ASPConferenceSeries,Vol.**VOLUME***,**YEAROFPUBLICATION****NAMESOFEDITORS**
TwomoreyearsofGRBopticalafterglows
NicolaMasetti
IASF/CNR,viaGobetti101,I-40129Bologna,Italy
Abstract.IreviewthemainobservationalachievementsconcerningGRBopticalafterglowsoccurredinthepasttwoyearsTheiremissionseemstobewellinterpretedintermsofthe‘fireball’model,althoughavarietyofbehavioursareobserved.Evidenceforsupernova-likelatetime‘bumps’intheopticallightcurvesisgrowing;however,otherin-terpretationsarepossible.Fastfollow-upobservationsallowedthedetec-tionoffaintopticalafterglows,andseemtosuggestthatafractionoftheso-called‘dark’burstsmightactuallybere-classifiedas‘dim’GRBs.Perspectivesforfutureobservationsinthisfieldarealsooutlined.1.Introduction:whathappenedinthepasttwoyears
IwillsummarizeherethediscoveriesaccomplishedduringthepasttwoyearsintheopticaldomainandconcerningtheGamma-RayBurst(GRB)afterglowphenomenon,alongwiththeinformationcarriedbythesedetections.
Istartbybrieflylisting,ina(nottootight)chronologicalorder,whatinmyopinionarethetechnicalandinstrumentallandmarksintheGRBfieldsinceOctober2000.First,bytheendof2000,HETE-2becamefullyoperationalandstarteddeliveringfastGRBtriggers;meanwhile,theGRBcommunitybeganmakingwideruseofrobotictelescopes(amongthemROTSE,LOTIS,BOOTES,BART,KAIT,Palomar48-inch,MountStromlo50-inch,RAPTOR).Inparal-lel,afterglowsearcheswereperformedalsobyapplyingnewtechniquessuchasthecolor-colordiagram(Rhoads2001)ortheimagesubtraction(Alard2000)methods.OnApril2002theMarsOdysseyspacecraftjoinedtheInterPlanetaryNetwork(IPN),makingthissatellitenetworkactiveandefficientagainforGRBlocalizationsaftertheendoftheNEARmissiononFebruary2001.Thebadnewsofthistwo-yearperiodistheendofBeppoSAXoperativelifeonApril30,2002,exactlysixyearsafteritslaunch.Ahappyappendixishoweverthesuccessfullaunch,onOctober17,2002,ofINTEGRAL,whichwillbeabletoprovidereal-timearcmin-sizedGRBlocalizations.
2.Thenewdiscoveries
InthissectionIwillreviewthemaincharacteristicsoftheOpticalTransients(OTs)discoveredintheperiodOctober2000-September2002,alongwithaGRB,explodedinMarch2000,whichcanpotentiallybeveryinterestingandforwhichacomprehensivestudywaspublishedonlyveryrecently.IwillalsobrieflymentionthenewOTdiscoveriesoccurredaftertheendofthisWorkshopandbeforetheproceedingssubmissiondeadline(January2003).InthefollowingIwilldenotewithαandβthetimedecayandspectralindicesofthe−afterglowemission,respectively,accordingtothecommonnotationF(t,ν)∝tαν−β.
GRB000313.Thisshort/hardGRBwasdetectedbyBATSEonly,thusithadnotanaccuratelocalization.Nevertheless,withinthe3σBATSEerrorbox,anobject(Fig.1,left)ofmagnitudeI=9.4±0.1wasdetectedbyBOOTES
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Figure1.Left:BOOTESimagesshowingthefadingofthepossibleGRB000313OT(adaptedfromCastro-Tiradoetal.2002).Right:Op-ticalandnear-infraredlightcurvesoftheGRB010222afterglow(fromMasettietal.2001).
about4minutesaftertheGRB.Subsequentobservationsshowedthatneitheranopticalafterglow,norahostdowntoalimitingmagnitudeR∼24.5weredetectedatthepositionofthetransient.Ifthiswasindeedtheopticalcounter-partofthisGRB,theearlylightcurvehadadecayindexα>2.2,similartothedecayobservedintheopticalflashofGRB990123(Akerlofetal.1999).Thus,BOOTESmayhavecaughtthefirstopticalflashassociatedwithashort/hardGRB.Theabsenceofanafterglowandofahostwouldsuggestthatindeedshort/hardGRBsoccurinlow-densityenvironmentsoutsidetheirhosts.Ade-tailedstudyonthisGRBcanbefoundinCastro-Tiradoetal.(2002).GRB001007.ThisGRBwasdetectedbytheIPNanditsOTwascharac-terizedbyaveryfast(α∼2)lightcurvedecay.Itsnon-detectioninLOTISdatasuggestedapossibleearlybreakinthelightcurve.ThespectralandtemporalbehaviouroftheOTwereplausiblymodeledassumingacollimatedemission.ThehostgalaxywasdetectedatR∼24.8andpresentedanirregularshape.MoreinformationisavailableinCastro-Cer´onetal.(2002).GRB001011.TheOTofthisGRBlocalizedbyBeppoSAXwasthefirsttobefoundwiththecolor-colordiagramtechnique(Rhoads2001)extendedtotheinfrareddata,andlaterconfirmedbyitsfading.Itshowedatimedecaywithα∼1.3andwaslocatedinahostwithR∼25.4.FurtherdetailsonthediscoveryofthisOTcanbefoundinGorosabeletal.(2002a).
GRB010222.ThisGRB,oneofthebestshotsbyBeppoSAX,wasaccompa-niedbyarelativelybrightOTwiththreeabsorptionsystemsinthespectrumatredshifts0.927,1.155and1.475(Jhaetal.2001;Masettietal.2001).Itslightcurveunderwentanachromaticbreak(withtheexceptionoftheKband;Fig.1,right)∼0.7daysaftertheGRB,whilethebroadbandspectrumsuggestedthatthecoolingfrequencyνcwasintheUVdomain∼1dayafterthepromptevent(Masettietal.2001).Nosignificantpolarizationwasdetected(Bj¨ornssonetal.2002).ApossiblefurtherlightcurvebreakwasfoundbyFruchteretal.
Twomoreyearsofopticalafterglows3
Figure2.Left:XMM-NewtonerrorboxofGRB020322.TheOTistheobjectmarkedwiththeletter‘b’(fromBloometal.2002a).Right:HSTimageoftheGRB020410OT.Theobjectisindicatedbythetickmarks.AdaptedfromFruchteretal.(2002c).
(2001a)∼10daysaftertheGRB.Nounanimousinterpretationofthefireballshape,physicsandemissionmechanismsatplayinthisOTwasreached:subrel-ativistictransition(Masettietal.2001),continuousenergyinjection(Bj¨ornssonetal.2002),lowelectrondistributionindexp(Sagaretal.2001)wereinvokedtoexplaintheOTbehaviour.High-resolutionspectraindicatedthattheGRBenvironmentwasnotparticularlydense(Mirabaletal.2002);thisresultisatvariancewiththefindingsofMasettietal.(2001)andSavaglioetal.(2003).GRB010921.ThiswasthefirstGRBdetectedbyHETE-2forwhichanOTwasfound,andthefirstOTfoundbymeansofimagesubtractiontechnique(Priceetal.2001);itwouldhaveprobablybeenoverlookedifthismethodwasnotapplied.Itwasatredshiftz=0.451(Priceetal.2001)anditslightcurveshowedtwobreaks:onewithin1hour(Parketal.2001)andtheotherabout35days(Priceetal.2002a)aftertheGRB.Indicationforsphericalfireballexpansionandforsubstantialabsorptioninthehostwerefound(Priceetal.2001).Anyunderlyingsupernova(SN),ifpresent,wasatleast1.3magfainterthanSN1998bw(Priceetal.2002a).
GRB011211.TheOTofthisBeppoSAXGRBwaslocatedatz=2.14(Fruchteretal.2001b).Thelightcurveshowedabreak∼2daysaftertheGRBandfluctuationswhichcouldbeduetocircumburstmedium(CBM)in-homogeneitiesonascaleoftensofAUs;observationsareconsistentwiththeOTemissionbeingproducedbyafireballexpandinginahomogeneousmedium(Hollandetal.2002).TheOTwasnotpolarized(Covinoetal.2002a).GRB020305.ThiswasthesecondOTdetectedfollowinganHETE-2alert.Itshoweddecayandspectralindicesα∼1.3andβ∼1,respectively(Leeetal.2002),andthepresenceofahostwasreported(Ohyamaetal.2002;Gorosabeletal.2002b).FurtherobservationscanbefoundinYoshidaetal.(theseproceedings).
GRB020322.TheOTofthisBeppoSAXGRBwasoneofthedimmestonesdetecteduptonow:8hoursaftertheGRBitwasalreadyatmagnitudeR∼
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23.3(Bloometal.2002a;Fig.2,left)andwasverylikelydiscoveredonlythankstothearcsecond-sizedpositionavailablethroughtheXMM-NewtondetectionoftheX–rayafterglow(Ehleetal.2002).Apossiblebreakaround∼1dayaftertheGRBwasdetectedinitsopticallightcurve(Burudetal.2002).
GRB020331.TheHETE-2‘EasterGRB’wasassociatedwithaveryslowlydecayingOT(α∼0.6;Foxetal.2002).Thiswaspossiblyduetothecontami-nationoftheunderlyinghost(Dullighanetal.2002).However,Castro-Cer´onetal.(theseproceedings)reportedaflatteningandapossiblesubsequentrestartofthe(slow)decayoccurring∼1weekaftertheGRB,whichwouldsuggestthatthisbehaviourmightbeintrinsicandnotduetothehostcontribution.
GRB020405.TheOTofthisIPNGRB,ataredshiftz=0.69(Masettietal.2002;Priceetal.2002b),showedabumpinthelightcurvebetween10and20daysaftertheGRB:thiswasinterpretedbyPriceetal.(2002b)asduetoanunderlyingSN.Besides,theOTwaspossiblyhighlypolarized(P∼10%;Bersieretal.,2002;however,seethereviewbyCovinointheseproceedings).AfairlycompletespectrophotometricandpolarimetricstudyofthisGRB,alongwithanalternativeinterpretationoftheopticallightcurvebump,canbefoundinMasettietal.(theseproceedings).
GRB020410.OneofthelastBeppoSAXlocalizationsallowedthedetectionofanOT,forthefirsttimeever,bymeansofHST(Fruchteretal.2002c;Fig.2,right).Thiswaspossiblethankstotheveryaccurate(20′′inradius)X–rayafterglowpositionbyBeppoSAXsothattheentireerrorboxcouldbefittedintheHST-WFPC2fieldofview.Comparisonwithground-basedobservationsshowedthattheOTlightcurveunderwentabreak;however,thepossibilitythatthiswasactuallyahigh-zSNcannotbeexcluded(Fruchteretal.2002c).
GRB020813.ThiswasoneofthebrightestGRBdetectedbyHETE-2uptonow.ItsOTshowedtwoabsorptionsystemsinitsspectrum,atredshifts1.122and1.2(Priceetal.2002c).Amarkedlightcurvebreakwasdetected∼1dayaftertheGRB(Bloometal.2002b),whileaflattening,mostlikelyduetothepresenceofthehost,wasobservedatlatetimes(Gorosabeletal.2002).ForthefirsttimeinthecaseofanOT,averyhighresolutionspectrumwasacquired(Fioreetal.2002).Opticalpolarization,possiblyvariable,wasalsodetected(Barthetal.2003;Covinoetal.2002b;Roletal.,theseproceedings).
OtherOTs.Aftertheendofthemeeting,andbeforethesubmissiondead-line(January15,2003),theOTsofthreemoreGRBs(021004,021211and030115,alllocalizedbyHETE-2)weredetected.Thefastlocalizationsof021004and021211allowedthedetection,bymeansofrobotictelescopes,ofthecorre-spondingOTswithinthefirstminutesaftertheGRBtrigger:theemissionwasinbothcasesinterpretedasthetailoftheopticalflashsimultaneouswiththeGRBitself(Kobayashi&Zhang2003;Wei2003).TheOTofGRB021004wasquitebrightandwellsampled;observationssuggestaclumpyCBM(e.g.Lazzatietal.2002).GRB021211wasinsteadassociatedwithaquitefaintOT(Foxetal.2003)and,possibly,anevenfainterOTwasfoundforGRB030115(Masettietal.2003)atthepositionoftheinfraredafterglow(Levanetal.2003).
AcontroversialcaseisthatoftheHETE-2X–rayflashXRF020903.InitserrorboxadecayingopticalsourceshowingaSN-likebumpafter∼30dayswasdetectedatz=0.25(Soderbergetal.2002).Thus,werethisindeedtheOTofXRF020903,itwouldbethefirstopticalcounterparteverdetectedfor
Twomoreyearsofopticalafterglows5
anXRF;moreover,itsbehaviourwouldnotsubstantiallydifferfromthatofGRBOTs,inspiteoftheirdifferenthigh-energyprompteventcharacteristics.Alternatively,anAGNinterpretationforthisOTwasgiven(Gal-Yam2002);recentHSTobservationshowedhoweverthatthevariableobjectisinacomplexofinteractinggalaxies,butnotatthecenterofanyofthem(Levanetal.2002).
IintentionallydidnotmentiontheOTsofGRB011121andGRB020124:theywillbediscussedintheseproceedingsbyGreiner,Zehetal.andPrice.3.Summary:thelessonswe(hopefully)learnt
ThediscoveriesofnewOTsduringthepasttwoyearsincreasedourknowledge(andinsomecasesouruncertainties)onthebehaviourandemissionmechanismsofGRBafterglowsintheoptical.WefoundthatOTsshowno“preference”intheirexpansiongeometry,fireballevolutionorenvironmentalcharacterstics.
Thankstotheavailabilityofaccurate(arcmin-sized)real-timeGRBalertandtotheincreasingnumberoffast-responserobotictelescopes,wearegettingcloserandclosertoseetheOTbehaviourinitsveryfirstphases.Recentobser-vationsaremoreoversuggestiveofanoteworthypoint:thereindeedseemstobewideaspreadintheOTbrightnessesandthismight,atleastpartially,explainthe‘dark’opticalafterglowconundrum.Thatis,quitelikelysomeofthemareactuallyintrinsically‘dim’OTs,ratherthanobscuredorhigh-zones.
Also,evidenceforlate-timebumpsintheOTlightcurvesisgrowing.ThesearecustomarilyassociatedwiththeemergingcontributionofaSNexplodedat,oraround,thetimeoftheGRB;however,thisbehaviourisnotuniversal(seee.g.thecaseofGRB010921),andotherinterpretationssuchasdustechoes(Esin&Blandford2000),lateshellcollisions(Kumar&Piran2000)orion-protonshellinteractions(Beloborodov2003)shouldbeexplored.
Whatisthelessonweshouldlearnfromallthis?Inmyopinion,itcanbelistedinsixpoints(someofthem,admittedly,abittautologic):1)weneedfasthigh-energyarcseclocalizationtopinpointtheGRBpositionandstartverydeepserchesassoonasthealertisdistributed.Thiscanalsohelptoquicklyfind‘dim’and/orrapidlydecayingOTs.TheSwiftsatellitewillbeabletofulfillthistask;2)asHETE-2andINTEGRALcanalreadyprovidereal-timeGRBalerts,weneedveryfast-responserobotictelescopes.TheREMtelescope(Zerbietal.,theseproceedings),startingonMarch2003,willsecureobservationalcoveragebeginningwithin10secondsaftertheGRBtrigger;3)sometimes,errorboxeswithsizesofseveralarcminofradiusareprovided.Inthiscase,theuseoftelescopeswithlarge-fieldimagersshouldbeencouraged;4)assomeafterglowscanbeopticallyfaintduetolocaldustobscurationorhigh-zeffects,wideruseofnear-infraredimagingshouldbemade.ThisalsowillbeaddressedbyREM;5)theuseofnewtechniquesforOTsearches(suchascolor-colordiagram,imagesubtraction,alongwiththedevelopmentofpipelinesforfastreductionofwide-fieldimages)shouldalsobeencouraged;6)finally,andwhenapplicable,co-operationamonggroups(andpossiblyfromGRBsalso...)wouldbedesirabletoexpandourunderstandingofthesestillenigmaticandthereforefascinatingphenomena.References
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