GRB 081029: Understanding Multiple Afterglow Components

0.18 MB·English

by  S. T. Holland

GRB 081029: Understanding Multiple Afterglow
Components
S. T. Holland∗,∗∗, M. De Pasquale‡, J. Mao§, T. Sakamoto∗,∗∗, P. Schadyk,‡, S.
1 Covino§, P. D’Avanzo§, A. Antonelli††, V. D’Elia††, G. Chincarini§, F. Fiore†† and S.
1
B. Pandey‡‡
0
2
n ∗CentreforResearchandExplorationinSpaceScienceandTechnology,NASA/GSFC,Greenbelt,MD20771,
a USA
J †UniversitiesSpaceResearchAssociation,10211WinicopinCircle,Columbia,MD21044,USA
1 ∗∗Code660.1,NASA/GSFC,Greenbelt,MD20771,USA
3 ‡MullardSpaceScienceLaboratory,UniversityCollegeLondon,Holmbury,StMary,Dorking,Surrey,RH56NT,
UK

] §INAF-OsservatorioAstronomicodiBrera,ViaEmilioBianchi46,I–23807Merate(LC),Italy
O
¶JointCentreforAstrophysics,UniversityofMaryland,BaltimoreCounty,1000HilltopCircle,Baltimore,MD
C 21250,USA
. kMax-PlanckInstitutfürExtraterrestrischePhysik,Giessenbachstraße,85748Garching,Germany
h
††INAF-OsservatorioAstronomicodiRoma,ViadeFrascati33,I–00040MonteporzioCatone(Roma),Italy
p
- ‡‡RandallLaboratoryofPhysics,UniversityofMichigan,450ChurchSt,AnnArbor,MI48109–1040,USA
o
r
t Abstract. Wepresentananalysisoftheunusualopticallightcurveofthegamma-rayburstGRB081029,whichoccurredata
s redshiftofz=3.8479.WecombineX-rayandopticalobservationsfromtheSwiftX-RayTelescopeandtheSwiftUltraViolet
a
OpticalTelescopewithopticalandinfrareddataobtainedusingtheREMandROTSEtelescopestoconstructadetaileddata
[
setextendingfrom86sto∼100000saftertheBATtrigger.Ourdataalsocoverawideenergyrange,from10keVto0.77eV

1 (1.24Åto16000Å).TheX-rayafterglowshowsashallowinitialdecayfollowedbyarapiddecaystartingatabout18000s.
v Theopticalandinfraredafterglow,however,showsanuncharacteristicriseatabout5000sthatdoesnotcorrespondtoany
2 featureintheX-raylightcurve.Ourdataarenotconsistentwithsynchrotronradiationfromasingle-componentjetinteracting
5 withanexternalmedium.Wedo,however,findthattheobservedlightcurvecanbeexplainedusingmulti-componentmodel
9 forthejet.
5
Keywords: gamma-rayburst:individual:GRB081029
.
1 PACS: 98.70s.Rz
0
1
1 INTRODUCTION
:
v
i Thereisgrowingevidencethattheclassicalpictureofasingleuniformjetcannotexplainthespectralenergydistribu-
X
tionsandlightcurvesofsomegamma-rayburst(GRB)afterglows.Forexample,theunusuallybrightopticalafterglow
r
of the “naked-eye”burst GRB 080319B was best explained using a two-componentjet [6] while GRB 030329 [1]
a
appearstorequireanarrow,ultra-relativisticinnerjetandawide,mildlyrelativisticouterjettoexplainitslightcurves.
This is in agreementwith results from magneto-hydrodynamicmodelling that show complex structure in GRB jets
[e.g.,9].GRBafterglowsappeartobemorecomplexthanoriginallythought.
An example of a GRB afterglow that appears to require a multi-component jet is GRB 081029. This burst was
detectedbySwift/BATat01:43:56UTon2008Oct29[8].ROTSE-IIIcidentifiedtheopticalafterglowat86s[7],and
theREMtelescopestartedobservingtheopticalafterglowat154s[2],sothereisawell-sampledR-bandlightcurve
startinglessthan90saftertheBATtrigger.DuetoanobservingconstraingSwiftwasunabletoslewtothisburstas
soonasitwasdetected.XRTandUVOTobservationsbeganabout45minuteaftertheBATtriggerandcontinuedfor
approximately10days.
TheVLT/UVESandGemini-Southmeasuredaredshiftofz=3.8479[4,3],whichcorrespondstoalookbacktime
of11.9Gyr.TheGemini-SouthGMOSspectrumshowsevidenceforadampedLyman-alphasystemaswellasseveral
metalabsorptionfeaturesinthehostgalaxyofGRB081029.
OBSERVATIONS
BAT:Prompt Emission
The Swift/BAT discovered and observed GRB 081029. The burst duration was T =280±50 s, the peak flux
90
was (2.8±1.3)×10−8 erg cm−2 s−1, and the spectrum was best fit by a simple power law with a photonindex of
G =1.5±0.2.TheBATlightcurvewassomewhatsmootherandweakerthanatypicalBAT-detectedGRB.Figure1
showstheBATlightcuvrvefortheprompeemissionfromGRB081029.
0.01 15−25 keV
5×10−3
0
0.01 25−50 keV
5×10−3
0
0.01
50−100 keV
5×10−3
0
4×10−3 100−150 keV
2×10−3
0
−2×10−3
0.03
0.02 15−150 keV
0.01
0
−0.01
−100 0 100 200 300 400
Time since the BAT trigger [sec]
FIGURE1. TheBATenergyresolvedlightcurvesofGRB081029with10sbinning.
XRT:X-Ray LightCurve and Spectrum
The Swift/XRT observed GRB 081029 from 41.4 minutes to approximately 10 days after the BAT trigger. The
X-raylightcurve(seeFigure2)iswellfitbya brokenpowerlaw withindices(fn (cid:181) t−a )ofa 1 =0.56±0.03until
18230±346s,anda =2.56±0.09afterthat.Thereissomeevidenceforflaringbetweenapproximately2500sand
2
5000s.ThetimescalesoftheseflaresareconsistentwithD t/t<1.TheX-raydataarenotunusual,andareconsistent
withthecanonicalX-raylightcurveforGRBafterglowsdescribedby[5]and[10].
FIGURE2. TheX-ray,optical,andinfrareddatapresentedasfluxdensities.
TheSwift/XRTspectrumcanbefitbyasinglepowerlaw(fn (cid:181) n −b )withanindexofb X =0.98±0.08.Thereis
noevidenceforanyevolutioninthepowerlawindexatX-rayenergies.TheestimatedGalacticcolumndensityinthe
directionoftheburstisN =2.8×10−20cm−2,andtheabsorptioninthehostisN =4.9×10−21cm−2.
H H
FIGURE3. Atwo-componentjetmodelprovidesareasonablefittotheoptical,infrared,andX-raylightcurvesoftheafterglow
ofGRB081029.
OpticalandInfrared Observations
TheSwiftUVOTbeganobservingtheafterglowofGRB081029at2689saftertheBATtrigger.Theafterglowwas
detectedintheUVOTv,b,andwhitebands,consistentwiththereportedredshiftofz=3.8479.Ground-baseddata
was obtained using REM and ROTSE. ROTSE began observations 86 s after the burst in the R band. REM began
observingGRB081029156saftertheBATtriggerintheR,J,andH bands.Theresultinglightcurvesarecomplex,
instarkcontrasttothesimpleX-raylightcurve.ThecombinedopticalandinfraredobservationsareshowninFigure2
alongwiththeSwift/XRTlightcurve.Theopticalandinfrareddatashowajumpinthefluxdensityofapproximately
afactoroftenatapproximately5000s.ThereisnocorrespondingincreaseintheX-rayfluxdensityatthattime.
DISCUSSION
The X-ray light curve is consistent with energy injection from ongoing central engine activity until about 15000 s
followed by a jet break at 18230 s. However, this scenario cannot explain the jump in the flux seen at optical and
infraredwavelengthsatabout5000s. Therefore,we donotthinkthata changein the energyinjectioniscapableof
explainingthelightcurvesforthisafterglow.Similarly,thejumpcannotbemodelledbyinvokingthepassageofthe
synchrotronpeakfrequencythroughtheopticalregime,orastheriseoftheforwardshockduetointeractionwiththe
circumburstmedium. We also examined the possibility that the jump is due to density structure in the surrounding
environment,butthisisunabletoreproducethespeedorthemagnitudeoftheincreaseinluminosity.
Ingeneralwefindthataone-componentjetcannotexplaintheobservedlightcurvesandspectralenergydistribution
oftheX-ray,optical,andinfraredafterglowsofGRB081029.However,atwo-componentjetmodel,similartowhatis
seeninsomeotherGRBafterglows,doesprovideareasonablefittothedataOurtwo-componentjetmodelisshown
inFigure3,andtheparametersofeachjetarelistedinTable1.Thehalf-openingangleofthejetisdenotedbyq ,G
j 0
istheLorentzfactor,E istheisotropicequivalentkineticenergyinthejet, p istheelectronindex,e ande are
K,iso e B
thefractionsoftheenergyinelectronsandmagneticfieldsrespectively,nisthedensityofthecircumburstmedium,
andzistheredshift.
Thenarrow,innerjethasahalf-openingangleofq =0.01radandaLorentzfactorof500.Thiscomponentgives
j,n
TABLE 1. Model parameters for the
best-fitting two-component jet model for
GRB081029.
Parameter NarrowJet WideJet
q j(rad) 0.01 0.02
G 500 60
0
EK,iso(erg) 2.5×1054 2.0×1054
p 2.2 2.2
ee 0.02 1/3
eB 0.0002 0.0002
n(cm−3) 10 10
z 3.8479 3.8479
rise totheX-rayfluxandthe pre-jumpopticalflux.Thewider,outerjethasq =0.02radanda Lorentzfactorof
j,w
60.Thiscomponentdominatestheafterglowafterabout10000s.Thetotalelectromagneticenergyintheafterglowis
approximatelyequallydividedbetweenthetwojets.
CONCLUSIONS
GRB 081029 was a long–soft GRB with a redshift of z=3.8479. It had a smooth gamma-ray light curve and did
notappearto haveanyunusualgamma-rayproperties.Neitherthe gamma-raynortheX-raypropertiesofthis burst
showed any sign of strange behaviour.The optical and infraredlight curves, on the other hand, were not typical of
GRBafterglows.Thereisabrighteningintheopticalandinfraredlightcurvesatabout5000sthatcannotbeexplained
usingasingle-componentjetmodel.However,wefindthatatwo-componentjetmodelfitsthedatareasonablywell.
WeconcludethattheafterglowofGRB081029wasprobablypoweredbyatwo-componentjetwiththeenergysplit
approximatelyequallybetweenanarrow(q =0.01rad)innerjetandawider(q =0.02rad)outerjet.Theinner
j,n j,w
jet has a Lorentz factor of G =500 while the outer jet has G =60. This result providesevidence that some (and
n w
perhapsall)GRBjetshavecomplexinternalstructure.
ACKNOWLEDGMENTS
WeacknowledgetheuseofpublicdatafromtheSwiftDataArchive.Thisworkisbasedinpartonobservationstaken
with the ROTSE-IIIctelescope in Namibia,the REM telescope atla Silla Observatory,andwith ESO Telescopesat
theParanalObservatories.
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