Implementation and testing of the first prompt search for gravitational wave transients with electromagnetic counterparts

0.62 MB·English

by  The LIGO Scientific Collaboration

Astronomy&Astrophysicsmanuscriptno.lvemfollowup (cid:13)c ESO20121
13January2012
Implementation and testing of the first prompt search for
gravitational wave transients with electromagnetic counterparts
TheLIGOScientificCollaborationandVirgoCollaboration:J.Abadie1,B.P.Abbott1,R.Abbott1,T.D.Abbott2,M.Abernathy3,T.Accadia4,F.Acernese5ac,C.Adams6,
R.Adhikari1,C.Affeldt7,8,P.Ajith1,B.Allen7,9,8,G.S.Allen10,E.AmadorCeron9,D.Amariutei11,R.S.Amin12,S.B.Anderson1,W.G.Anderson9,K.Arai1,M.A.Arain11,
M.C.Araya1,S.M.Aston13,P.Astone14a,D.Atkinson15,P.Aufmuth8,7,C.Aulbert7,8,B.E.Aylott13,S.Babak16,P.Baker17,G.Ballardin18,S.Ballmer19,D.Barker15,
F.Barone5ac,B.Barr3,P.Barriga20,L.Barsotti21,M.Barsuglia22,M.A.Barton15,I.Bartos23,R.Bassiri3,M.Bastarrika3,A.Basti24ab,J.Batch15,J.Bauchrowitz7,8,
Th.S.Bauer25a,M.Bebronne4,B.Behnke16,M.G.Beker25a,A.S.Bell3,A.Belletoile4,I.Belopolski23,M.Benacquista26,J.M.Berliner15,A.Bertolini7,8,J.Betzwieser1,
N.Beveridge3,P.T.Beyersdorf27,I.A.Bilenko28,G.Billingsley1,J.Birch6,R.Biswas26,M.Bitossi24a,M.A.Bizouard29a,E.Black1,J.K.Blackburn1,L.Blackburn30,D.Blair20,
B.Bland15,M.Blom25a,O.Bock7,8,T.P.Bodiya21,C.Bogan7,8,R.Bondarescu31,F.Bondu32b,L.Bonelli24ab,R.Bonnand33,R.Bork1,M.Born7,8,V.Boschi24a,S.Bose34,
L.Bosi35a,B. Bouhou22,S.Braccini24a,C.Bradaschia24a,P.R.Brady9,V.B.Braginsky28,M.Branchesi36ab,J.E.Brau37,J.Breyer7,8,T.Briant38,D.O.Bridges6,A.Brillet32a,
M.Brinkmann7,8,V.Brisson29a,M.Britzger7,8,A.F.Brooks1,D.A.Brown19,A.Brummit39,T.Bulik40bc,H.J.Bulten25ab,A.Buonanno41,J.Burguet–Castell9,O.Burmeister7,8,
2 D.Buskulic4,C.Buy22,R.L.Byer10,L.Cadonati42,G.Cagnoli36a,E.Calloni5ab,J.B.Camp30,P.Campsie3,J.Cannizzo30,K.Cannon44,B.Canuel18,J.Cao45,C.D.Capano19,
1 F.Carbognani18,S.Caride46,S.Caudill12,M.Cavaglia`43,F.Cavalier29a,R.Cavalieri18,G.Cella24a,C.Cepeda1,E.Cesarini36b,O.Chaibi32a,T.Chalermsongsak1,E.Chalkley13,
0 P.Charlton47,E.Chassande-Mottin22,S.Chelkowski13,Y.Chen48,A.Chincarini49,A.Chiummo18,H.Cho50,N.Christensen51,S.S.Y.Chua52,C.T.Y.Chung53,S.Chung20,
2 G.Ciani11,F.Clara15,D.E.Clark10,J.Clark54,J.H.Clayton9,F.Cleva32a,E.Coccia55ab,P.-F.Cohadon38,C.N.Colacino24ab,J.Colas18,A.Colla14ab,M.Colombini14b,
n A.Conte14ab,R.Conte56,D.Cook15,T.R.Corbitt21,M.Cordier27,N.Cornish17,A.Corsi1,C.A.Costa12,M.Coughlin51,J.-P.Coulon32a,P.Couvares19,D.M.Coward20,
a D.C.Coyne1,J.D.E.Creighton9,T.D.Creighton26,A.M.Cruise13,A.Cumming3,L.Cunningham3,E.Cuoco18,R.M.Cutler13,K.Dahl7,8,S.L.Danilishin28,R.Dannenberg1,
J S.D’Antonio55a,K.Danzmann7,8,V.Dattilo18,B.Daudert1,H.Daveloza26,M.Davier29a,G.Davies54,E.J.Daw57,R.Day18,T.Dayanga34,R.DeRosa5ab,D.DeBra10,
G.Debreczeni58,J.Degallaix7,8,W.DelPozzo25a,M.delPrete59b,T.Dent54,V.Dergachev1,R.DeRosa12,R.DeSalvo1,V.Dhillon57,S.Dhurandhar60,L.DiFiore5a,
2
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1
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M] F.Fidecaro24ab,L.S.Finn31,I.Fiori18,R.P.Fisher31,R.Flaminio33,M.Flanigan15,S.Foley21,E.Forsi6,L.A.Forte5a,N.Fotopoulos1,J.-D.Fournier32a,J.Franc33,
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I M.Fyffe6,M.Galimberti33,L.Gammaitoni35ab,M.R.Ganija66,J.Garcia15,J.A.Garofoli19,F.Garufi5ab,M.E.Ga´spa´r58,G.Gemme49,R.Geng45,E.Genin18,A.Gennai24a,
h. L.A´.Gergely67,S.Ghosh34,J.A.Giaime12,6,S.Giampanis9,K.D.Giardina6,A.Giazotto24a,C.Gill3,E.Goetz7,8,L.M.Goggin9,G.Gonza´lez12,M.L.Gorodetsky28,
p S.Goßler7,8,R.Gouaty4,C.Graef7,8,M.Granata22,A.Grant3,S.Gras20,C.Gray15,N.Gray3,R.J.S.Greenhalgh39,A.M.Gretarsson68,C.Greverie32a,R.Grosso26,H.Grote7,8,
- S.Grunewald16,G.M.Guidi36ab,C.Guido6,R.Gupta60,E.K.Gustafson1,R.Gustafson46,T.Ha69,B.Hage8,7,J.M.Hallam13,D.Hammer9,G.Hammond3,J.Hanks15,
o C.Hanna1,70,J.Hanson6,J.Harms62,G.M.Harry21,I.W.Harry54,E.D.Harstad37,M.T.Hartman11,K.Haughian3,K.Hayama71,J.-F.Hayau32b,T.Hayler39,J.Heefner1,
r A.Heidmann38,M.C.Heintze11,H.Heitmann32,P.Hello29a,M.A.Hendry3,I.S.Heng3,A.W.Heptonstall1,V.Herrera10,M.Hewitson7,8,S.Hild3,D.Hoak42,K.A.Hodge1,
st K.Holt6,J.Homan21,T.Hong48,S.Hooper20,D.J.Hosken66,J.Hough3,E.J.Howell20,B.Hughey9,S.Husa72,S.H.Huttner3,T.Huynh-Dinh6,D.R.Ingram15,R.Inta52,
a T.Isogai51,A.Ivanov1,K.Izumi71,M.Jacobson1,H.Jang73,P.Jaranowski40d,W.W.Johnson12,D.I.Jones74,G.Jones54,R.Jones3,L.Ju20,P.Kalmus1,V.Kalogera63,
[ I.Kamaretsos54,S.Kandhasamy61,G.Kang73,J.B.Kanner41,30,E.Katsavounidis21,W.Katzman6,H.Kaufer7,8,K.Kawabe15,S.Kawamura71,F.Kawazoe7,8,W.Kells1,
D.G.Keppel1,Z.Keresztes67,A.Khalaidovski7,8,F.Y.Khalili28,E.A.Khazanov75,B.Kim73,C.Kim76,D.Kim20,H.Kim7,8,K.Kim77,N.Kim10,Y.-M.Kim50,P.J.King1,
2 M.Kinsey31,D.L.Kinzel6,J.S.Kissel21,S.Klimenko11,K.Kokeyama13,V.Kondrashov1,R.Kopparapu31,S.Koranda9,W.Z.Korth1,I.Kowalska40b,D.Kozak1,V.Kringel7,8,
v S.Krishnamurthy63,B.Krishnan16,A.Kro´lak40ae,G.Kuehn7,8,R.Kumar3,P.Kwee8,7,M.Laas-Bourez20,P.K.Lam52,M.Landry15,M.Lang31,B.Lantz10,N.Lastzka7,8,
8 C.Lawrie3,A.Lazzarini1,P.Leaci16,C.H.Lee50,H.M.Lee78,N.Leindecker10,J.R.Leong7,8,I.Leonor37,N.Leroy29a,N.Letendre4,J.Li45,T.G.F.Li25a,N.Liguori59ab,
9 P.E.Lindquist1,N.A.Lockerbie79,D.Lodhia13,M.Lorenzini36a,V.Loriette29b,M.Lormand6,G.Losurdo36a,J.Luan48,M.Lubinski15,H.Lu¨ck7,8,A.P.Lundgren31,
4 E.Macdonald3,B.Machenschalk7,8,M.MacInnis21,D.M.Macleod54,M.Mageswaran1,K.Mailand1,E.Majorana14a,I.Maksimovic29b,N.Man32a,I.Mandel21,V.Mandic61,
3 M.Mantovani24ac,A.Marandi10,F.Marchesoni35a,F.Marion4,S.Ma´rka23,Z.Ma´rka23,A.Markosyan10,E.Maros1,J.Marque18,F.Martelli36ab,I.W.Martin3,R.M.Martin11,
9. J.N.Marx1,K.Mason21,A.Masserot4,F.Matichard21,L.Matone23,R.A.Matzner64,N.Mavalvala21,G.Mazzolo7,8,R.McCarthy15,D.E.McClelland52,P.McDaniel21,
0 S.C.McGuire80,G.McIntyre1,J.McIver42,D.J.A.McKechan54,G.D.Meadors46,M.Mehmet7,8,T.Meier8,7,A.Melatos53,A.C.Melissinos81,G.Mendell15,D.Menendez31,
1 R.A.Mercer9,S.Meshkov1,C.Messenger54,M.S.Meyer6,H.Miao20,C.Michel33,L.Milano5ab,J.Miller52,Y.Minenkov55a,V.P.Mitrofanov28,G.Mitselmakher11,
1 R.Mittleman21,O.Miyakawa71,B.Moe9,P.Moesta16,M.Mohan18,S.D.Mohanty26,S.R.P.Mohapatra42,D.Moraru15,G.Moreno15,N.Morgado33,A.Morgia55ab,T.Mori71,
S.Mosca5ab,K.Mossavi7,8,B.Mours4,C.M.Mow–Lowry52,C.L.Mueller11,G.Mueller11,S.Mukherjee26,A.Mullavey52,H.Mu¨ller-Ebhardt7,8,J.Munch66,D.Murphy23,
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i J.O’Dell39,E.Oelker21,G.H.Ogin1,J.J.Oh69,S.H.Oh69,R.G.Oldenburg9,B.O’Reilly6,R.O’Shaughnessy9,C.Osthelder1,C.D.Ott48,D.J.Ottaway66,R.S.Ottens11,
X H.Overmier6,B.J.Owen31,A.Page13,G.Pagliaroli55ac,L.Palladino55ac,C.Palomba14a,Y.Pan41,C.Pankow11,F.Paoletti24a,18,M.A.Papa16,9,M.Parisi5ab,A.Pasqualetti18,
r R.Passaquieti24ab,D.Passuello24a,P.Patel1,M.Pedraza1,P.Peiris82,L.Pekowsky19,S.Penn83,C.Peralta16,A.Perreca19,G.Persichetti5ab,M.Phelps1,M.Pickenpack7,8,
a F.Piergiovanni36ab,M.Pietka40d,L.Pinard33,I.M.Pinto84,M.Pitkin3,H.J.Pletsch7,8,M.V.Plissi3,R.Poggiani24ab,J.Po¨ld7,8,F.Postiglione56,M.Prato49,V.Predoi54,
L.R.Price1,M.Prijatelj7,8,M.Principe84,S.Privitera1,R.Prix7,8,G.A.Prodi59ab,L.Prokhorov28,O.Puncken7,8,M.Punturo35a,P.Puppo14a,V.Quetschke26,F.J.Raab15,
D.S.Rabeling25ab,I.Ra´cz58,H.Radkins15,P.Raffai65,M.Rakhmanov26,C.R.Ramet6,B.Rankins43,P.Rapagnani14ab,S.Rapoport52,92,V.Raymond63,V.Re55ab,
K.Redwine23,C.M.Reed15,T.Reed85,T.Regimbau32a,S.Reid3,D.H.Reitze11,F.Ricci14ab,R.Riesen6,K.Riles46,N.A.Robertson1,3,F.Robinet29a,C.Robinson54,
E.L.Robinson16,A.Rocchi55a,S.Roddy6,C.Rodriguez63,M.Rodruck15,L.Rolland4,J.Rollins23,J.D.Romano26,R.Romano5ac,J.H.Romie6,D.Rosin´ska40cf,C.Ro¨ver7,8,
S.Rowan3,A.Ru¨diger7,8,P.Ruggi18,K.Ryan15,H.Ryll7,8,P.Sainathan11,M.Sakosky15,F.Salemi7,8,A.Samblowski7,8,L.Sammut53,L.SanchodelaJordana72,
V.Sandberg15,S.Sankar21,V.Sannibale1,L.Santamar´ıa1,I.Santiago-Prieto3,G.Santostasi86,B.Sassolas33,B.S.Sathyaprakash54,S.Sato71,P.R.Saulson19,R.L.Savage15,
R.Schilling7,8,S.Schlamminger87,R.Schnabel7,8,R.M.S.Schofield37,B.Schulz7,8,B.F.Schutz16,54,P.Schwinberg15,J.Scott3,S.M.Scott52,A.C.Searle1,F.Seifert1,
D.Sellers6,A.S.Sengupta1,D.Sentenac18,A.Sergeev75,D.A.Shaddock52,M.Shaltev7,8,B.Shapiro21,P.Shawhan41,D.H.Shoemaker21,A.Sibley6,X.Siemens9,D.Sigg15,
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D.Talukder34,D.B.Tanner11,S.P.Tarabrin7,8,J.R.Taylor7,8,R.Taylor1,P.Thomas15,K.A.Thorne6,K.S.Thorne48,E.Thrane61,A.Thu¨ring8,7,C.Titsler31,
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andW.Zheng46
(Affiliationscanbefoundafterthereferences)
ABSTRACT
Aims. A transientastrophysicaleventobservedin both gravitationalwave (GW) and electromagnetic(EM) channelswouldyield
richscientificrewards.AfirstprograminitiatingEMfollow-upstopossibletransientGWeventshasbeendevelopedandexercised
bytheLIGOandVirgocommunityinassociationwithseveralpartners.Inthispaper,wedescribeandevaluatethemethodsusedto
promptlyidentifyandlocalizeGWeventcandidatesandtorequestimagesoftargetedskylocations.
Methods.Duringtwoobservingperiods(Dec172009toJan82010andSep2toOct202010),alow-latencyanalysispipelinewas
usedtoidentifyGWeventcandidatesandtoreconstructmapsofpossibleskylocations.AcatalogofnearbygalaxiesandMilkyWay
globularclusterswasusedtoselectthemostpromisingskypositionstobeimaged,andthisdirectionalinformationwasdeliveredto
EMobservatorieswithtimelagsofaboutthirtyminutes.AMonteCarlosimulationhasbeenusedtoevaluatethelow-latencyGW
pipeline’sabilitytoreconstructsourcepositionscorrectly.
Results. For signals near the detection threshold, our low-latency algorithms often localized simulated GW burst signals to tens
of square degrees,while neutronstar/neutronstar inspiralsand neutronstar/blackhole inspirals were localized to a few hundred
square degrees. Localization precision improves for moderately stronger signals. The correct sky location of signals well above
thresholdandoriginatingfromnearbygalaxiesmaybeobservedwith∼50%orbetterprobabilitywithafewpointingsofwide-field
telescopes.
Keywords.gravitationalwaves-methods:observational
LSC+Virgo+others:FirstpromptsearchforGWtransientswithEMcounterparts 3
1. Introduction consisting of two neutron stars (NS-NS) or a neutron star and
a stellar-mass black hole (NS-BH), the inspiral stage produces
The Laser Interferometer Gravitational-Wave Observatory
the most readily detectable GW signal. The energyflux reach-
(LIGO) (Abbottetal. 2009a) and Virgo (Accadiaetal. 2011)
ing Earth depends on the inclination angle of the binary orbit
have taken significant steps toward gravitational wave (GW)
relative to the line of sight. The initial LIGO-Virgonetwork is
astronomy over the past decade. The LIGO Scientific
sensitivetooptimallyorientedNS-NSmergersfromasfaraway
Collaboration operates two LIGO observatories in the U.S.
as30Mpc,andmergersbetweenaNSanda10M⊙ blackhole
alongwiththeGEO600detector(Lu¨cketal.2010)inGermany.
outto70Mpc(Abadieetal.2010c).Modelsofthestellarcom-
Together with Virgo,located in Italy, they form a detector net-
pactobjectpopulationin thelocaluniverseestimatethe rateof
work capable of detecting GW signals arriving from all direc-
NS-NS mergers detectable with initial detectors to be between
tions.TheirmostrecentjointdatatakingrunwasbetweenJuly 2×10−4and0.2peryear.Withadvanceddetectors,theserange
2009andOctober2010.GEO600andVirgoarecurrentlyoper-
limitsareexpectedtoincreaseto440and930Mpc,respectively.
atingduringsummer2011,whiletheLIGOinterferometershave
TheenergeticsofthesesystemssuggestthatanEMcounter-
been decommissioned for the upgrade to the next-generation partislikely.Thefinalplungeradiatesoforder1053ergsofgrav-
Advanced LIGO detectors (Harryetal. 2010), expected to be
itationalbindingenergyas gravitationalwaves. If evena small
operational around 2015. Virgo will also be upgraded to be-
fractionofthisenergyescapesasphotonsintheobservingband,
comeAdvancedVirgo(Acerneseetal. 2008).Additionally,the
theresultingcounterpartcouldbeobservabletolargedistances.
newLCGTdetector(Kuroda&TheLCGTCollaboration2010)
TheEMtransientsthatarelikelytofollowaNS-BHorNS-NS
isplannedinJapan.These“advancedera”detectorsareexpected
mergeraredescribedbelow.
to detect compact binary coalescences, possibly at a rate of
Short-hardgamma-raybursts(SGRBs),whichtypicallyhave
dozensper year,after reachingdesign sensitivity (Abadieetal.
durations of 2 seconds or less, may be powered by NS-NS
2010c).
or NS-BH mergers (Nakar 2007; Me´sza´ros 2006; Piran 2004).
Detectable, transient GW signals in the LIGO/Virgo fre-
Afterglows of SGRBs have been observed in wavelengths
quencybandrequirebulkmotionof mass onshorttime scales.
from radio to X-ray, and out to Gpc distances (Nakar 2007;
Emissioninotherchannelsisalsopossibleinmanysuchrapidly
Gehrelsetal. 2009). Optical afterglows have been observed
changingmassivesystems.Thisleadstotheprospectthatsome
fromafewtensofsecondstoafewdaysaftertheGRBtrigger
transientGWsourcesmayhavecorrespondingelectromagnetic
(see, for example, Klotzetal. (2009a)), and fade with a power
(EM)counterpartswhichcouldbediscoveredwithalowlatency lawt−α,whereαisbetween1and1.5.At1dayafterthetrigger
response to GW triggers (Sylvestre 2003; Kanneretal. 2008;
time, theapparentopticalmagnitudewouldbebetween12 and
Stubbs2008;Kulkarni&Kasliwal2009;Bloometal.2009).
20forasourceat50Mpc(Kannetal.2011),comparabletothe
Finding these EM counterparts would yield rich scientific
distancetowhichLIGOandVirgocoulddetectthemerger.
rewards(see Sect. 2), but is technically challengingdue to im-
Even if a compact binary coalescence is not observable in
perfect localization of the gravitationalwave signal and uncer-
gamma-rays,thereisreasontoexpectitwillproduceanobserv-
tainty regarding the relative timing of the GW and EM emis-
ableopticalcounterpart.Li&Paczyn´ski(1998) suggestedthat,
sions.Thispaperdetailsourrecentefforttoconstructaprompt
during a NS-NS or NS-BH merger, some of the neutron star’s
search for joint GW/EM sources between the LIGO/Virgo de-
mass is tidally ejected. In their model, the ejected neutron-rich
tectornetworkandpartnerEM observatories(see Sect. 3).The
matter produces heavy elements through r-process nucleosyn-
searchwasdemonstratedduringtwoperiodsofliveLIGO/Virgo
thesis, whichsubsequentlydecayandheattheejecta,powering
running:the “winter” observing period in December 2009 and
anopticalafterglowknownasakilonova.Thepredictedoptical
January2010andthe“autumn”observingperiodinSeptember
emissionisroughlyisotropic,andsoisobservableregardlessof
andOctober2010.Wefocushereonthedesignandperformance
the orientation of the original binary system. This emission is
of software developed for rapid EM follow-ups of GW candi-
expectedto peakafteraboutoneday,aroundmagnitude18 for
dateevents,aswellastheproceduresusedtoidentifysignificant
asourceat50Mpc(Metzgeretal.2010),andthenfadeoverthe
GW triggersand to communicatethe mostlikely sky locations
courseofafewdaysfollowingthemerger.
to partner EM observatories. The analysis of the observational
dataisinprogress,andwillbethesubjectoffuturepublications.
2.1.2. StellarCoreCollapse
Beyondthecompactobjectmergersdescribedabove,someother
2. Motivation
astrophysicalprocessesareplausiblesourcesofobservableGW
2.1.Sources emission.GWtransientswithunknownwaveformsmaybedis-
coveredbysearchingtheLIGOandVirgodataforshortperiods
AvarietyofEMemissionmechanisms,bothobservedandthe- of excess power (bursts). The EM counterparts to some likely
oretical,mayoccurinassociationwithobservableGWsources. sourcesofGWburstsignalsaredescribedbelow.
Characteristicsofafewscenarioshelpedinformthedesignand Core-collapsesupernovaearelikelytoproducesomeamount
executionofthissearch.Here,somelikelymodelsarepresented, of gravitational radiation, though large uncertainties still exist
alongwithcharacteristicsoftheassociatedEMemission. intheexpectedwaveformsandenergetics.Mostmodelspredict
GWspectrathatwouldbeobservablebyinitialLIGOandVirgo
from distanceswithin some fractionof the Milky Way, but not
2.1.1. CompactBinaryCoalescence
from the Mpc distances needed to observe GWs from another
Compactbinarysystemsconsistingofneutronstarsand/orblack galaxy(Ott2009).NeutrinodetectorssuchasSuperKamiokande
holesarethoughttobethemostcommonsourcesofGWemis- andIceCubeshouldalsodetectalargenumberofneutrinosfrom
siondetectablewithground-basedinterferometers.Radiationof aGalacticsupernova(Beacom&Vogel1999;Halzen&Raffelt
energy and angular momentum causes the orbit to decay (in- 2009;Leonoretal.2010).Galacticsupernovaenormallywould
spiral)untiltheobjectsmerge(Cutleretal.1993).Forasystem beverybrightintheopticalband,butcouldbelessthanobvious
LSC+Virgo+others:FirstpromptsearchforGWtransientswithEMcounterparts 4
ifobscuredbydustorbehindtheGalacticcenter.Opticalemis- (Cutleretal.1993;Vallisneri2000;Flanagan&Hinderer2008;
sion wouldfirst appearhoursafter the GW and neutrinosignal Anderssonetal. 2011; Pannaraleetal. 2011; Hindereretal.
and would peak days to weeks later, fading over the course of 2010).
weeksormonths. Observing EM counterparts of NS-NS and NS-BH merger
Long-softgamma-raybursts(LGRBs)arebelievedtobeas- eventswill give strong evidenceas to which class of source, if
sociatedwiththecorecollapseofmassivestars(Woosley1993; either,isthesourceofSGRBs(Bloometal.2009).Inaddition,
MacFadyen&Woosley 1999; Piran 2004; Woosley&Bloom ifsomeneutronstarmergersarethesourcesofSGRBs,acollec-
2006; Metzgeretal. 2011). A large variety of possible GW tion of joint EM/GW observations would allow an estimate of
emitting mechanisms within these systems have been pro- theSGRBjetopeninganglebycomparingthenumberofmerger
posed, with some models predicting GW spectra that would events with and without observable prompt EM emission, and
be observable from distances of a few Mpc with initial LIGO some informationwouldbeobtainableevenfroma single loud
and Virgo (Fryeretal. 2002; Kobayashi&Me´sza´ros 2003a; event(Kobayashi&Me´sza´ros2003b;Seto2007).
Corsi&Me´sza´ros 2009; Piro&Pfahl 2007; Korobkinetal. An ensemble of these observations could provide a novel
2011; Kiuchietal. 2011). The afterglows of LGRBs, like the measurementofcosmologicalparameters.Analysisofthewell-
afterglows of SGRBs, typically show power law fading with modeled GW signal will provide a measurement of the lumi-
α = 1−1.5. However, the peak isotropic equivalent luminos- nositydistancetothesource,whiletheredshiftdistanceismea-
ityofLGRBafterglowsistypicallyafactorof10brighterthan surable from the EM data. Taken together, they provide a di-
SGRBafterglows(Nakar2007;Kannetal.2010). rect measurement of the local Hubble constant (Schutz 1986;
Anoff-axisorsub-energeticLGRBmayalsobeobservedas Markovic1993;Dalaletal.2006;Nissankeetal.2010).
anorphanafterglowordirtyfireball(Granotetal.2002;Rhoads Finally,alloftheaboveassumethatgeneralrelativityisthe
2003).Thesetransientsbrightenoverthecourseofseveraldays correcttheoryof gravity on macroscopicscales. Joint EM/GW
orevenweeks,dependingontheobservingbandandviewingan- observationscanalsobeusedtotestcertainpredictionsofgen-
gle.Identifyingorphanafterglowsinlargeareasurveys,suchas eralrelativity,suchasthepropagationspeedandpolarizationsof
Rykoffetal.(2005),hasprovendifficult,butaGWtriggermay GWs(Will2005;Yunesetal.2010;Kahya2011).
helpdistinguishorphanafterglowsfromotherEMvariability. In the case that the transient GW source is not a binary
mergerevent,thecombinationofGWandEMinformationwill
again prove very valuable. In this scenario, the gravitational
2.1.3. OtherPossibleSources waveform will not be known a priori. Any distance estimate
would be derived from the EM data, which would then set the
Cosmic string cusps are another possible joint source of
overallscalefortheenergyreleasedasGWs.
GW (Siemensetal. 2006; Damour&Vilenkin 2000) and EM
As in the merger case, the linking of a GW signal with a
(Vachaspati 2008) radiation. If present, their distinct GW sig-
known EM phenomenonwill provide insight into the underly-
nature will distinguish them from other sources. On the other
ingphysicalprocess.Forexample,thedetailsofthecentralen-
hand,evenunmodeledGWemissionscanbedetectedusingGW
ginethatdrivesLGRBsareunknown.TheGWsignalcouldgive
burstsearchalgorithms,andsucheventsmayinsomecasespro-
crucial clues to the motion of matter in the source, and poten-
duceEMradiationeither throughinternaldynamicsorthrough
tially distinguish between competingmodels. A similar insight
interactionwiththesurroundingmedium.Thus,ourjointsearch
into the source mechanism could be achieved for an observa-
methodsshouldallowforawiderangeofpossiblesources.
tionofGWemissionassociatedwithasupernova.Rapididenti-
fication may also allow observation of a supernova in its ear-
liest moments, an opportunity that currently depends on luck
2.2.InvestigationsenabledbyjointGW/EMobservations
(Soderbergetal.2008).
A variety of astrophysicalinformationcould potentially be ex-
tracted from a joint GW/EM signal. In understanding the pro-
2.3.ExtendGWDetectorReach
genitor physics, the EM and GW signals are essentially com-
plementary.TheGWtimeseriesdirectlytracesthebulkmotion FindinganEMcounterpartassociatedwithaLIGO/Virgotrigger
ofmassinthesource,whereasEMemissionsarisingfromout- would increase confidence that a truly astrophysical event had
flowsortheirinteractionwiththeinterstellarmediumgiveonly beenobservedintheGWdata.UsingEMtransientstohelpdis-
indirect information requiring inference and modeling. On the tinguish low amplitude GW signals fromnoise eventsallows a
otherhand,observinganEMcounterparttoaGWsignalreduces loweringofthedetectionthreshold,aswasdoneinsearchessuch
the uncertainty in the source position from degrees to arcsec- asAbbottetal.(2010).Kochanek&Piran(1993)estimatedthat
onds.Thisprecisedirectionalinformationcanleadtoidentifica- thedetectableamplitudecouldbereducedbyasmuchasafac-
tionofahostgalaxy,andameasurementofredshift.Somespe- torof1.5,increasingtheeffectivedetectorhorizondistance(the
cific questionsthatmay be addressedwith a collection of joint maximum distance at which an optimally oriented and located
GW/EMsignalsarediscussedbelow. system couldbe detected)by the same factorand thusincreas-
IftheGWsourceisidentifiedasaNS-NSorNS-BHmerger, ingthedetectionratebyafactorof3.Inpractice,theactualim-
additional investigations are enabled with an EM counterpart. provementin GW sensitivity achievedby pairingEM and GW
The observation of the EM signal will improve the estimation observationsdependsonmanyfactorsuniquetoeachsearch,in-
ofastrophysicalsourceparameters.Forexample,whenattempt- cludingdetailsofthesourcemodelanddataset,andsoisdiffi-
ing parameter estimation with a bank of templates and a sin- culttopredictinadvance.
gle data stream, the source’s distance, inclination angle, and InthecaseofacoincidencebetweenaGWsignalandadis-
angular position are largely degenerate. A precise source po- covered EM transient, the joint significance may be calculated
sition from an EM counterpartwould help break this degener- byassumingthatthebackgroundsoftheEMandGWsearchare
acy(Dalaletal.2006;Nissankeetal.2010).Highprecisionpa- independent. The False Alarm Rate (FAR) of a GW/EM coin-
rameterestimationmayevenconstraintheNSequationofstate cidenceistheFARoftheGWsignal,asdescribedinSect.4.2,
LSC+Virgo+others:FirstpromptsearchforGWtransientswithEMcounterparts 5
timesα,theexpectedfractionofobservationsassociatedwitha The problem may be partially mitigated by making use of
falseorunrelatedEMtransient.Thefalsealarmfractionαmay theknownmassdistributioninthenearbyuniverse.Asearchfor
beestimatedusingfieldsfromsurveysnotassociatedwithGW GW counterpartscan dramatically reduce the needed sky cov-
triggers. The measured value of α will depend heavily on the erage by focusing observationson galaxieswithin the distance
telescopebeingused,thecutsselectedinimageanalysis,galac- limitsoftheGWdetectors(Kanneretal.2008;Nuttall&Sutton
ticlatitudeofthesourceandotherfactors.Forexample,surveys 2010). Limiting the search area to known galaxies may also
withthePalomarTransientFactoryrequireasophisticatedclas- improve the feasibility of identifying the true counterpart
sificationmechanismforrejectingcontaminants.Eachsetofim- from among other objects with time-varying EM emissions
agesubtractionscovering100-200squaredegreesyields∼105 (Kulkarni&Kasliwal 2009). Even within the Milky Way, a
candidates.Ofthese,30-150sourcesareselectedafterimpos- search may emphasize known targets by seeking counterparts
ing cuts optimized for the detection of fast evolving transients within globular clusters, where binary systems of compact ob-
(Bloometal. 2011). Using classification software designed for jectsmayformefficiently(O’Learyetal.2007).
PTFdata(Oarical)(Bloometal.2011),theselectedsourcesun- An emphasis on extragalactic and globular-cluster sources
dergoanautomaticclassificationastype“transient”or“variable hasthepotentialdrawbackthatanycounterpartsintheplaneof
star” based on time-domain and context properties. Promising the MilkyWay may be missed. Also, neutronstar mergersthat
candidates are selected for additional, spectroscopic observa- occuratlargedistancesfromtheirhostgalaxiesmaynotbeob-
tion.Ofthesourcesthatareclassifiedastransients,andthenfol- served,thoughthe populationwith large kicksshouldbe small
lowedupspectroscopically,∼82%aresupernovae(Bloometal. (Berger2010;Kelleyetal.2010).
2011). To use EM transience to improve confidence in a GW Ourselectionoffieldstoobservewasweightedtowardsar-
signal, the time-domainsky in the wavelength of interest must eascontainingknowngalaxieswithin50Mpc.Theutilizedcata-
bewellunderstood.Transientsthatarefoundindirectionaland logofnearbygalaxiesandglobularclusters,andtheprocessfor
time coincidence with GW triggers would increase confidence selectingfieldstoobserve,isdescribedinSect.5.
intheGWsignalonlyifthechanceofasimilar,incidentalcoin-
cidenceisunderstoodtobelow(Kulkarni&Kasliwal2009).
3. GWandEMInstruments
3.1.GravitationalWaveDetectorNetwork
2.4.ImplicationsforSearchDesign
The LIGO and Virgo detectors are based on Michelson-type
Characteristicsofthetargetsourceshelpeddeterminewhenand interferometers, with Fabry-Perot cavities in each arm and a
wheretoseektheEMcounterpartstoGWeventcandidates.For power recycling mirror between the laser and beamsplitter to
reasons discussed in this section, the search strategy presented dramaticallyincreasethepowerinthearmsrelativetoasimple
in this paper emphasizes capturing images as soon as possible Michelsondesign.TheGEO600detectorusesafoldedinterfer-
after the GW trigger, along with follow-upimages over subse- ometerwithoutFabry-Perotarmcavitiesbutwith anadditional
quentnights.Theratesofstellarcore-collapseandcompactob- recyclingmirrorattheoutputtoresonantlyenhancetheGWsig-
jectmergerswithin ourowngalaxyaremuchless thanoneper nal.Asagravitationalwavepassesthrougheachinterferometer,
year, and so field selection was strongly weighted towards re- itinducesa“strain”(aminusculechangeinlengthontheorder
gionscontainingnearbygalaxies. of1partin 1021 orless) oneacharmoftheinterferometerdue
The observations and theoretical models of EM transients tothequadrupolarperturbationofthespacetimemetric.Thein-
discussed aboveprovidedguidance when choosingthe observ- terferometersare designed to measure the differential strain on
ing cadence. GRB optical afterglows have been observed dur- the two arms through interference of the laser light when the
ing the prompt emission phase (Klotzetal. 2009b) and up to two beams are recombined at the beam splitter, with the rela-
manyhoursafterthetrigger.Forthissearch,thefirstattemptto tive optical phase modulated by the passing gravitationalwave
image the source position was made as soon as possible after (Abbottetal.2009a).
validating a GW trigger. In both the kilonova (Li&Paczyn´ski In 2009–2010 there were two operating LIGO interfer-
1998) and supernova (Ott 2009) models, some time lag exists ometers, each with 4-km arms: H1, located near Hanford,
betweenthe release ofGW andEM emission,primarilydueto Washington, and L1, located in Livingston Parish, Louisiana.1
thetimeittakestheoutflowingmaterialtobecomeopticallythin. Virgo(V1)hasarmsoflength3kmandislocatednearCascina,
This time lag may be from several hours for a kilonova, up to Italy.GEO600datawasnotusedintheonlinesearchdescribed
days for a core-collapsesupernova.Furthermore,Cowardetal. inthispaper,butwasavailableforofflinereanalysisofpromis-
(2011)showthatforGRBsthatareoff-axis,theopticalafterglow ingeventcandidates.Thelargephysicalseparationbetweenthe
maynotbe visible untildaysafter theburst. Forthese reasons, instrumentsmeansthattheeffectsoflocalenvironmentalback-
repeatedobservationsoverseveralnightsaredesirable.Also,the groundcanbemitigatedbyrequiringacoincidentsignalinmul-
lightcurvesobtainedbyobservingthesamefieldsovermultiple tiple interferometers. Each interferometer is most sensitive to
nightsarecriticalcluesfordiscoveringandclassifyingtransient GW signals traveling parallel or anti-parallelto zenith, but the
sources. antennapatternvariesgraduallyoverthesky,sothatthe detec-
torsareessentiallyall-skymonitors.
Knowing where to look for the counterpart to a GW trig-
TheEMfollow-upprogramdescribedinthispaperwasexer-
ger is challenging.Directionalestimates of low signal to noise
cisedduringthe2009–2010scienceruns.Whilesingle-detector
ratio(SNR)binaryinspiralsourceswiththe2009–10GWdetec-
triggershad been generatedwith low latency in earlier science
tornetworkhaveuncertaintiesofseveraltensofsquaredegrees
(Fairhurst2009). Thissuggestsusing telescopeswith a field of
1 Earlierscience runsincluded asecond interferometer at Hanford,
view (FOV) of at least a few square degrees if possible. Even
calledH2,with2-kmarms.H2willreappearaspartofAdvancedLIGO,
withsucha“widefield”instrument,thereisastrikingmismatch either as a second 4-km interferometer at Hanford or else at a sitein
betweenthelargeareaneedingtobesearched,andthesizeofa WesternAustralia.Thelatteroptionwouldgreatlyimprovethesource
singleFOV. localizationcapabilitiesofthenetwork(Fairhurst2011;Schutz2011).
LSC+Virgo+others:FirstpromptsearchforGWtransientswithEMcounterparts 6
runs for diagnostic and prototyping purposes, 2009–2010 was
the first time that a systematic search for GW transients using
the full LIGO-Virgo network was performedwith low latency,
andthefirsttimethatalertsweresenttoexternalobservatories.
3.2.OpticalandOtherElectromagneticObservatories
Inanefforttoexplorevariousapproaches,thetelescopenetwork
used in 2009–10 was intentionally heterogeneous. However,
most of instruments had large fields of view to accommodate
the imprecise GW position estimate. The approximatelocation
ofeachEMobservatoryisshownin Fig.1, andTables1and2
Fig.1.A mapshowingthe approximatepositionsoftelescopes
showsomeofthepropertiesofeachobservatory.
thatparticipatedintheproject.TheSwiftsatelliteobservatoryis
notedatanarbitrarylocation.Theimageisadaptedfromablank
3.2.1. OpticalInstruments worldmapplacedinthepublicdomainbyP.Dlouhy´.
The Palomar Transient Factory (PTF) (Lawetal. 2009;
Rauetal. 2009) operates a 7.3 square degree FOV camera
ZadkoTelescope(Cowardetal.2010)isa1mtelescopelo-
mountedonthe1.2mOschinTelescopeatPalomarObservatory.
cated in Western Australia. The current CCD imager observes
A typical 60 s exposure detects objects with a limiting magni-
fields of 0.15 square degrees down to magnitude ∼ 20 in the
tudeR = 20.5.Fortheautumnobservingperiod,thePTFteam
R band for a typical 180 s exposure.For each accepted trigger
devotedtenfieldsoverseveralnightsatatargetrateof1trigger
in theautumnrun,Zadkorepeatedlyobservedthefive galaxies
foreverythreeweeks.
consideredmostlikelytohostthesourceoverseveralnights.The
Pi of the Sky (Maleketal. 2009) observed using a camera
targettriggerrateforZadkowasonetriggerperweek.
with a 400 square degree FOV and exposuresto limiting mag-
The Liverpool telescope (Steeleetal. 2004) is a 2 m
nitude 11–12. It was located in Koczargi Stare, near Warsaw.
robotictelescopesituatedattheObservatoriodelRoquedeLos
The camera was a prototype for a planned system that will si-
Muchachoson La Palma. For this project the RATCam instru-
multaneously image two steradians of sky. The target rate was
ment,with a 21squarearcminutefield ofview, wasused. This
approximately1perweek inthe autumnrun,followedupwith
instrumentation allows a five minute exposure to reach magni-
hundredsof10sexposuresoverseveralnights. tude r′ = 21. This project was awarded 8 hours of target-of-
TheQUEST camera(Baltayetal.2007),currentlymounted opportunitytime,whichwassplitinto8observationsof1hour
on the 1 m ESO Schmidt Telescope at La Silla Observatory, each,withatargetrateof1triggerperweek.
views9.4squaredegreesofskyineachexposure.Thetelescope
is capable of viewing to a limiting magnitude of R ∼ 20. The
QUESTteamdevotedtwelve60sexposuresoverseveralnights 3.2.2. RadioandX-rayInstruments
for each trigger in both the winter and autumn periods, with a
LOFAR (Fenderetal. 2006; deVosetal. 2009; Stappersetal.
targetrateof1triggerperweek.
2011)isadipolearrayradiotelescopebasedintheNetherlands
ROTSEIII(Akerlofetal.2003)isacollectionoffourrobotic but with stations across Europe. The array is sensitive to fre-
telescopesspreadaroundtheworld,eachwitha0.45maperture quenciesintherangeof30to80MHzand110to240MHz,and
and 3.4 square degree FOV. No filters are used, so the spectral can observemultiple simultaneousbeams, each with a FWHM
responseisthatoftheCCDs, spanningroughly400to900nm. varyingwithfrequencyuptoamaximumofaround23o.During
TheequivalentRbandlimitingmagnitudeisabout17ina20s the autumn run, LOFAR accepted triggers at a target rate of 1
exposure.TheROTSEteamarrangedforaseriesofthirtyimages perweekandfollowedupeachwithafour-hourobservationin
for the first night, and several images on following nights, for its higher frequencyband, providinga ∼25 square degree field
eachautumnruntrigger,withatargetrateof1triggerperweek. ofview.
SkyMapper(Kelleretal.2007)isasurveytelescopelocated Although not used in the prompt search during the science
at Siding Spring observatory in Australia. The mosaic camera run, the Expanded Very Large Array (Perleyetal. 2011) was
covers 5.7 square degrees of sky in each field, and is mounted used to follow up a few triggers after the run with latencies of
on a 1.35 m telescope with a collecting area equivalent to an 3and5weeks.
unobscured 1.01 m aperture. It is designed to reach a limiting TheSwift satellite (Gehrelsetal. 2004) carriesthree instru-
magnitudeg ∼ 21(>7sigma)ina 110sexposure.SkyMapper ments, each in different bands. Swift granted several target of
accepted triggers in the autumn run with a target rate of 1 per opportunityobservationswithtwoofthese,theX-rayTelescope
week,withseveralfieldscollectedforeachtrigger. (XRT) and UV/Optical Telescope (UVOT), for the winter and
autumn observing periods. The XRT is an imaging instrument
TAROT(Klotzetal.2009a)operatestworobotic25cmtele-
witha0.15squaredegreeFOV,sensitivetofluxesaround10−13
scopes,oneatLaSillainChileandoneinCalern,France.Like
ergs/cm2/sinthe0.5-10keVband.Afewfieldswereimagedfor
theROTSEIIIsystem,eachTAROTinstrumenthasa3.4square
eachtriggerthatSwiftaccepted.
degreeFOV. A 180 second image with TAROT in ideal condi-
tionshasalimitingRmagnitudeof17.5.Duringthewinterrun,
TAROT observed a single field during one night for each trig-
4. TriggerSelection
ger. In the autumn run, the field selected for each trigger was
observed over several nights. TAROT accepted triggers with a TheonlineanalysisprocesswhichproducedGWcandidatetrig-
targetrateof1perweek. gerstobesenttotelescopesisoutlinedinFig.2.Afterdataand
LSC+Virgo+others:FirstpromptsearchforGWtransientswithEMcounterparts 7
cWBandρ forMBTA arerelatedtotheamplitudeSNR
combined
ofthesignalacrossallinterferometerswhileΩisrelatedtothe
BayesianlikelihoodofaGWsignalbeingpresent.Triggerswith
adetectionstatisticaboveanominalthreshold,andoccurringin
times where all three detectors were operating normally, were
recorded in the Gravitational-wave Candidate Event Database
(GraCEDb).
The trigger generators also produced likelihood maps over
thesky(skymaps),indicatingthelocationfromwhichthesignal
wasmostlikelytohaveoriginated.Abriefintroductiontoeach
triggergeneratorispresentedinSects.4.1.1–4.1.3.
Fig.2.Asimplifiedflowchartoftheonlineanalysiswithapprox-
imate time requirements for each stage. Data and information
on dataquality were generatedatthe Hanford,Livingston,and 4.1.1. CoherentWaveBurst
Virgo interferometers (H1, L1, and V1) and copied to central-
CoherentWaveBursthasbeenusedinprevioussearchesforGW
izedcomputercenters.Theonlineeventtriggergeneratorspro-
bursts, such as Abbottetal. (2009b) and Abadieetal. (2010b).
ducedcoincidenttriggerswhichwerewrittenintotheGraCEDb
Thealgorithmperformsatime-frequencyanalysisofdatainthe
archive.TheLUMINandGEMalgorithmsselectedstatistically
waveletdomain.Itcoherentlycombinesdatafromalldetectors
significant triggers from the archive and chose pointing loca-
to reconstruct the two GW polarization waveforms h (t) and
tions. Significant triggers generated alerts, and were validated +
h (t) and the source coordinates on the sky. A statistic is con-
×
manually. If no obvious problem was found, the trigger’s esti-
structedfromthecoherenttermsofthemaximumlikelihoodra-
mated coordinateswere sentto telescopesfor potentialfollow-
tiofunctional(Flanagan&Hughes1998;Klimenkoetal.2005)
up.
for each possible sky location, and is used to rank each lo-
cation in a grid that covers the sky (skymap). A detailed de-
scriptionofthelikelihoodanalysis,theskylocalizationstatistic
and the performance of the cWB algorithm is published else-
informationon data quality were copied from the interferome-
where(Klimenkoetal.2011).
tersitestocomputingcenters,threedifferentdataanalysisalgo-
The search was run in two configurations which differ in
rithms identified triggers and determined probability skymaps.
their assumptions about the GW signal. The “unconstrained”
Theprocessofdownselectingthislargecollectionoftriggersto
searchplacesminimalassumptionsontheGWwaveform,while
thefeweventcandidatesthatreceivedEMfollow-upisdescribed
the“linear”searchassumesthesignalisdominatedbyasingle
inthissection.
GWpolarizationstate(Klimenkoetal.2011).Whiletheuncon-
Aftereventcandidateswereplacedinacentralarchive,addi-
strainedsearchismoregeneral,andistheconfigurationthatwas
tionalsoftwareusedthelocationsofnearbygalaxiesandMilky
usedinpreviousburstanalyses,thelinearsearchhasbeenshown
Wayglobularclusterstoselectlikelysourcepositions(Sect.5).
to better estimate source positions for some classes of signals.
Triggers were manually vetted, then the selected targets were
Fortheonlineanalysis,thetwosearcheswereruninparallel.
passed to partnerobservatorieswhich imaged the sky in an at-
tempttofindanassociatedEMtransient.Studiesdemonstrating
theperformanceofthispipelineonsimulatedGWsarepresented 4.1.2. OmegaPipeline
inSect.7.
In the Omega Pipeline search (Abadieetal. 2010b), triggers
are first identified by performing a matched filter search with
4.1.TriggerGeneration
a bank of basis waveforms which are approximately (co)sine-
Gaussians.ThesearchassumesthataGWsignalcanbedecom-
SendingGWtriggerstoobservatorieswithlessthananhourla-
tencyrepresentsamajorshiftfrompastLIGO/Virgodataanal- posed into a small numberof these basis waveforms,and so is
most sensitive to signals with a small time-frequency volume.
yses, which were reported outside the collaboration at soonest
Coincidence criteria are then applied, requiring a trigger with
several monthsafter the data collection. Reconstructing source
consistent frequency in another interferometer within a physi-
positions requires combiningthe data streams from the LIGO-
callyconsistenttimewindow.AcoherentBayesianpositionre-
Virgo network using either fully coherentanalysis or a coinci-
construction code (Searleetal. 2008, 2009) is then applied to
denceanalysisofsingle-detectortriggertimes.Akeystepinla-
remainingcandidates.ThecodeperformsBayesianmarginaliza-
tencyreductionwastherapiddatareplicationprocess,inwhich
tionoverallparameters(timeofarrival,amplitudeandpolariza-
datafromallthreeGWobservatorysiteswerecopiedtoseveral
tion)otherthandirection.Thisresultsinaskymapprovidingthe
computingcenterswithinaminuteofcollection.
probabilitythatasignalarrivedatanytime,withanyamplitude
For the EM follow-up program,three independentGW de-
andpolarization,asafunctionofdirection.Furthermarginaliza-
tection algorithms (trigger generators), ran promptly as data
tionisperformedoverthisentireprobabilityskymaptoarriveat
became available, generating candidate triggers with latencies
a single number,the estimated probabilitythat a signal arrived
between five and eight minutes. Omega Pipeline and coherent
fromanydirection.TheΩstatisticisconstructedfromthisnum-
WaveBurst (cWB), which are both described in Abadieetal.
berandothertriggerproperties.
(2010b), searched for transients (bursts) with only loose as-
sumptions regarding waveform morphology. The Multi-Band
Template Analysis (MBTA) (Marion 2004), searched for sig-
4.1.3. MBTA
nalsfromcoalescingcompactbinaries.Triggerswererankedby
their “detection statistic”, a figure of merit for each analysis, The Multi-Band Template Analysis (MBTA) is a low-latency
known as Ω, η, and ρ , respectively. The statistics η for implementationofthematchedfiltersearchthatistypicallyused
combined
LSC+Virgo+others:FirstpromptsearchforGWtransientswithEMcounterparts 8
to search for compact binary inspirals (Marion 2004; Buskulic 4.3.OnlineDataQuality
2010). In contrast to burst searches which do not assume any
Anumberofcommonoccurrencesmaymakeastretchofinter-
particularwaveformmorphology,MBTAspecificallytargetsthe
ferometerdataunsuitableforsensitiveGWsearches.Examples
waveforms expected from NS-NS, NS-BH and BH-BH inspi-
include times of large seismic disturbance, non-standard inter-
rals.Inthiswayitprovidescomplementarycoveragetotheburst
ferometer configurations, and temporary saturations of various
searchesdescribedabove.
photodiodes in the interferometer sensing and control system.
The search uses templates computed from a second order Tomarksuchtimes,monitorprogramsanalyzeauxiliarydatato
post-Newtonian approximation for the phase evolution of the producelistsofabnormaltimesegmentswithlowlatency.When
signal,withcomponentmassesintherange1–34M⊙andatotal a trigger was identified, it was automatically checked against
mass of < 35M⊙. However,triggersgeneratedfromtemplates theselists;triggerswhichoccurredinstretchesofunacceptable
withbothcomponentmasseslargerthantheplausiblelimitofthe data were automatically rejected. During this search, all three
NS mass—conservatively taken to be 3.5M⊙ for this check— GW detectors were simultaneously collecting science quality
were not considered for EM follow-up, since the optical emis- dataforroughly45%ofthetime.
sionisthoughttobeassociatedwiththemergeroftwoneutron
stars or with the disruption of a neutron star by a stellar-mass
blackhole. 4.4.ManualEventValidation
Triggersfromeachinterferometerareclusteredandusedto Inadditiontoautomatedchecksondataquality,significanttrig-
searchforcoincidenceamongthe individualdetectors.To gen- gersweremanuallyvetted.Triggeralertswerebroadcasttocol-
erate a candidate event for follow-up, triggers with consistent laborationmembersvia e-mail,textmessage,a website, andin
physicalparametersmustbepresentinallthreeLIGO/Virgoin- theinterferometercontrolroomsasaudioalarms.Foreachalert,
terferometers.Foreachtriplecoincidenttrigger,theskylocation alow-latencypipelineexpertconferredwithpersonnelateachof
was estimated using the time delay between detector sites and thethreeobservatorysitestovalidatetheevent.Pipelineexperts
theamplitudeofthesignalmeasuredineachdetector(Fairhurst andscientistsmonitoringdataon-siteprovided24/7coveragein
2009). Before the observingperiod, a set of simulated gravita- 8hourshifts.Assignedpersonnelconfirmedtheautomateddata
tional wave signals was used to measure the distribution of er- qualityresults,checkedplotsforobviousabnormalities,andver-
rors in recovering the time delays and signal amplitudes. The ified thatthere were no knowndisturbancesat any of the three
skylocalizationalgorithmthenusesthesedistributionstoassign observatorysites.
probabilitiestoeachpixelonthesky. Theintentionofmanualeventvalidationwasto vetospuri-
ouseventscausedbyknownnon-GWmechanismsthathavenot
beencaughtbylow-latencydataqualitycuts,nottoremoveev-
4.2.EstimatingFalseAlarmRates ery non-GWtrigger.In fact, atcurrentsensitivities, most orall
ofthetriggersareunlikelytorepresenttrueastrophysicalevents.
Theprimaryquantityusedtodecidewhetheraneventshouldbe Thetrade-offforthisadditionalcheckonthequalityofthetrig-
consideredacandidateforfollow-upswasitsFAR,theaverage gerswasaddedlatency(usually10to20minutes)betweentrig-
rate at which noise fluctuations create events with an equal or ger identification and reporting to the EM observatories. It is
greatervalueofthedetectionstatistic.Forthewinterrun,aFAR possiblethatasthesearchmaturesintheAdvancedLIGO/Virgo
oflessthan1eventperdayoflivetimewasrequiredtosendan erathevalidationprocesscanbefullyautomated.
imaging request to the ground-based telescopes, with a higher
thresholdforSwift.Fortheautumnrun,theFARthresholdwas
0.25eventsperdayoflivetimeformosttelescopes,withstricter 5. ChoosingFieldstoObserve
requirementsforsendingtriggersto PalomarTransientFactory
andSwift.Livetimeisheredefinedastimeallthreeinterferom- The uncertainty associated with GW position estimates, ex-
etersweresimultaneouslycollectingusablesciencedata. pected to be severaltens of square degrees(Fairhurst2009), is
large compared to the FOV of most astronomical instruments.
As in previous all-sky burst searches, e.g. Abbottetal.
Moreover,thelikelyskyregionscalculatedfrominterferometer
(2009b) and Abadieetal. (2010b), the FAR for the two burst
datamaybeirregularlyshaped,orevencontainseveraldisjoint
pipelines was evaluated using the time-shift method. In this
regions. It is impractical to image these entire regions given a
method,artificialtimeshifts,betweenonesecondandafewhun-
limitedamountofobservingtimeforagiveninstrument.There
dredseconds,areappliedtothestrainseriesofoneormorein-
isthusaneedtocarefullyprioritizefields,ortiles,ofaninstru-
terferometers,andtheshifteddatastreamsareanalyzedwiththe
menttooptimizethelikelihoodofimagingthetruegravitational
regular coherent search algorithm. The shifted data provide an
wavesource.
estimate of the background noise trigger rate without any true
The LUMIN software package was created to gather GW
coincident gravitational wave signals. During the online anal-
triggersfrom the threetrigger generators,anduse the skymaps
ysis, at least 100 time shifts were continuously evaluated with
and locations of known galaxies to select fields for each opti-
latencies between 10 minutes and several hours. The FAR of
calorradioinstrumenttoobserve.Inaddition,LUMINincludes
each eventcandidatewas evaluatedwith the most recentavail-
toolsthatwereusedtofacilitatetriggervalidation(Sect.4.4)and
abletimeshifts.
communication with robotic telescopes. Fields for observation
TheMBTApipelineevaluatedtheFARanalyticallybasedon withtheSwiftXRT andUVOTwereselectedwithslightlydif-
singleinterferometertriggerrates,ratherthanusingtimeshifts. ferentcriteriabyaseparatesoftwarepackage,theGravitational
Thisiscomputationallysimplerthantheburstmethod.Itisvalid to Electro-Magnetic Processor (GEM). During the testing pro-
sinceMBTAisacoincidentratherthanacoherentanalysis,and cess, GEM also appliedthe tilingcriteriaforopticaltelescopes
allowstheFARtobeevaluatedwithdatafromtheminutesim- tosimulatedskymaps,andsoprovidedanimportantconsistency
mediatelyprecedingthetriggertime(Marion2004). checkbetweenLUMINandGEM.
LSC+Virgo+others:FirstpromptsearchforGWtransientswithEMcounterparts 9
5.1.GalaxyCatalog Unliketheburstalgorithms,MBTAassumestheGWsource
isamergingbinary,andestimatessomeofthesource’sphysical
The GravitationalWave GalaxyCatalog (GWGC) (Whiteetal.
parametersforeachtrigger.Thisallowsthegalaxycatalogtobe
2011)wascreatedtohelpthisandfuturesearchesquicklyiden-
appliedinaslightlydifferentway.Eachinterferometermeasures
tifynearbygalaxies.
aquantityknownaseffectivedistance
The catalogcontainsup-to-dateinformationcompiledfrom
the literature on sky position, distance, blue magnitude, ma-
jor and minor diameters, position angle and galaxy type for 1+cos2ι 2 −1/2
53,225galaxiesranging out to 100 Mpc, as well as 150 Milky D =D F2 +F2cos2ι , (2)
eff + 2 ×
Wayglobularclusters.Whiteetal.(2011)comparedthecatalog  ! 
wanidthcaonnecxlupdeecdtedthbaltuteheligGhWtdGisCtribisutnioenardlyercivoemdpflreotme oSuDtStoSd∼a4t0a  
where D istheactualdistancetothesource,ιistheinclination
Mpc. The catalog improveson the issue of multiple entries for
anglebetweenthedirectiontotheobserverandtheangularmo-
the same galaxy suffered by previous catalogs by creating the
mentumvectorofthebinary,andF andF aretheantennare-
+ ×
GWGC from a subset of 4 large catalogs, each of which lists
sponsefunctionsoftheparticularinterferometer.Theimportant
a unique Principal Galaxy Catalogue (PGC) number for every featureoftheeffectivedistanceisthatitisalwaysgreaterthanor
galaxy(Patureletal.1989).Thecatalogsusedwere:anupdated
equaltothetruedistancetothesource.ForeachMBTAtrigger
versionoftheTullyNearbyGalaxiesCatalog(Tully1987),the
thegalaxycatalogisthenonlyconsideredouttothesmallestef-
CatalogofNeighboringGalaxies(Karachentsevetal.2004),the
fectivedistancemeasuredforthattrigger,withamaximumpos-
V8kcatalog(Tullyetal. 2009), andHyperLEDA(Patureletal. sibleeffectivedistanceof50Mpc.Afterthecatalogisdownse-
2003).Alsoincludedisalistof150knownMilkyWayglobular
lectedinthisway,eachpixelis weightedbythefractionofthe
clusters(Harris1996).Theseareallavailablefreelyonline,buta
catalog’stotalmasscontainedinthatpixel,i.e.
local,homogeneouslistisessentialforrapidfollow-uppurposes.
P= MfracL, (3)
5.2.WeightingandTilingAlgorithm i
i
X
Tomakeuseofthegalaxycatalog,andchoosetilesforeachGW
trigger,similar algorithmshavebeen implementedin the GEM
with the sum over all galaxies associated with the pixel, and
andLUMINsoftwarepackages. Mfrac =1forasumoverthedownselectedcatalog.
The position information from the trigger generators (see k k
Theseproceduresrequireapixel’scoordinatestobeconsis-
Sect.4.1)isencodedinskymapsthatassignalikelihoodtoeach P
tentwithaknowngalaxy’slocationtobetargetedbytelescopes.
0.4◦ × 0.4◦ pixel in a grid covering the sky. In practice, only
However, in the case that the skymap does not intersect with
the1000mostlikelypixelsareretained,limitingtheskyareato
anygalaxiesinthecatalog,thelikelihoodfromtheGWskymap
roughly160 square degrees. The search volume is further lim-
aloneisusedaseachpixel’slikelihood(P= L).Inpractice,this
itedbykeepingonlyobjectsinthecatalogwithanestimateddis-
isaveryrareoccurrenceandonlyhappensinthecaseofavery
tanceof lessthan50Mpc, asthecurrentsensitivityofthe GW
well-localizedskymap.
detectorsmakesitunlikelythatbinariescontaininganeutronstar
wouldbedetectablebeyondthisdistance.Approximately8%of Theactualpointingcoordinatesrequestedforeachtelescope
thepixelsinanaverageskymapcontainalocalgalaxyorglobu- areselectedtomaximizethetotalcontainedPsummedoverpix-
larclusterlistedintheGWGCcatalog. els within the FOV. If multiple pointings are allowed with the
For bursttriggers, the tiling algorithmstreatthe luminosity same instrument, additional tiles with the next highest ranking
of each galaxy or globular cluster as a prior for its likelihood arethenselected.ThetileselectionprocessisillustratedinFig.
to host a GW emitting event.The blue lightluminosityis used 3.
as a proxy for star formation, indicating the presence of mas-
sivestarsthatmaybeGWburstprogenitorsthemselvesandmay
5.3.GalaxyTargetingforSmall-FieldInstruments
evolveintocompactbinariesthateventuallymerge.Inaddition,
weak sources of GWs are assumed to be more numerous than ThelogicusedforselectingpointingsfortheSwiftsatellitewas
strongsources,so thata closergalaxyshouldcontainmorede- similartothatofground-basedtelescopes,exceptthat,because
tectable sources than a more distant galaxy of the same mass the narrower Swift FOV required greater precision, care was
(Nuttall&Sutton 2010). This leads to assigning the following takentoensurethetargetgalaxieswerewithintheselectedfield.
likelihoodtoeachpixel: The coordinates supplied to Swift for follow-up were those of
ML thematchedgalaxyitselfincaseswheretherewasonlyasingle
P∝ i (1) galaxyin a pixel,butthe center ofthe 0.4◦×0.4◦ pixelin cases
D
i i wherethecentralcoordinatesofanextendedsourcewereoutside
X
where L is the likelihood based only on the GW data, and M thepixelorthereweremultiplegalaxiesinthepixel.Sincefewer
andDarethebluelightluminosity(aroughproxyformass)and follow-upswere allowed using Swift than with other scopes, a
distanceoftheassociatedgalaxyorglobularcluster.Thesumis minimum requirement was placed on the statistic P contained
overalltheobjectsassociatedwithaparticularpixel(whichwill withinthepixelsselectedforX-rayobservation.
be 0 or 1 galaxy for the majority of pixels). Extended nearby Zadkoand LiverpoolTelescope also have relativelynarrow
sourceswhichhaveamajoraxislargerthanthepixelsize have fields.Forthesetelescopes,noattemptwasmadetocapturemul-
their mass dividedevenly overeach pixelfalling within the el- tiplegalaxiesinasinglefield.Instead,theweightingschemein
lipse of the disk defined by their major and minor axes. Once Eqn.1wasappliedtoeachgalaxyratherthaneachpixel,andthe
thiscalculationisperformedforeachpixel,theentireskymapis centercoordinatesofthetoprankedgalaxieswerepassedtothe
renormalizedtoatotallikelihoodequaltounity. observatories.
LSC+Virgo+others:FirstpromptsearchforGWtransientswithEMcounterparts 10
30 30
20 20
10 10
0 0
s) s)
ee−10 ee−10
gr gr
e−20 e−20
D D
c (−30 c (−30
e e
D D
−40 −40
−50 −50
−60 −60
−70 −70
125 120 115 110 105 100 95 90 85 125 120 115 110 105 100 95 90 85
RA (Degrees) RA (Degrees)
Fig.3.TheweightingandtilingprocessforasimulatedsignalreconstructedbycWB.Theskymapisshownintheleftpanelwith
thehighestlikelihoodregionsinred,andlowerrankedpixelsinblue,alongwithgalaxylocationsmarkedasblackcircles.Theright
panelshowsthelocationandapproximatesizeofthethreechosenQUESTtiles,alongwiththelocationsofpixelsthatareretained
afterweightingbythegalaxycatalog.Theinjectionlocationiscaughtbythesouthernmosttile,andismarkedwithanasterisk.
6. ObservingStrategy humanintervention.Inafewcasesthisallowedresponsetimes
of less than a minute after an alert was sent, though response
6.1.Communication timesofafewhoursweremoretypicalduetowaittimefortar-
getstobeoverhead.
Afteraneventcandidatepassedmanualinspection,ascriptwas
launched to pass the GPS time and selected field center loca-
tions to the QUEST, ROTSE III, SkyMapper, TAROT, Zadko,
Duringthe winterrun, QUEST respondedto three triggers,
LiverpoolTelescope,andLOFARobservatories.Duringtheau-
making 2 exposures of each field on the night of the request.
tumnrun,a totalof fivesuch alerts weresent. Duringthe (ear-
TAROT respondedto one winter run trigger,takingsix images
lier) winter run,8 eventcandidateswere passed to the TAROT
onthe nightoftherequest.Swiftalso respondedto onetrigger
andQUESTobservatories.Thenumberoffieldlocationspassed
inthewinterrun,takingoneexposureofeachfieldfollowingthe
to each telescope for each GW event candidate are listed as
request,andthenasecondsetofexposuresonalaterdatetobe
the “Tiles per Trigger” in Table 2. During the autumn run, in
usedasreferenceimages.
caseswhere the fieldsselected fora particularinstrumentwere
unobservable due to daylight or latitude, no alert was sent to
the observatory. In most cases, alerts were sent via a direct
Formostobservatoriesinthesummerrun,theobservingplan
socket connection from a LIGO computing center at Caltech
calledforcapturingafirstimageoftheselectedfieldsasrapidly
with IP mask protection. Alerts to ROTSE III, SkyMapper,
as possible, with follow-up observations every night or every
TAROT, and Zadko used the format of GCN notices. Alerts to
other night out to five days after the trigger time. For the op-
LOFARandtheLiverpoolTelescopeusedtheVOEventformat
tical observatories, any night’s observation included 2 or more
(Williams&Seaman2006).ForQUEST,theGPStimeandfield
exposures for each field, to help eliminate asteroids, CCD ar-
positions were posted as ASCII tables to a password protected
tifacts, and other contaminants from the data set. In addition,
websitewhichwasregularlypolledbytheQUESTscheduler.
some fieldswereimagedat latertimes, up to a monthafter the
The Palomar TransientFactory received field locationsand
triggertime,toprovidereferenceimages,orpossiblytocapture
GPS times using the VOEvent formatvia a socket connection,
alightcurvewithalatebrighteningtime.TAROT,Zadko,PTF,
butwithamorerestrictiveFARthresholdthantheotheroptical
QUEST,andPioftheSkyallfollowedthisrecipe.ROTSEexe-
telescopes,and so triggerswere onlysent to PTF if the on-call
cutedamoreaggressiveobservingplan,collectingasetof30im-
teamexecutedaseparatescript.AlertstoSwiftalsorequiredex-
agesinrapidsuccessiononthefirstnight,andthensetsofeight
tra actionby theon-callteam, whoenteredfield coordinatesin
imagesoneachof15nightsfollowingthetriggerwithintervals
an online form. The Pi of the Sky prototypetelescope was en-
of two days on average. As in the winter run, Swift made one
gagedthroughautomatede-mailsandmanualchecksofapass-
exposureof each field followingthe trigger,and then collected
wordprotectedwebpage.
a reference image after a lag of several weeks. The Liverpool
Telescopedevotedroughlyonehourofobservationuponreceiv-
6.2.TelescopeResponse ing a trigger,and then collected reference images a few weeks
afterthetriggertime.TheLOFARresponsewasnotautomated.
The wide variety of telescopes involved in the search led to a A telescope operator made a single, four hour observation one
diversityofobservingstrategies,witheachpartneringgroupap- tofourdaysafterdeliveryofatrigger.SkyMapperalsorequired
plyingadifferentcadence.Bydesign,mostofthetelescopesin manualinterventiontorespondtoatrigger,andsorespondedon
the network were robotic, and could respond to alerts without abesteffortbasis.