stratedirectlythatr-processelementsareproducedincompactmergersandprovideanestimateofther-processyield.
Althoughtherehasbeensometentativeevidenceforkilonovaefollowingshortgamma-raybursts(24,25),noconclusiveeventhasyetbeenobserved.
Ultraviolet,optical,andinfraredobservationsofaneutronstarmergerOn17August2017,theLIGOandVirgointer-ferometersjointlydetectedandlocalizedthegravitationalwavesourceGW170817,whichwasidentifiedasabinaryneutronstarmergerbasedonthewaveform(26–28).
At23:33UTCon17August2017(10.
86hourspostmerger),anopticaltransient,SwopeSupernovaSurvey2017a(SSS17a),wasidentifiedinthegalaxyNGC4993bythe1M2Hcollaborationandwasdeterminedtobeassociatedwiththisevent(29,30).
Withinanhouroftheidentification,webeganobservingthespectralenergydistribution(SED)ofSSS17afromtheopticaltonear-infrared(near-IR)withtheMagellantelescopes(31).
Earlyspectraofthesource,alsoobtainedwithinanhouroftheop-ticaldiscovery,wereblueandsmooth,indicat-ingthatthetransienteventwasinitiallyveryhot(32,33).
Overthefollowingweeks,weacquiredopticalandnear-IRimagingofSSS17aatLasCampanasObservatoryandW.
M.
KeckOb-servatorywiththeSwope,duPont,Magellan,andKeck-Itelescopes,whichareanalyzedbelow(34).
AcompanionpaperpresentsopticalspectroscopyofSSS17aforanoverlappingtimeperiod(33).
Figure1Ashowsthediscoveryimage,composedofdataobtainedwiththeMagellan/Swopetel-escopesonthenightof17Augustwithg-,i-,andH-bandspectralfilters(transmissionfunctionsforallbroadbandfiltersusedinthismanuscriptaresummarizedinFig.
3B).
Forcomparison,Fig.
1Bshowsacolorimagefromobservationsobtained4dayslater.
ThechangeincolorofSSS17abetweentheseimagesdemonstratestherapidevolutionofthistransient.
RESEARCHDroutetal.
,Science358,1570–1574(2017)22December20171of5Fig.
1.
PseudocolorimagesofSSS17ainthegalaxyNGC4993.
Imagesare1by1arcminutesandcenteredonNGC4993;SSS17aisindicatedbyabluearrowineachpanel.
Thered,green,andbluechannelscorrespondtotheH-,i-,andg-bandimagesdescribedin(33).
(A)Imagestakenonthenightof17August2017,0.
5daysafterthemerger.
(B)Imagestakenonthenightof21August2017,4.
5daysafterthemerger.
Over4days,SSS17abothfadedandbecameredder.
onMarch7,2021http://science.
sciencemag.
org/DownloadedfromTheresultinglightcurvesareshowninFig.
2,augmentedwithmeasurementsmadefrompub-licSwiftimagingatultraviolet(UV)wavelengths,andEuropeanSouthernObservatory(ESO)imagesintheopticalandnear-IR(33).
SSS17aundergoesarapidriseonatimescalethatvarieswithwavelength,from8.
5daysafterthemerger,weonlydetectthesourceineitherH-orK-bandnear-IRimagesatanygiventime,sowecannotdirectlymeasurethetemperature.
Toestimatebolometricluminositiesandphotosphericradiiattheselaterepochs,weassumeaneffectivetem-peratureof25005001000K.
Althoughthephysicalmotivationforthischoiceisfurtherdetailedbelow,observationally,themeasurablecolortemperatureisapproachingthisvaluefrom5.
5to8.
5dayspostmerger.
Further,theH-andK-bandsfallnearthepeakoftheSEDforblack-bodiesinthetemperaturerange1500to3000K.
Asaresult,bolometriccorrectionsforeithertheH-orK-bandsoverthisentiretemperaturerangeleadtoavariationintheestimatedluminosityoflessthanafactorof1.
6.
ErrorbarsrepresentingthisfullrangeareincludedinFig.
4,AandC.
Thepseudo-bolometriclightcurvehasapeakvalueof~1042ergs–1at0.
5dayspostmerger,correspondingtoourfirstepochofobservations,andthetotalradiatedenergyover18daysis~1.
7*1047erg.
Between0.
5and5.
5dayspostmerger,thebolometriclightcurveisconsistentwithapower-lawdeclineofLbolt0:85T0:01.
After5.
5days,thebest-fittingpowerlawissteeper,withLbolt1:33T0:15between7.
5and13.
5days.
TheenergysourcepoweringSSS17aWeusetheevolutionofLbol,Tc,andRphottoconstraintheenergysourcepoweringtheemis-sionfromSSS17a.
WefirstexplorewhetherthephysicalpropertiesofSSS17aareconsistentwithatransientpoweredbytheradioactivedecayofr-processelements.
Modelsforr-processpoweredtransientspredictthattheenergygenerationrate,qr,isproportionaltot–1.
3(14,15,34).
Thispowerlawissimilartotheslopeobservedinthelate-timebolometriclightcurveofSSS17a.
Todirectlycomparethepredictionsforr-processheatingtoourobservedluminosities,wemulti-plythisintrinsicheatingratebyatime-dependentthermalizationefficiency(60to25%)(34)andfitourdata.
AccordingtoArnett'slaw,thepeakluminosityofaradioactivelypoweredtransientshouldcorrespondtotheinstantaneousheatingrate(36).
Underthehypothesisthattheluminosityat0.
5dayspostmergerisduetor-processheat-ing,thisimpliesthat~0.
01solarmasses(M⊙)ofr-processmaterialwasgenerated.
TheheatingDroutetal.
,Science358,1570–1574(2017)22December20172of5Rest–frametimefrommerger(days)Apparentmag(AB)+offsetAbsolutemag(AB)+offset–15–10–51520253005101520K–4.
0H–3.
0J–2.
0Y–1.
0z–0.
8i+0.
0r+3.
0V+1.
3g+2.
2B+3.
0U,u+4.
5w1+5.
5m2+6.
0w2+7.
5Fig.
2.
UVtonear-IRphotometryofSSS17a.
Observationsbegin10.
9hoursaftermergerandcontinueto+18.
5rest-framedays.
SSS17aexhibitsbotharapidriseandarapiddeclineandbecomessubstantiallyredderwithtime.
Detectionsareshownascirclesandconnectedbysolidlinesforagivenphotometricband.
Upperlimitsareshownastrianglesandconnectedbydottedlines.
Thetimeofmergerisindicatedbyaverticaldashedline.
Theright-handverticalaxisaccountsonlyforthedistancetothehostgalaxy,NGC4993.
ForabsolutemagnitudescorrectedforforegroundMilkyWayreddening,see(34).
1TheObservatoriesoftheCarnegieInstitutionforScience,813SantaBarbaraStreet,Pasadena,CA91101,USA.
2InstituteforAstronomy,UniversityofHawai'i,2680WoodlawnDrive,Honolulu,HI96822,USA.
3DepartmentofAstronomyandAstrophysics,UniversityofCalifornia,SantaCruz,CA95064,USA.
4LasCampanasObservatory,CarnegieObservatories,Casilla601,LaSerena,Chile.
5DepartmentsofPhysicsandAstronomy,366LeConteHall,UniversityofCalifornia,Berkeley,CA94720,USA.
6NuclearScienceDivision,LawrenceBerkeleyNationalLaboratory,Berkeley,CA94720,USA.
7DepartmentofAstronomyandAstrophysics,ThePennsylvaniaStateUniversity,525DaveyLaboratory,UniversityPark,PA16802,USA.
8DarkCosmologyCenter,NielsBohrInstitute,UniversityofCopenhagen,Blegdamsvej17,2100Copenhagen,Denmark.
9SpaceTelescopeScienceInstitute,3700SanMartinDrive,Baltimore,MD21218,USA.
10DepartmentofPhysicsandAstronomy,TheJohnsHopkinsUniversity,3400NorthCharlesStreet,Baltimore,MD21218,USA.
11DivisionofPhysics,Mathematics,andAstronomy,CaliforniaInstituteofTechnology,Pasadena,CA91125,USA.
12MassachusettsInstituteofTechnology,Cambridge,MA,USA.
13NúcleodeAstronomíadelaFacultaddeIngenieríayCiencias,UniversidadDiegoPortales,AvenidaEjército441,Santiago,Chile.
14DepartmentofPhysics,UniversityofNotreDame,NotreDame,IN46556,USA.
15JointInstituteforNuclearAstrophysics,CenterfortheEvolutionoftheElements,EastLansing,MI48824,USA.
16InstitutodeRadioastronomíayAstrofísica,UniversidadNacionalAutónomadeMéxico,C.
P.
58190,Morelia,Mexico.
17DepartamentodeFísicayAstronomía,FacultaddeCiencias,UniversidaddeLaSerena,Cisternas1200,LaSerena,Chile.
18UniversityofVictoria,Victoria,BritishColumbia,Canada.
19NationalResearchCouncilHerzbergInstituteofAstrophysics,5071WestSaanichRoad,Victoria,BritishColumbiaV9E2E7,Canada.
20InstitutodeFísicayAstronomía,UniversidaddeValparaíso,AvenidaGranBretaa1111,Casilla5030,Valparaíso,Chile.
21GeorgeP.
andCynthiaWoodsMitchellInstituteforFundamentalPhysicsandAstronomy,andDepartmentofPhysicsandAstronomy,TexasA&MUniversity,CollegeStation,TX77843,USA.
22InstitutodeAstronomía,UniversidadCatólicadelNorte,AvenidaAngamos0610,Antofagasta,Chile.
23MillenniumInstituteofAstrophysics,Santiago,Chile.
24SydneyInstituteforAstronomy,SchoolofPhysics,A28,UniversityofSydney,NSW2006,Australia.
*Correspondingauthor.
Email:mdrout@carnegiescience.
eduRESEARCH|RESEARCHARTICLEonMarch7,2021http://science.
sciencemag.
org/Downloadedfromrateforthismassofr-processmaterial,Mr–p,isplottedinFig.
4A.
Althoughheatingfrom~0.
01M⊙ofr-processejectacouldexplainthepeakobservedluminosity,itwouldhaveseveralfurtherconsequences.
First,thefastrise(5daysandsupportsourassumptionofaroughlyconstanttemper-aturethroughouttheremainderoftheevolution.
Otherprocesseshavebeenconsideredforpro-vidinganopticalcounterparttoneutronstarmergers,includingmagneticdipolespin-down,heatingfromradioactivenickel,andcocoonemission[e.
g.
,(47–49)].
Thesemodelsmustbecomparedwithourdetailedobservationsaswell.
Forinstance,luminositypoweredbythespin-downDroutetal.
,Science358,1570–1574(2017)22December20173of5Fig.
3.
EvolutionoftheUVtonear-IRSEDofSSS17a.
(A)Theverticalaxis,logFl,o,isthelogarithmoftheobservedflux.
FluxeshavebeencorrectedforforegroundMilkyWayextinction(34).
Detectionsareplottedasfilledsymbols,andupperlimitsforthethirdepoch(1.
0dayspostmerger)asdownwardpointingarrows.
Less-constrainingupperlimitsatotherepochsarenotplottedforclarity.
Between0.
5and8.
5daysafterthemerger,thepeakoftheSEDshiftsfromthenear-UV(1mm)andfadesbyafactor>70.
TheSEDisbroadlyconsistentwithathermaldistribution,andthecoloredcurvesrepre-sentbest-fittingblackbodymodelsateachepoch.
In24hoursafterthediscoveryofSSS17a,theobservedcolortemperaturefallsfrom10,000Kto~5000K.
Theepochandbest-fittingblackbodytemper-ature(roundedto100K)arelisted.
SEDsforeachepocharealsoplottedindividuallyinfig.
S2anddescribedin(34).
(B)Filtertransmissionfunctionsfortheobservedphotometricbands.
Restwavelength()logFλ,o(ergss–1cm–2–1)200050001000020000–15.
0–15.
5–16.
0–16.
5–17.
0–17.
5200050001000020000+0.
5d+0.
7d10000K7600K6600K5100K3700K3300K2800K2500K2400K2500K+1.
0d+1.
5d+2.
5d+3.
5d+4.
5d+5.
5d+7.
5d+8.
5dw2m2w1uUBgVrizYJHKFig.
4.
PhysicalparametersderivedfromtheUVtonear-IRSEDsofSSS17.
Verticaldashedlinesindicatethetimeofmergerand4dayspostmerger,betweenwhichSSS17aundergoesaperiodofrapidexpansionandcooling.
(A)Pseudo-bolometriclightcurveevolution;repre-sentativer-processradioactiveheatingcurvesarealsoshown.
Althoughtheinitialobservedpeakisconsistentwith~0.
01M⊙ofr-processmaterial(bluecurve),thisunderpredictstheluminosityatlatertimes.
Instead,thelate-time(>4days)lightcurvematchesradioactiveheatingfrom0.
05±0.
02M⊙ofr-processmaterial(redcurve).
(B)Best-fittingblackbodymodeltemperatures.
At11hoursafterthemerger,SSS17aisconsistentwithablackbodyof10,000K.
Between4.
5and8.
5days,thetemperatureasymptoticallyapproaches~2500K,thetemperatureatwhichopenf-shelllan-thanideelementsareexpectedtorecombine.
Radiiandluminositiesbeyond8.
5daysarecomputedassumingatemperatureof25005001000Kandareplottedassquares.
Thistemperaturerangeishighlightedbytheorangehorizontalband.
(C)Best-fittingblackbodymodelradii.
Curvedlinesrepresenttheradiusofmaterialmovingat10,20,and30%thespeedoflight.
Atearlytimestheincreaseinradiuswithtimeimpliesthattheejectaareexpandingrelativistically.
After~5days,themeasuredradiidecrease,likelyduetorecombination.
Rest–frametimefrommerger(days)Lbol(1040ergs–1)Teff(103K)R(1015cm)051015205.
01.
00.
3246810100101102Mr–p=0.
01MMr–p=0.
05MRESEARCH|RESEARCHARTICLEonMarch7,2021http://science.
sciencemag.
org/DownloadedfromofamagneticdipoleispredictedtoscaleasLbolt2,steeperthanthemeasuredbolometriclightcurveofSSS17a,andshouldproducestrongx-rayemission(47).
Then,similartor-processheating,powerfromradioactivenickelcannotself-consistentlyreproducetheentirephotometricevolutionofSSS17a;fittingboththepeaklumi-nosityandfastdeclineleadstotheunphysicalrequirementthatthemassofradioactivenickelapproachesorexceedsthetotalejectamass(34).
Still,wefindthatitispossibletoreproducethebolometricevolutionbetween7.
5and18dayspostmergerwithheatingdueto~0.
002M⊙ofradioactivenickel(34)ifanotheremissionprocessdominatesatearlytimes.
However,nickelheat-ingdoesnotnaturallyexplainthetemperatureevolutionobservedinSSS17a.
Arapidevolutiontoveryredcolorsisnotobservedinotherknowntransientspoweredbyradioactivenickel(35).
Implicationsforr-processnucleosynthesisWeconcludethatthelate-time(5days)decayrateandcolorevolutionofSSS17aareconsistentwithatransientpoweredbytheradioactivedecayofr-processelements.
Iftheearlyemissionisalsopoweredbyr-processheating,multipleejectacom-ponentswithdifferinglanthanideabundancesarerequired.
Overall,weestimatethatatleast~0.
05M⊙ofr-processmaterialisgeneratedinthiseventfromthelate-timelightcurve.
Thepredictedmassfractionoflanthanidesinthismaterialis~0.
1to0.
5,dependingonYe(42).
Typicalsolarabundance(bymassfraction)forther-processelementswithmassnumberA>100is~8*10–8(50),resultinginaMilkyWayr-processproductionrateof~3*10–7M⊙year–1(48,51).
Ifneutronstarmergersdominater-processproduc-tion,thisproductionraterequiresaneventlikeGW170817/SSS17ainourGalaxyevery20,000to80,000years,oravolumedensityof~(1to4)*10–7Mpc–3year–1.
Attheirdesignsensitivity,Ad-vancedLIGO,AdvancedVirgo,andtheKamiokaGravitationalWaveDetector(KAGRA)willbeabletodetectbinaryneutronstarmergersoutto200Mpc(52),leadingtoapossibledetectionrateof~3to12peryear.
ThisratetranslatestolessthanoneeventperyearasnearbyasGW170817/SSS17a.
Thisnumberwouldincreaseifther-processmassthatwecalculateforSSS17aisoveresti-mated.
Suchanoverestimatecouldoccurifourassumedheatingefficiencyistooloworifthiseventproducedmoreejectathananaverageneutronstarmerger.
Empiricalexplanationfortheportionoftheperiodictableexpectedtoresultfromr-processnucleosynthesishasbeenelusive.
TheUVtonear-IRlightcurvesoftheneutronstarmergerGW170817/SSS17aprovideevidenceforbinaryneutronstarmergersasanoriginfortheseele-ments.
Observationsofmoreeventsarenowre-quiredtopreciselymapr-processyieldsfromthischannel.
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ACKNOWLEDGMENTSWethankJ.
Mulchaey(CarnegieObservatories),L.
Infante(LasCampanasObservatory),andtheentireLasCampanasObservatorystafffortheirdedication,professionalism,andexcitement,whichwereallcriticalinobtainingtheobservationsusedinthisstudy.
WealsothankI.
ThompsonandtheCarnegieObservatoryTimeAllocationCommitteeforapprovingtheSwopeSupernovaSurveyandschedulingourprogram.
WethanktheUniversityofCopenhagen,DarkCosmologyCentre,andtheNielsBohrInternationalAcademyforhostingD.
A.
C.
,R.
J.
F.
,A.
M.
B.
,E.
R.
,andM.
R.
S.
duringthiswork.
R.
J.
F.
,A.
M.
B.
,andE.
R.
wereparticipatingintheKavliSummerPrograminAstrophysics,"Astrophysicswithgravitationalwavedetections.
"ThisprogramwassupportedbytheKavliFoundation,DanishNationalResearchFoundation,theNielsBohrInternationalAcademy,andtheDarkCosmologyCentre.
M.
R.
D.
,B.
J.
S.
,K.
A.
A.
,andA.
P.
J.
weresupportedbyNASAthroughHubbleFellowshipsawardedbytheSpaceTelescopeScienceInstitute,whichisoperatedbytheAssociationofUniversitiesforResearchinAstronomy,Inc.
,forNASA,undercontractNAS5-26555.
M.
R.
D.
isaHubbleandCarnegie-DunlapFellow.
M.
R.
D.
acknowledgessupportfromtheDunlapInstituteattheUniversityofToronto,andthanksM.
W.
B.
Wilson,L.
Z.
Kelly,C.
McCully,andR.
Marguttiforhelpfuldiscussions.
TheUniversityofCalifornia,SantaCruz(UCSC)groupissupportedinpartbyNSFgrantAST-1518052;theGordonandBettyMooreFoundation;theHeising-SimonsFoundation;generousdonationsfrommanyindividualsthroughaUCSCGivingDaygrant;andfellowshipsfromtheAlfredP.
SloanFoundation(R.
J.
F),theDavidandLucilePackardFoundation(R.
J.
F.
andE.
R.
),andtheNielsBohrProfessorshipfromtheDNRF(E.
R.
).
D.
K.
issupportedinpartbyaDepartmentofEnergy(DOE)EarlyCareerawardDE-SC0008067,aDOEOfficeofNuclearPhysicsawardDE-SC0017616,andaDOESciDACawardDE-SC0018297,andbytheDirector,OfficeofEnergyResearch,OfficeofHighEnergyandNuclearPhysics,DivisionsofNuclearPhysics,oftheU.
S.
DepartmentofEnergyundercontractNo.
DE-AC02-05CH11231.
SupportforJ.
L.
P.
isinpartprovidedbyFONDECYTthroughgrant1151445andbytheMinistryofEconomy,Development,andTourism'sMillenniumScienceInitiativethroughgrantIC120009,awardedtoTheMillenniumInstituteofAstrophysics,MAS.
C.
M.
B.
wassupportedbyFONDECYTthroughregularproject1150060.
G.
M.
acknowledgessupportfromCONICYT,ProgramadeAstronomía/PCI,FONDOALMA2014,Proyectono.
31140024.
A.
M.
B.
acknowledgessupportfromaUCMEXUS-CONACYTDoctoralFellowship.
C.
A.
wassupportedbytheCaliforniaInstituteofTechnologythroughaSummerUndergraduateResearchFellowship(SURF)withfundingfromtheAssociatesSURFEndowment.
T.
C.
B.
,K.
C.
R.
,andD.
D.
W.
acknowledgepartialsupportforthisworkfromgrantPHY14-30152;PhysicsFrontierCenter/JointInstituteforNuclearAstrophysicsCenterfortheEvolutionoftheElements(JINA-CEE),awardedbytheU.
S.
NationalScienceFoundation,andfromtheLuksicFoundation.
J.
X.
P.
isalsoaffiliatedwiththeKavliInstituteforthePhysicsandMathematicsoftheUniverse.
Thispaperincludesdatagatheredwiththe6.
5-mMagellanTelescopeslocatedatLasCampanasObservatory,Chile.
ThisworkisbasedinpartonobservationscollectedattheEuropeanOrganisationforAstronomicalResearchintheSouthernHemisphere,Chile,aspartofPESSTO(thePublicESOSpectroscopicSurveyforTransientObjectsSurvey)throughESOprogram199.
D-0143.
SomeofthedatapresentedhereinwereobtainedattheW.
M.
KeckObservatory,whichisoperatedasascientificpartnershipamongtheCaliforniaInstituteofTechnology,theUniversityofCalifornia,andNASA.
TheObservatorywasmadepossiblebythegenerousfinancialsupportoftheW.
M.
KeckFoundation.
TheauthorswishtorecognizeandacknowledgetheveryimportantculturalroleandreverencethatthesummitofMaunakeahasalwayshadwithintheindigenousHawaiiancommunity.
Wearemostfortunatetohavetheopportunitytoconductobservationsfromthismountain.
ThisresearchhasmadeuseoftheNASA/IPACExtragalacticDatabase(NED)whichisoperatedbytheJetPropulsionLaboratory,CaliforniaInstituteofTechnology,undercontractwithNASA.
ThispublicationmakesuseofdataproductsfromtheTwoMicronAllSkySurvey,whichisajointprojectoftheUniversityofMassachusettsandtheInfraredProcessingandAnalysisCenter/CaliforniaInstituteofTechnology,fundedbyNASAandNSF.
TheMagellan/duPontdatapresentedinthisworkandcodeusedDroutetal.
,Science358,1570–1574(2017)22December20174of5RESEARCH|RESEARCHARTICLEonMarch7,2021http://science.
sciencemag.
org/Downloadedfromtoperformtheanalysisareavailableviahttporanonymousftpat[http|ftp]://data.
obs.
carnegiescience.
edu/SSS17a.
TheSwopedataofSSS17aareavailableathttps://ziggy.
ucolick.
org/sss17a/.
ESOandSwift-UVOTdataanalyzedinthisworkareavailableathttp://archive.
eso.
org/eso/eso_archive_main.
html(programID199.
D-0143)andhttps://archive.
stsci.
edu/swiftuvot/search.
php(targetIDs12167,12978,and12979),respectively.
ReducedphotometryispresentedintableS1andisalsoavailableontheOpenKilonovaCatalog(53)(https://kilonova.
space).
SUPPLEMENTARYMATERIALSwww.
sciencemag.
org/content/358/6370/1570/suppl/DC1MaterialsandMethodsFigs.
S1toS3TablesS1andS2References(54–89)20September2017;accepted11October2017Publishedonline16October201710.
1126/science.
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1126/science.
aaq0049(6370),1570-1574.
358Science,thisissuep.
1556,p.
1570,p.
1574,p.
1583;seealsop.
1554Scienceexplainedbyanexplosionknownasakilonova,whichproduceslargequantitiesofheavyelementsinnuclearreactions.
showhowtheseobservationscanbeetal.
unlikepreviouslydetectedastronomicaltransientsources.
Kilpatrickreporttheirspectroscopyoftheevent,whichisetal.
measurementsofitsopticalandinfraredbrightness,andShappeepresentthe1M2Hetal.
Two-Hemispheres(1M2H)collaborationwasthefirsttolocatetheelectromagneticsource.
DroutdescribehowtheOne-Meteretal.
forthesourceusingconventionaltelescopes(seetheIntroductionbySmith).
Coulterelectromagneticradiation.
WhenthegravitationalwaveeventGW170817wasdetected,astronomersrushedtosearchTwoneutronstarsmergingtogethergenerateagravitationalwavesignalandhavealsobeenpredictedtoemitPhotonsfromagravitationalwaveeventARTICLETOOLShttp://science.
sciencemag.
org/content/358/6370/1570MATERIALSSUPPLEMENTARYhttp://science.
sciencemag.
org/content/suppl/2017/10/13/science.
aaq0049.
DC1CONTENTRELATEDhttp://science.
sciencemag.
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fullhttp://science.
sciencemag.
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fullhttp://science.
sciencemag.
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fullhttp://science.
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fullhttp://science.
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fullhttp://science.
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fullhttp://science.
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fullhttp://science.
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fullhttp://science.
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