Spectral evolution and the onset of the X-ray GRB afterglow P.A. Evans, J.P. Osborne, R. Willingale and P.T. O’Brien X-rayandObservationalAstronomyGroup,Dept.ofPhysics&Astronomy,UniversityofLeicester,Leicester, 1 UK,LE17RH 1 0 Abstract. BasedonlightcurvesfromtheSwiftBurstAnalyser,weinvestigatewhethera‘dip’featurecommonlyseeninthe 2 early-timehardnessratiosofSwift-XRTdatacouldarisefromthejuxtapositionofthedecayingpromptemissionandrising n afterglow.Weareabletomodel thedipassuch afeature,assuming theafterglow risesaspredicted bySariandPiran[1]. a UsingthismodelwemeasuretheinitialbulkLorentzfactorofthefireball.Forasampleof23GRBswefindamedianvalue J ofG =225,assumingaconstant-densitycircumburstmedium;orG =93ifweassumeawind-likemedium. 0 0 1 Keywords: GammaRayBursts 3 PACS: 98.70.Rz,95.75.Wx ] E H INTRODUCTION . h The early Swift-XRT observations of GRBs have long been seen to exhibit spectral evolution ([2]; [3]). A feature p seen in a number of GRBs is a ‘dip’ in the hardness ratio (i.e. a softening followed by a hardening;see Fig. 1a) at - o theendofthesteepdecayphase.TheSwiftBurstAnalyserlightcurves([4]),whichareinunitsoftheintrinsic(i.e. r unabsorbed)flux and accountfor spectral evolution,suggestthat this may correspondto the ‘turn-on’on the X-ray t s afterglow(Fig.1b). a Theriseoftheafterglowwasconsideredby[1],whopredictedthatitriseast2 untilsometimet atwhichpointit [ p peaksandbeginstodecay.Thistimetpcorrespondstothedecelerationradius:theradiusatwhichtherestmassenergy 1 ofthematerialsweptupbythefireballequalstheenergyofthefireball.FromthisonecandeterminethebulkLorentz v factoratthedecelerationradiusofthefireball(e.g.[1];[5]). 3 2 1 9 G 3E [1+z]3 8 0 iso = (1) 5 2 32p nm c5h t3 . p p! 1 0 WhereEisoistheisotropicequivalentenergyoutputoftheGRB,nisthenumberdensityofthecircumburstmedium, 1 andh istheradiativeefficiencyofthefireball. 1 Thisassumesthatthedensityisconstant,i.e.theGRBisinanISM-likeenvironment.Ifweinsteadassumethatthe : environmentiswind-like(i.e.r (cid:181) r−2)wefind: v i X G 0 Eiso[1+z] 14 ar 2 =(cid:18)8p Ampc3h tp(cid:19) (2) whereAisthedensitynormalisation.FollowingMolinarietal.weassumeh =0.2,n=1cm−3 andA=3×1035 cm−1;notingthatdependenceontheseunknownsisweak. In this paper we will thereforeconsiderwhether the X-ray hardnessratio dip can be used to constrainthe rise of theX-rayafterglowandhencetheinitialbulkLorentzfactor,giventheSari&Piranmodel.Itshouldbenotedthatthe presenceofaplateauphaseinX-raydataisofteninterpretedasasignofcontinuedenergyinjectionintothefireball insomeformorother(e.g.[6]).Itislikelythatthiswillmodifythefireballdynamicssomewhat,howeversince the rate of energyinjection is muchlower than the rate of energyoutputin the promptGRB, we believewe can safely disregardthiseffect. Hardness ratio of GRB 060729 BAT−XRT data of GRB 060729 1000 0.01 s 100 eV c/ 110 10−3 k 0.1 −10 01.00−13 V) 10−4 1.5 11000−04 0 ke 10−5 1.5 keV c/s 010.010.11100 sity (Jy @ 1 1100−−76 0.3− 1100−−243 x den 10−8 1.5 Flu 10−9 o 10−10 ati 1 R 0.5 10−11 0 1 10 100 1000 104 105 106 107 100 1000 104 105 106 107 Time since BAT trigger (s) Time since BAT trigger (s) FIGURE1. The(1.5–10)/(0.3–1.5)keVhardnessratioofGRB060729(leftpanel),showingthe‘dip’featurearound100–200 s after the trigger. The reconstructed 10 keV flux density light curve from theBurst Analyser light curve (right panel) –which accountsforthisspectralevolution–showsafeatureatthissametime,suggestiveoftheoverlapofthedecayingpromptemission andrisingafterglow. MODELFITTING Toinvestigatewhetherthehardnessratiodipcanindeedbecausedbytheriseoftheafterglow,wefitthehard(1.5–10 keV)andsoft(0.3–10keV)XRTbanddatafromtheXRTlightcurverepository([7],[8])simultaneously.Wefittwo components:the first modelsthe decayingpromptemission which we parameterise with the pulse modeldescribed byWillingaleetal. (2005);thehardnessratioofthiscomponentdecreases(i.e.softens)asa power-law.Thesecond componentmodelistheafterglow,whichwetreatasspectrallyinvariant.Thiscomponentiszerountilsometimet at a whichtheafterglow‘starts’,thereafteritrisesast2untilt whereafteritbreakstoagenerict−a decay. p We do not fit the entire light curve, since we are only interested in the transition from the prompt to afterglow emission.Iftheafterglowlightcurveshowsevidenceforafurtherbreakafterthatreferredtoabove,thedataafterthis breakareexcludedfromthefit.Similarlyanybreaksorflareswhichoccurbeforethepromptemissionfollowsasingle power-lawareexcluded;iftheburstshowsnoneofthesefeaturestheentiredatasetisfitted.Examplefitsareshownin Fig.2. RESULTS We identified GRBs with knownredshift, a clear dip feature in the hardnessratio, no flares close to or overlapping thedip,andwhereSwiftobservedthroughoutthedipfeature.Thisgaveanlistof23GRBs.Histogramsoftheinitial LorentzfactorsdeterminedfromthesefitsaregiveninFig.3.ForanISM-likeenvironmentthemedianvalueis225, withmostvaluesbeingabove100,inagreementwithmeasurementsintheliterature(e.g.[5],[9],[10],[11]).Onthe otherhand,ifweassumeawind-likeenvironmentthemedianvalueoftheinitialLorentzfactorisonly93,andisbelow 50forseveralbursts;thisisclosetothelimitimposedbythecompactnessproblemandmaysuggestthattheseGRBs cannothaveoccurredinawind-likeenvironment.WenotethatallofourvaluesarewellbelowG =1000,whichhas 0 beenproposedasalowerlimitonthebulkLorentzfactorgiventheGeVemissiondetectedbyFermi-LAT(e.g.[12]). Howevernone of the GRBs in our sample are those which were also observedby the LAT. Also note that [13] and Haoscoëtetal.(theseproceedings)havearguedthatGeVphotonscanescapefromaFireballwithbulkLorentzfactors substantiallybelow1000. GRB 061110A GRB 060729 Dark Grey: Hard −− Light Grey: Soft 0 Dark Grey: Hard −− Light Grey: Soft 0 100 0 1 0 0 10 1 0 s)−1 1 s)−1 1 e ( e ( 1 Count rat 00.0.11 Count rat 0.010.1 10−3 10−3 0−4 10−44 14 3 3 R 2 R 2 H H 1 1 0 0 100 1000 104 105 1000 104 Time since trigger (s) Time since trigger (s) FIGURE2. Examplesfits.Thedot-dashedlinesshowtheindividualcomponents.Thetoppanelshowsthefitteddata,thebottom panel the hardness ratio. The latter is not fitted, but is calculated from the top panel, and is shown to illustrate how the model reproducesthedipfeature.Leftpanel:GRB061110A.Rightpanel:GRB060729 CONCLUSIONS WehaveshownthatadipfeatureseenintheX-rayhardnessratiosofGRBsattheendofthesteepdecayphasecanbe interpretedastheturn-onoftheX-rayafterglow,whichrisesinaccordancewiththepredictionsof[1].Bymodelling thisrisewehavedeterminedtheinitialbulkLorentzfactorfor23GRBs,findingamedianvalueof225assumingan ISM-likecircumburstmedium,or93forawind-likemedium. REFERENCES 1. R.Sari,andT.Piran,ApJ520,641–649(1999),arXiv:astro-ph/9901338. 2. R. L. C. Starling, P. M. Vreeswijk, S. L. Ellison, E. Rol, K. Wiersema, A. J. Levan, N. R. Tanvir, R. A. M. J. Wijers,C.Tadhunter, J.Rodriguez Zaurin, R.M.Gonzalez Delgado, andC.Kouveliotou, A&A442, L21–L24 (2005), arXiv:astro-ph/0508237. 3. N.R.Butler,andD.Kocevski,ApJ663,407–419(2007),arXiv:astro-ph/0612564. 4. 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