NEUTRON STAR MERGER

Early spectra of the gravitational wavesource GW170817 Evolution of aneutron star mergerB J Shappee12 J D Simon1 M R Drout1 A L Piro1 N Morrell3 J L Prieto45

D Kasen67 T W-S Holoien1 J A Kollmeier1 D D Kelson1 D A Coulter8

R J Foley8 C D Kilpatrick8 M R Siebert8 B F Madore1 A Murguia-Berthier8

Y-C Pan8 J X Prochaska8 E Ramirez-Ruiz89 A Rest1011 C Adams12 K Alatalo110

E Bantildeados1 J Baughman413 R A Bernstein1 T Bitsakis14 K Boutsia3 J R Bravo3

F Di Mille3 C R Higgs1516 A P Ji111 G Maravelias17 J L Marshall18

V M Placco19 G Prieto3 Z Wan20

On 17 August 2017 Swope Supernova Survey 2017a (SSS17a) was discovered as theoptical counterpart of the binary neutron star gravitational wave event GW170817Wereport time-series spectroscopy of SSS17a from 1175 hours until 85 days after themerger Over the first hour of observations the ejecta rapidly expanded and cooledApplying blackbody fits to the spectra we measured the photosphere cooling from11000thorn3400

900 to 9300thorn300300 kelvin and determined a photospheric velocity of roughly 30

of the speed of lightThe spectra of SSS17a began displaying broad features after 146 daysand evolved qualitatively over each subsequent day with distinct blue (early-time) andred (late-time) components The late-time component is consistent with theoreticalmodels of r-processndashenriched neutron star ejecta whereas the blue component requireshigh-velocity lanthanide-free material

Short gamma-ray bursts (GRBs) have longbeen hypothesized to be produced by neu-tron star mergers (1 2) and thus they arethe most likely electromagnetic counter-parts to the gravitational wave signals

from binary neutron star coalescence Unfor-tunately because GRB emission is highly non-isotropic (3) in many cases the beam of gammarays will not be seen by an observer This hasmotivated studies of electromagnetic counter-parts that are more isotropic with one of themost popular cases being the so-called macro-novae or kilonovae (4ndash8) These transients wouldresult from the outflow of ~001 solar massesof neutron-richmaterial ejected from themerg-ing neutron stars at ≳10 of the speed of lightThis neutron-rich material is expected to syn-thesize heavy elements that power a fast tran-sient peaking at red optical or near-infrared(near-IR) wavelengths via their radioactive de-cay Furthermore the r-process nucleosynthe-sis in these outflowsmdashnamed from the captureof neutrons onto lighter seed nuclei on a time-

scale more rapid than b decays (9 10)mdashmayexplain the origin of half of the elements heavierthan iron in the periodic table (11 12) Althougha handful of candidate kilonovae followingshort GRBs have been identified (13ndash15) nonehave been studied in detail or conclusivelyconfirmedThe overall heating rate from r-process nu-

cleosynthesis is generally agreed upon and fairlyrobust with respect to the composition (6 7)but the expected spectroscopic appearance ofa kilonova is much less clear Kilonova spectradepend strongly on the nuclear yields neutrinoflux geometric orientations mass and velocityof the ejecta The neutron-rich outflow is ex-pected to produce elements in the lanthanideseries which have a large effect on the emergentradiation because of the opacity generated bytheir numerous bound-bound electronic tran-sitions (16) Despite considerable theoreticaleffort there is no consensus on the expectedspectrum of a kilonova (16ndash18) There could bemultiple components to the ejecta with dif-

ferent compositions opacities geometries andvelocities (19) In the absence of observationalmeasurements of kilonovae especially spectros-copy this variety of theoretical models remainslargely unconstrainedOn 17 August 2017 the Laser Interferometer

Gravitational-Wave Observatory (LIGO) andVirgo Collaboration (LVC) detected GW170817a gravitational wave (GW) signal from a binaryneutron star merger (20) A contemporaneousand weak short GRB was detected by the Fermispacecraft and International Gamma-Ray As-trophysics Laboratory (INTEGRAL) telescopes(21 22) Following the GW trigger our One-Meter Two-Hemispheres (1M2H) collaborationidentified an optical counterpart Swope Super-nova Survey 2017a (SSS17a) 1087 hours afterthe merger (23 24) SSS17a was located in thegalaxyNGC 4993 (23 24) at a distance of 40Mpc(25) which is an order of magnitude closer thanprevious gravitational wave detections This dis-covery was immediately announced to the LVCand we began a comprehensive follow-up cam-paign that extended for nearly 3 weeks A com-panion paper presents extensive ultraviolet (UV)optical and near-IR photometry of SSS17a fol-lowing the light curve of SSS17a as it reddenedand faded (26)At 1175 hours after the GW170817 trigger

we obtained an optical spectrum of SSS17awith the Low Dispersion Survey Spectrograph 3(LDSS-3) on the Magellan-Clay telescope at theLas Campanas Observatory in Chile This earlyspectrum shows a smooth blue continuum ex-tending over the entire optical-wavelength range(Fig 1) (27) In the next hour we obtained threeadditional spectra at higher resolution usingLDSS-3 and the Magellan Echellette (MagE)spectrograph on the Magellan-Baade telescopebefore SSS17a set and was no longer observablefrom Chile The transient faded measurably atthe bluest wavelengths during the short timeinterval covered by these initial spectra whereasthe spectra redward of 5000 Aring did not evolve inthis time span (Fig 1)Motivated by the fact that the UV-optical spec-

tral energy distribution observed 3 to 4 hourslater (at t = 067 days after the merger) iswell approximated by a blackbody (26) we fitblackbody models to the observed rest-framedereddened spectra to quantify the very earlyspectral evolution (28) The best-fitting modelresults in a temperature of 11000thorn3400

900 K andradius of 33thorn03

08 1014 cm at t = 049 days The

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Shappee et al Science 358 1574ndash1578 (2017) 22 December 2017 1 of 5

1The Observatories of the Carnegie Institution for Science 813 Santa Barbara Street Pasadena CA 91101 USA 2Institute for Astronomy University of Hawairsquoi 2680 Woodlawn DriveHonolulu HI 96822 USA 3Las Campanas Observatory Carnegie Observatories Casilla 601 La Serena Chile 4Nuacutecleo de Astronomiacutea de la Facultad de Ingenieriacutea y Ciencias UniversidadDiego Portales Avenida Ejeacutercito 441 Santiago Chile 5Millennium Institute of Astrophysics Santiago Chile 6Departments of Physics and Astronomy 366 LeConte Hall University ofCaliforniandashBerkeley Berkeley CA 94720 USA 7Nuclear Science Division Lawrence Berkeley National Laboratory Berkeley CA 94720 USA 8Department of Astronomy and AstrophysicsUniversity of CaliforniandashSanta Cruz Santa Cruz CA 95064 USA 9Dark Cosmology Center Niels Bohr Institute University of Copenhagen Blegdamsvej 17 2100 Copenhagen Denmark10Space Telescope Science Institute 3700 San Martin Drive Baltimore MD 21218 USA 11Department of Physics and Astronomy The Johns Hopkins University 3400 North Charles StreetBaltimore MD 21218 USA 12Division of Physics Mathematics and Astronomy California Institute of Technology Pasadena CA 91125 USA 13Massachusetts Institute of TechnologyCambridge MA 02139 USA 14Instituto de Radioastronomiacutea y Astrofiacutesica Universidad Nacional Autoacutenoma de Meacutexico CP 58190 Morelia Mexico 15University of Victoria Victoria BC V8P5C2 Canada 16National Research Council (NRC) Herzberg Institute of Astrophysics 5071 West Saanich Road Victoria BC V9E 2E7 Canada 17Instituto de Fiacutesica y Astronomiacutea Universidadde Valparaiacuteso Valparaiacuteso Chile 18George P and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy and Department of Physics and Astronomy Texas AampMUniversity College Station TX 77843 USA 19Department of Physics and Joint Institute for Nuclear Astrophysics (JINA) Center for the Evolution of the Elements University ofNotre Dame Notre Dame IN 46556 USA 20Sydney Institute for Astronomy School of Physics A28 University of Sydney NSW 2006 AustraliaCorresponding author Email shappeehawaiiedu

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listed uncertainties represent 90 confidenceintervals Although the peak of the blackbodyis located at UV wavelengths that we do notobserve the combination of the known lumi-nosity and the spectral slope from 3800 to10000 Aring provides sufficient constraints on thetemperature For material to reach this radiusso quickly requires an expansion velocity of77000thorn7000

20000 km sndash1 or 026thorn002007c where c is

the speed of light By comparison typical super-novae have bulk velocities of 10000 km sndash1Even the most energetic supernovae which areassociated with long-duration GRBs have peakmeasured photospheric velocities of ~20000to 50000 km sndash1 at 2 to 3 days postexplosion(29) less than what we infer here for the GWcounterpart Although the velocity of thesesystems may be even higher in the first daypostexplosion early spectra within 24 hours ofexplosion are not widely available for GRBsupernovaeFor the MagE spectrum at t = 053 days we

fit a blackbody temperature of 9300thorn300300 K and

radius of 41thorn0202 1014 cm The uncertainties on

the temperature fits are not normally distrib-uted but we find that the difference in tem-perature between the LDSS-3 andMagE spectrais significant at 5s confidence This drop in tem-perature over only 1 hour indicates that the ex-panding ejecta are cooling rapidly A similarvelocity of 90000thorn4000

4000 km sndash1 (030thorn001001c) is

inferred from this spectrumOn subsequent nights we acquired optical

spectroscopy of SSS17a covering more than1 week postexplosion using both Magellan tel-escopes Most of the spectra were obtained withLDSS-3 but we also observed the source withthe Inamori Magellan Areal Camera and Spec-trograph (IMACS) and the Magellan InamoriKyocera Echelle (MIKE) spectrograph Our spec-troscopic time series of SSS17a spans from 049to 846 days after the GW170817 trigger (Fig 2)

A description of the acquisition and reductionof these spectra and a log of all spectroscopicobservations are presented in the supplemen-tary materials (28)We searched the higher-resolutionMIKE and

MagE spectra for absorption or emission linesfrom the host galaxy as well as for Na I D ab-

sorption from theMilkyWay Using the strengthof the Milky Way Na I D absorption we con-firmed the foreground reddening that was de-termined using other spatially coarser methods(28) Host-galaxy features have been detected inall available short GRB spectra (30ndash33) How-ever we did not detect any host-galaxy lines

Shappee et al Science 358 1574ndash1578 (2017) 22 December 2017 2 of 5

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) +

con

stan

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375

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Fig 1 Early optical spectra of SSS17aMagellan-LDSS-3 and Magellan-MagE spectraof SSS17a acquired 1175 and 1275 hours afterthe LVC trigger respectively The overall slopeof the continuum evolved subtly but substan-tially in this 1-hour interval demonstrating achange in the effective temperature of thesource Blackbody models and uncertaintiesare shown by the shaded green (LDSS-3) andbrown (MagE) regions The thick and thin solidlines in the fit regions indicate the median and90 confidence interval for the fits respectivelyThese fits are described in (28) The shadedgray outlines surrounding each spectrum indicatethe uncertainties on the flux-calibrated spectraAlthough theMagE spectrum extends to 10100Aringwe only use the data blueward of 7000 Aring for theblackbody fit because of the difficulty of flux-calibrating the data at redder wavelengths wherethe overlap between adjacent spectral orders isminimal and telluric absorption bands can cover alarge fraction of an order A vertical offset ofndash035dex has been applied to the MagE spectrum forclarity

049d

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[erg

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Aring]

Fig 2 Spectroscopic time series of SSS17aThe vertical axis is observed flux (fl) Observationsbegan ~05 days after the merger and were obtained with the LDSS-3 MagE spectrograph and theInamori Magellan Areal Camera and Spectrograph (IMACS) on theMagellan telescopesThese spectrahave been calibrated to the photometry of (24 26) Colored bands indicate the wavelength rangesof the g r i z and Y photometric filters

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our 2s limits onNa ID absorption aremore thanfive times stricter than the absorption detectedin GRB130603B (28 33)After being observed as a rapidly cooling black-

body observed on the first night later spectra showthat the appearance of SSS17a changed not justquantitatively but also qualitatively each nightAt 146days a broad feature extends from~5000 to7000Aring and the spectrumdeclines toward shorterwavelengths At 249 days the peak wavelengthof the emission has shifted to ~7500 Aring and thespectrum has a distinct triangular shape At 346and 451 days the fall-off at blue wavelengthssteepens the peak of the optical emission con-tinues evolving redward and a new featurethat increases with wavelength develops in thenear-IR part of the spectrum By 745 days afterthe merger the spectrum consists of a smoothred continuum with very little flux detectedbelow 6500 Aring These data reveal that the pho-tosphere continued expanding and cooling withits temperature declining by a factor of ~4withina week (26)The spectral features of SSS17a become more

complex over time After the first night theyare not well described by either single-blackbodyfits or the sum of two blackbodies All featuresin the data are smooth and very broad ~2500 Aringwide for the peak centered at ~7500 Aring in the249- and 346-day spectra ~2000 Aring wide forthe trough centered at ~9000 Aring in the 249-and 346-day spectra and gt1000 Aring for the near-IR feature at 346 and 451 days It is not clear

from the data whether these features shouldbe interpreted as emission centered at thewave-length of the fluxmaxima or absorption centeredon the flux minima In either case from theirwidth and the Doppler effect we can estimatethat the material in SSS17a must be moving ata velocity of ~02c to 03c which is consistentwith the observed lack of narrow linesThe photometric evolution of SSS17a strongly

suggests two distinct components to the ejecta(26 34) The spectral evolution as well as theinability to match the early- and late-time spec-tra with a single kilonova model for which wemake direct comparisons below also favorstwo components The largely featureless bluecomponent dominates the initial spectrumbut quickly fades within the first few daysAfter 3 days the spectrum becomes dominatedby a red component corresponding to coolertemperatures which fades much more slowlyThe spectral evolution of SSS17a is unlike

known astronomical transients as can be seenin Fig 3 which compares the Magellan spec-tra taken at t = 049 346 and 745 days afterthe GW170817 trigger to other classes of tran-sients Because SSS17a was associated with ashort gamma-ray transient (21 22) it is natu-ral to investigate whether the SSS17a spectraare consistent with afterglow emission Onlythree short GRBs (all at redshifts z gt 03) haveavailable optical spectroscopy (30ndash33) of whichonly GRB130603B (32 33) is unambiguouslyclassified as a short GRB In Fig 3 we show

spectra of GRB130603B taken ~8 hours afterthe GRB (33) which do not resemble those ofSSS17a Unfortunately there are no spectra ofshort GRBs reported in the literature at epochslater than 1 day We therefore cannot compareour later spectra against other short GRBs atsimilar epochs Despite the limited comparisonsample we conclude that the early blue spec-trum of SSS17a does not resemble previouslydetected short GRB spectra which are likelydominated by afterglows from a jet projectedalong the line of sight Instead we argue thatthe observations probe emission that is inde-pendent of the GRB This conclusion is bolsteredby the lack of x-ray and radio emissions at earlytimes which rules out a broadband synchrotronspectrum as the primary driver of the opticalemission (35)In Fig 3 we also compare SSS17a to super-

novae and other rapid transients Type Ia andtype Ibc supernovae develop emission lineswith characteristic widths and velocities of10000 km sndash1 (003c) and evolve over weeks tomonths rather than days Young type II super-novae can have blue smooth spectra not dis-similar to the earliest spectra of SSS17a but thisphase lasts for many days Later in their evolu-tion they settle to a temperature of ~6000 K for~100 days as determined by hydrogen recom-bination and show hydrogen absorption linesboth of which are properties very much unlikethose of SSS17a Early spectra of rapid bluetransients (36) can be as blue as SSS17a but

Shappee et al Science 358 1574ndash1578 (2017) 22 December 2017 3 of 5

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4000 6000 8000 10000 1400012000

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4000 6000 8000 10000 1400012000

t ~ 3d t ~ 7dt lt 1d~

SSS17a 745d

GRBSN 8d

SSS17a 346d

GRBSN 27d

Fast SN Ic 5-9d

SN Ia 756d

SN II 3d

SN Ia 32d

SSS17a 049d

short GRB 031-035d

SN Ia 118d

Rapid blue transient 5-9d

Fig 3 Spectra of SSS17a compared with a broad range of otherastronomical transients at several evolutionary phases Althoughthe ~05-day spectrum of SSS17a has few features and is potentially anextreme version of some other hot andor fast transients it evolves rapidlyin comparisonWithin 3 days of the LIGO trigger the optical spectrumof SSS17a is no longer similar to other known transients Phases listed arerelative to the time of explosion for all objects All spectra are dividedby their median value and displayed with arbitrary additive offsets forclarity (A) SSS17a compared to the type Ia supernova (SN Ia) SN2011fe

(42) and the afterglow spectrum of the short gamma-ray burst GRB130603B(33) Few observations of other transients within 1 day of explosion areavailable (B) SSS17a at 346 days after explosion compared to theSN Ia ASASSN-14lp (43) the type II supernova SN2006bp (44) and thelong GRB and its associated afterglow and broad-lined type Ic supernovaGRB030329SN2003dh (38) at similar times relative to explosion(C) SSS17a at 745 days after explosion compared to SN2011fe (45) therapid blue transient PS1-12bv (36) the fast type Ic supernova SN2005ek(46) and the GRBSN GRB980425SN1998bw (37)

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they maintain their blue color for days where-as SSS17a had become much redder by day249 GRB980425SN1998bw and GRB030329SN2003dh supernovae associated with longGRBs revealed the spectrum of a type Ic super-nova as the afterglow faded and remained detect-able for months (37 38) However the supernovafeatures seen inGRB980425 are absent in SSS17aand SSS17a again evolves and fades much fasterbecoming undetectable in weeksDetailed comparison of the spectral features

of SSS17a to theoretical models is challengingCurrent models use only a small subset of theelements synthesized in the neutron-rich out-flows and even for the elements that are in-cluded there is considerable uncertainty inthe details of their line transitions (16 18) Thefeatures that are found in the optical spectra ofcurrent models are composed of many differenttransitions and cannot be reliably associatedwithspecific elementsWe therefore focus on themostrobust physical features (such as color velocityand temporal evolution) in our comparisonThe ejecta from a neutron star merger can

have different compositions and velocities de-pending on their origin This motivates com-parisons to a few characteristic models InFig 4A we present theoretical models of 01solar masses of lanthanide-rich material thathas been dynamically ejected with a velocity of02c (17) Such a model is consistent with thepower-law evolution of the bolometric lumi-nosity of SSS17a at times ≳4 days which isconsistent with the expectations for r-process

heating (26) although it is not currently pos-sible to identify the particular r-process ele-ments responsible Modest scaling by a factor≲7 was required to match the overall lumi-nosity of each epoch but there are qualitativesimilarities to the spectra from 451 days onwardHowever this model alone does not reproducethe rapidly evolving early blue phaseThe material generating the early compo-

nent is likely lanthanide-free to reproduce theblue emission Such material can be driven byaccretion disk winds (39) or dynamical ejectionfrom the neutron starndashneutron star mergerinterface (40) We consider both cases withthe main difference being the velocity of thematerial In Fig 4B we compare SSS17a witha disk-wind model (19) which although themodel can account for the blue colors at earlytimes has a number of problems These includea luminosity that is much too low (the modelspectra have been scaled by over one order ofmagnitude) velocities (≲01c) that are muchsmaller than those we infer from the temper-ature evolution and absorption features thatare not seen in our early smooth spectra Wetherefore disfavor a disk-wind originFor the case of lanthanide-free dynamical

ejecta there are fewer theoretical predictionsof spectra available for direct comparison Wereplicate the main features of this scenariowith a model composed of fast-moving (≳02c)lanthanide-free material (41) shown in Fig 4CUnlike the previous scenarios such a modelmay explain the spectra blueward of 9000 Aring

for times up to 346 days At later epochs theobserved spectra have large red excesses rela-tive to the model which could be explained ifthe red component is obscuring the lanthanide-free material as the ejecta evolve (34) Suchobscuration could occur because the red dy-namical ejecta are more concentrated in equa-torial regions whereas the lanthanide-freematerial would be launched perpendicular tothe binary midplane This geometry suggests aviewing angle that is neither edge nor pole onHowever the large velocity (gt02c) we infer forthe blue component might make obscurationdifficult and it is unclear if certain viewing anglesand ejecta distributions can satisfy all the con-straints from the spectral evolution we observeThe detailed spectral kinematic and chem-

ical data obtained for SSS17a provide multipleconstraints for understanding binary neutronstar mergers We find that existing modelsrelated to neutron star ejecta may explain someaspects of the spectral evolution we presentbut no single model matches all of the mainproperties The ejecta likely have a complicatedthree-dimensional structure with large inter-nal variations in velocity and composition andthis complexity may be required to account forthe full time-series spectral data in greater de-tail However alternative physical mechanismsshould not be discounted especially for the earlyhot thermal emission The smooth spectrum ofthe very early optical emission allows us to ruleout models namely disk winds that would bedegenerate with photometry alone

Shappee et al Science 358 1574ndash1578 (2017) 22 December 2017 4 of 5

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Red dynamical model Wind model Blue dynamical model

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Fig 4 Comparison of SSS17a to theoretical modelsThe vertical axisis observed flux (fl) The models shown are for three possible physicalinterpretations of SSS17a Although the red kilonova model provides areasonable likeness to the data at late times the early-time spectra andkinematics require lanthanide-free relativistic material No single modelshown here or described in the current literature can self-consistentlyreproduce the full spectroscopic time series of SSS17a (A) Lanthanide-

rich red kilonovamodel from a neutron star merger and dynamical ejection(17) At each epoch the absolute luminosity is scaled to match the data(B) Disk-wind model (19) with a neutron star that immediately collapsesafter the merger The absolute luminosity is scaled by a factor of ≳10 ateach epoch to match the data (C) Lanthanide-poor blue kilonova modelfrom a neutron star merger and dynamical ejection (41) The model hasbeen crafted to match the observations at early times (34)

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REFERENCES AND NOTES

1 B Paczynski Astrophys J 308 L43 (1986)2 D Eichler M Livio T Piran D N Schramm Nature 340

126ndash128 (1989)3 W Fong E Berger R Margutti B A Zauderer Astrophys J

815 102 (2015)4 L-X Li B Paczyński Astrophys J 507 L59ndashL62 (1998)5 S R Kulkarni arXivastro-ph0510256 [astro-ph] (10 October

2005)6 B D Metzger et al Mon Not R Astron Soc 406 2650ndash2662

(2010)7 L F Roberts D Kasen W H Lee E Ramirez-Ruiz Astrophys J

736 L21 (2011)8 B D Metzger Living Rev Relativ 20 3 (2017)9 E M Burbidge G R Burbidge W A Fowler F Hoyle Rev

Mod Phys 29 547ndash650 (1957)10 A G W Cameron Publ Astron Soc Pac 69 201 (1957)11 Y-Z Qian G J Wasserburg Phys Rep 442 237ndash268

(2007)12 M Arnould S Goriely K Takahashi Phys Rep 450 97ndash213

(2007)13 N R Tanvir et al Nature 500 547ndash549 (2013)14 E Berger W Fong R Chornock Astrophys J 774 L23

(2013)15 M M Kasliwal O Korobkin R M Lau R Wollaeger C L Fryer

Astrophys J 843 L34 (2017)16 D Kasen N R Badnell J Barnes Astrophys J 774 25

(2013)17 J Barnes D Kasen Astrophys J 775 18 (2013)18 M Tanaka K Hotokezaka Astrophys J 775 113 (2013)19 D Kasen R Fernaacutendez B D Metzger Mon Not R Astron

Soc 450 1777ndash1786 (2015)20 LIGOVirgo collaboration GRB Coordinates Network 21509

(2017)21 GBM-LIGO GRB Coordinates Network 21506 (2017)22 INTEGRAL GRB Coordinates Network 21507 (2017)23 One-Meter Two-Hemisphere (1M2H) collaboration GRB

Coordinates Network 21529 (2017)24 D A Coulter et al Science 358 1556ndash1558 (2017)25 W L Freedman et al Astrophys J 553 47ndash72 (2001)26 M R Drout et al Science 358 1570ndash1574 (2017)27 M R Drout et al GRB Coordinates Network 21547 (2017)28 Materials and methods are available as supplementary

materials29 Z Cano S-Q Wang Z-G Dai X-F Wu Adv Astron 2017

8929054 (2017)30 E M Levesque et al Mon Not R Astron Soc 401 963ndash972

(2010)31 N R Tanvir et al GRB Coordinates Network Circular Service

11123 (2010)32 A Cucchiara et al Astrophys J 777 94 (2013)33 A de Ugarte Postigo et al Astron Astrophys 563 A62

(2014)

34 C D Kilpatrick et al Science 358 1583ndash1587(2017)

35 A Murguia-Berthier et al Astrophys J 848 L34 (2017)36 M R Drout et al Astrophys J 794 23 (2014)37 F Patat et al Astrophys J 555 900ndash917 (2001)38 K Z Stanek et al Astrophys J 591 L17ndashL20 (2003)39 B D Metzger R Fernaacutendez Mon Not R Astron Soc 441

3444ndash3453 (2014)40 S Wanajo et al Astrophys J 789 L39 (2014)41 D Kasen B Metzger J Barnes E Quataert E Ramirez-Ruiz

Nature 551 80ndash84 (2017)42 P E Nugent et al Nature 480 344ndash347 (2011)43 B J Shappee et al Astrophys J 826 144 (2016)44 R M Quimby et al Astrophys J 666 1093ndash1107

(2007)45 R Pereira et al Astron Astrophys 554 A27 (2013)46 M R Drout et al Astrophys J 774 58 (2013)

ACKNOWLEDGMENTS

We thank J Mulchaey (Carnegie Observatories director)L Infante (Las Campanas Observatory director) and the entireLas Campanas staff for their dedication professionalism andexcitement which were critical for obtaining the observationsused in this study BJS MRD KA and APJ were supportedby NASA through Hubble Fellowships awarded by the SpaceTelescope Science Institute which is operated by the Associationof Universities for Research in Astronomy Inc for NASAunder contract NAS 5-26555 BJS and EB were supportedby Carnegie-Princeton Fellowships MRD was supported by aCarnegie-Dunlap Fellowship and acknowledges support fromthe Dunlap Institute at the University of Toronto TW-SH wassupported by a Carnegie Fellowship Support for JLP was inpart provided by the Fundo Nacional de Desarrollo Cientiacutefico yTecnoloacutegico (FONDECYT) through grant 1151445 and by theMinistry of Economy Development and Tourismrsquos MillenniumScience Initiative through grant IC120009 awarded to theMillennium Institute of Astrophysics DK is supported in partby a US Department of Energy (DOE) Early Career awardDE-SC0008067 a DOE Office of Nuclear Physics awardDE-SC0017616 and a DOE SciDAC award DE-SC0018297 and bythe Director Office of Energy Research Office of High Energy andNuclear Physics Divisions of Nuclear Physics of the DOE undercontract no DE-AC02-05CH11231 VMP acknowledges partialsupport for this work from grant PHY 14- 30152 from the PhysicsFrontier Center JINAndashCenter for the Evolution of the Elements(JINA-CEE) awarded by the NSF The University of CaliforniandashSanta Cruz (UCSC) group was supported in part by NSF grantAST-1518052 the Gordon and Betty Moore Foundation theHeising-Simons Foundation generous donations from manyindividuals through a UCSC Giving Day grant and fromfellowships from the Alfred P Sloan Foundation (RJF) theDavid and Lucile Packard Foundation (RJF and ER-R) and the

Niels Bohr Professorship from the Danish National ResearchFoundation (ER-R) TB acknowledges support from theConsejo Nacional de Ciencia y Tecnologia (CONACYT) ResearchFellowships program GM acknowledges support from ComisioacutenNacional de Investigacioacuten Cientiacutefica y Tecnoloacutegica (CONICYT)Programa de AstronomiacuteandashPrograma de Cooperacioacuten InternacionalFONDO ALMA 2014 project no 31140024 AM-B acknowledgessupport from a University of California Institute for Mexicoand the United States (UC MEXUS)ndashCONACYT DoctoralFellowship CA was supported by Caltech through a SummerUndergraduate Research Fellowship (SURF) with fundingfrom the Associates SURF Endowment JXP is also affiliatedwith the Kavli Institute for the Physics and Mathematics of theUniverse This paper includes data gathered with the 65-mMagellan Telescopes located at Las Campanas ObservatoryChile Part of this work is based on a comparison to observationsof GRB130603B that were obtained from the EuropeanOrganisation for Astronomical Research in the SouthernHemisphere (ESO) Science Archive Facility under requestnumber vmplacco308387 We thank A Cucchiara forsending us the spectrum of GRB130603B We thank J van Sadersfor useful suggestions We thank the University ofCopenhagen Dark Cosmology Centre and the Niels BohrInternational Academy for hosting DAC RJF AM-BER-R and MRS during a portion of this work RJFAM-B and ER-R were participating in the Kavli SummerProgram in Astrophysics ldquoAstrophysics with gravitational wavedetectionsrdquo This program was supported by the Kavli FoundationDanish National Research Foundation the Niels BohrInternational Academy and the Dark Cosmology Centre Thisresearch has made use of the NASAIPAC Extragalactic Database(NED) which is operated by the Jet Propulsion LaboratoryCalifornia Institute of Technology under contract with NASA Thedata presented in this work and the code used to perform theanalysis are available at httpsdataobscarnegiescienceeduSSS17a Calibrated rest-wavelength spectra are alsomade available at the Weizmann Interactive Supernova DataRepository (WISeREP) (doi101086666656 httpswiserepweizmannacil)

SUPPLEMENTARY MATERIALS

wwwsciencemagorgcontent35863701574supplDC1Materials and MethodsFig S1Tables S1 and S2References (47ndash63)

22 September 2017 accepted 11 October 2017Published online 16 October 2017101126scienceaaq0186

Shappee et al Science 358 1574ndash1578 (2017) 22 December 2017 5 of 5

RESEARCH | RESEARCH ARTICLEon N

ovember 8 2020

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Early spectra of the gravitational wave source GW170817 Evolution of a neutron star merger

Boutsia J R Bravo F Di Mille C R Higgs A P Ji G Maravelias J L Marshall V M Placco G Prieto and Z WanProchaska E Ramirez-Ruiz A Rest C Adams K Alatalo E Bantildeados J Baughman R A Bernstein T Bitsakis KKelson D A Coulter R J Foley C D Kilpatrick M R Siebert B F Madore A Murguia-Berthier Y-C Pan J X B J Shappee J D Simon M R Drout A L Piro N Morrell J L Prieto D Kasen T W-S Holoien J A Kollmeier D D

originally published online October 16 2017DOI 101126scienceaaq0186 (6370) 1574-1578358Science

this issue p 1556 p 1570 p 1574 p 1583 see also p 1554Scienceexplained by an explosion known as a kilonova which produces large quantities of heavy elements in nuclear reactions

show how these observations can beet alunlike previously detected astronomical transient sources Kilpatrick report their spectroscopy of the event which iset almeasurements of its optical and infrared brightness and Shappee

present the 1M2Het alTwo-Hemispheres (1M2H) collaboration was the first to locate the electromagnetic source Drout describe how the One-Meteret alfor the source using conventional telescopes (see the Introduction by Smith) Coulter

electromagnetic radiation When the gravitational wave event GW170817 was detected astronomers rushed to search Two neutron stars merging together generate a gravitational wave signal and have also been predicted to emit

Photons from a gravitational wave event

ARTICLE TOOLS httpsciencesciencemagorgcontent35863701574

MATERIALSSUPPLEMENTARY httpsciencesciencemagorgcontentsuppl20171013scienceaaq0186DC1

CONTENTRELATED

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REFERENCES

httpsciencesciencemagorgcontent35863701574BIBLThis article cites 50 articles 3 of which you can access for free

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is a registered trademark of AAASScienceScience 1200 New York Avenue NW Washington DC 20005 The title (print ISSN 0036-8075 online ISSN 1095-9203) is published by the American Association for the Advancement ofScience

Copyright copy 2017 American Association for the Advancement of Science

on Novem

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PERMISSIONS httpwwwsciencemagorghelpreprints-and-permissions

Terms of ServiceUse of this article is subject to the

is a registered trademark of AAASScienceScience 1200 New York Avenue NW Washington DC 20005 The title (print ISSN 0036-8075 online ISSN 1095-9203) is published by the American Association for the Advancement ofScience

Copyright copy 2017 American Association for the Advancement of Science

on Novem

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nloaded from