Swift X-ray Afterglows and the Missing Jet Break Problem J. L. Racusin∗, E.-W. Liang†,∗∗, D. N. Burrows∗, A. Falcone∗, D. C. Morris∗, B. B. Zhang† and B. Zhang† 8 0 ∗DepartmentofAstronomy&Astrophysics,ThePennsylvaniaStateUniversity,525DaveyLab, 0 UniversityPark,PA16802 2 †DepartmentofPhysics,UniversityofNevada,LasVegas,NV89154 n ∗∗DepartmentofPhysics,GuangxiUniversity,Nanning530004,China a J 0 Abstract. WepresentasystematicsurveyofthetemporalandspectralpropertiesofallGRBX-ray 3 afterglows observed by Swift-XRT between January 2005 and July 2007. We have constructed a catalogofalllightcurvesandspectraandinvestigatethephysicaloriginofeachafterglowsegment ] intheframeworkoftheforwardshockmodelsbycomparingthedatawiththeclosurerelations.We h searchforpossiblejet-likebreaksinthelightcurvesandtrytoexplainsomeofthe"missing"X-ray p - jetbreaksinthelightcurves. o r Keywords: g -raybursts,X-rays t PACS: 98.70.Rz,95.85.Nv,95.55.Ka s a [ INTRODUCTION 1 v 9 Studies of the presence or absence of jet breaks in GRB X-ray afterglows have been 4 7 recently undertaken with several different approaches yielding differing results [1, 2, 4 3, 4]. The importance of this work lies in that the results have vital implications on . 1 the energetics, geometry, and frequency of GRBs. The fact that they do not behave 0 as expected from pre-Swift observations is not surprising in the context of how much 8 we do not understand and have only recently learned about all of the aspects of X-ray 0 : afterglows. To understand the jet break phenomena we must understand it in the global v i context of GRB and afterglow properties. Therefore, the goal of our study is to do a X census of X-ray afterglow properties by fitting a variety of physical models to each r a componentof theafterglows and understand how the jet breaks fit in as one component inthelargercoherent pictureofthisphenomenon. ANALYSIS Our sample consists of all GRBs observed by Swift-XRT between January 2005 and July 2007 with enough counts to make and fit light curves and spectra. Our resulting sample contains 212 X-ray afterglows, 14 of which were not originally discovered by Swift-BAT, and 80 of which have redshifts. We created light curves for each afterglow using the Penn State XRT light curve tools, removing all significant flares, and fit them topower-lawsand(multiply-)brokenpower-laws. We attempt to categorize these light curves in the context of the canonical model [5,6].Thecanonicalmodelcontains5segments;I:theinitialsteepdecay oftenreferred toasthehigh-latitudeemissionorcurvatureeffect [7];II: theplateau whichisbelieved to be due to continuous energy injection from the central engine [8] ; III: the normal decay due to adiabatic evolution of the forward shock [9] ; IV: the post-jet break phase [10, 11] ; V: flares, which are seen in ∼1/3 of all Swift GRB X-ray afterglows and are believedtobecaused bycontinuoussporadicemissionfromthecentralengine[12,13]. Weclassifythelightcurvesdependinguponcriteriaofthenumberofsegmentsandtheir relativedecayindices,leadingtounambiguouscategoriesofsegmentsI-II-III-IVandII- III-IV that contain jet breaks in IV, segments I-II and I-II-III that are apparently pre-jet break with some ambiguity in the segments III, the ambiguous segments II-III/III-IV, andsinglepower-laws.Theambiguousgroupsmaywellcontainmanyofthemissingjet breaksand requirefurtherdistinguishingcriteria. Tofurtherinvestigatethepropertiesofthestraightforwardjetbreaksandtheambigu- ouscases, wecreated spectraforeach ofthesesegmentsofthelightcurvesand fit them toabsorbedpower-laws.Thesetemporalandspectral propertiesareusedinconjunction tocharacterize theafterglows. The closure relations describe the temporal and spectral evolution of the afterglows with dependence on the physical mechanisms at work in the GRB and its environment. We assembled many permutations of the closure relations that depend on the circum- GRB environment, the frequency regime, slow or fast cooling, electron spectral index regime, presence of energy injection, isotropicor collimatedemission, and jet structure from the literature [14, 5, 15, 16, 17]. We applied these relations to each light curve segment, where appropriate, using the compiled temporal and spectral indices. The resulting fits allowed us to distinguish those light curves with potential jet breaks that areconsistentwiththepost-jetbreakclosurerelationsandthosethatarenot.Werequire closure relation consistency between the segments of each light curve and use the corresponding information to eliminate models that cannot appropriately be applied throughout. Unfortunately, due to the large number of possible models, often many relationswereconsistentand furtherdistinguishingcriteriawererequired. RESULTS Using the temporal fit criteria and the closure relations fits, we classified our sample into several categories of potential jet breaks based upon their likelihood of containing jet breaks. Those afterglow light curves that distinctly contain a segment IV that is consistentwithatleastonepost-jetbreakclosurerelationsarecategorizedasProminent jet breaks and constitute ∼ 13% of our total sample. GRB 050315 (shown in the left panel of Figure 1) is an example of a burst in this category. Those ambiguous light curves(segmentsI-II-III,II-III, singlepower-laws)thatareconsistentwithonlypost-jet breakandnotpre-jetbreakclosurerelationsarecategorizedasHiddenjetbreaks,which constitute∼3% or our total sample. The remaining temporally ambiguous light curves that are consistent with both pre- and post-jet break closure relations require further distinction. To further distinguish post-jet break from pre-jet break light curves we compare the GRB 050315 GRB 051008 102 1 10 −1V) (s) −1V) (s) 0.1 −10.0 ke 1 −10.0 ke 10−2 Rate (0.3 0.1 Rate (0.3 Count Count 10−3 10−2 10−3 10−4 10 1 10 2 10 3 10 4 10 5 10 6 10 3 10 4 10 5 10 6 Time since BAT trigger (s) Time since BAT trigger (s) FIGURE 1. Left - Example of Prominent jet break in GRB 050315 with fit showing all 4 segments. Right-Exampleofambiguous2segmentlightcurveforGRB051008withaProbablejetbreakclassified usinga comparisontechnique. relative decay slopes of the apparent II-III transition of the ambiguous sample to the Prominent jet break sample. Though this technique we find that an additional 25% of oursamplecontain apparent jet break transitionslikethat of GRB 051008shown in the rightpanelofFigure1. For those ambiguous light curves that do not contain even the apparent II-III transi- tion, namely the single power-laws, we evaluate their temporal decay slopes and start andstoptimesrelativetotheProminentjetbreaks,findingthatmostofthosewithsteep decays begin during the time frame where we would expect to find jet breaks. There- fore, those that start late and are steep are probably post-jet break and those that start earlyandendearlyareprobablypre-jetbreak.Throughthesecriteriawesuggestthatan additional∼3% ofoursamplecontainjet breaks orarepost-jetbreak. The other logical explanation for not seeing jet breaks for every GRB is that the observationssimplyendtooearly.Weevaluatethisbycalculatingthelasttimeforwhich abreakcouldoccurandbeburiedwithintheerrors.Ifthistimeisinsidethetimeinterval forwhichweexpectajetbreaktooccurbaseduponthebehaviortheProminentsample, then it is feasible that a jet break occurred around or after this time and would still be consistent with expectations. We compare these distributions in Figure 2, and find that thesecriteriaare metfor∼80%oftheremainingafterglows. CONCLUSIONS Although the jet break phenomena is not fully understood, we are beginning to be able to better explain the paucity of expected observations of them. We are able to iden- tify a sizeable group of afterglows that likely contain jet breaks or post-jet break data eventhoughtheydonotpresent themselvesintheclassical contextofthefullcanonical model. Due to observational limitations and additional possible physical model varia- tions, even more afterglows may be consistent with the expectations from those that do contain confident jet breaks, but are currently indistinguishable. While we are begin- 30 30 Prominent t Unlikely t tbreak tbreak t last det t last det 20 last possible break 20 last possible break N N 10 10 0 0 30 30 Hidden & Probable Non−Jet B reak 20 20 N N 10 10 0 0 2 4 6 2 4 6 8 2 4 6 2 4 6 8 log t (s) log t /(1+z) (s) log t (s) log t /(1+z) (s) obs obs obs obs FIGURE2. Distributionsofpotentialjetbreaktimes, time oflastdetection,andtimeof lastpossible breakforourcategoriesoflightcurveswith potentialjetbreaksandnon-jetbreaksin the observedand restframeforthosewithredshifts. ning to understand or at least be able to explain the majority of our sample, we are also finding an interesting small subset of outliers that confidently do not contain jet breaks duringthetimeintervalinwhichwewouldexpecttoseethem.Theseafterglowsrequire further investigation and perhaps are somehow fundamentally different in their jet and afterglowproperties. ACKNOWLEDGMENTS JLR,DNB, DCM,and AFacknowledgesupportunderNASAcontract NAS5-00136. REFERENCES 1. 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