DraftversionFebruary5,2008 PreprinttypesetusingLATEXstyleemulateapjv.04/03/99 IDENTIFICATION OF TWO CATEGORIES OF OPTICALLY BRIGHT γ-RAY BURSTS Enwei Liang1,2 and Bing Zhang1 1Department ofPhysics,UniversityofNevada,LasVegas,NV89154, USA Email:[email protected];[email protected] 2PhysicsDepartment, GuangxiUniversity,Nanning530004,P.R.China Draft version February 5, 2008 ABSTRACT We present the results of a systematical analysis of the intrinsic optical afterglow light curves for a 6 complete sample of gamma-ray bursts (GRBs) observed in the period from Feb. 1997 to Aug. 2005. 0 These lightcurvesaregenerallywell-sampled,with atleastfour detections in theR band. The redshifts 0 of all the bursts in the sample are available. We derive the intrinsic R band afterglow lightcurves 2 (luminosity versus time within the cosmic proper rest frame) for these GRBs, and discover a fact that n theyessentiallyfollowtwouniversaltracksafter2hourssincetheGRBtriggers. Theopticalluminosities a at 1 day show a clear bimodal distribution, peaking at 1.4×1046 ergs s−1 for the luminous group and J 5.3×1044 ergs s−1 forthe dimgroup. About75%oftheGRBs areinthe luminous group,andthe other 0 25% belong to the dim group. While the luminous group has a wide range of redshift distribution, the 1 bursts in the dim group all appear at a redshift lower than 1.1. Subject headings: gamma rays: bursts—gamma rays: observations—methods: statistical 2 v 0 1. INTRODUCTION more than 70 optically-bright GRBs have been detected, 1 amongwhich44burstshavewell-sampledlightcurvesand 5 Gamma-raybursts(GRBs)arebelievedtobethebright- redshift measurements (§2). In this Letter we present a 8 est electromagnetic explosions in the universe after the systematical analysis to these 44 optical afterglow light 0 identification of their cosmic origin (Metzger et al. 1997). curves in the cosmic rest frame. We find a fact that 5 Twocategoriesoftheseerratic,transienteventshavebeen their late-time lightcurves follow two apparent universal 0 identified, i.e. long-soft and short-hard (Kouveliotou et / al. 1993). The association of long GRBs with very en- tracks (§3). We then conclude that within the optically h bright GRBs there exist two sub-categories, the luminous p ergetic core-collapse supernovae has now been well estab- group and the dim group (§4). Cosmological parameters o- leitshael.d1(9G9a9l;amStaaneetkale.t1a9l9.82;0M03a;cHFajodrytehneettaall..21090939;;TBhlooomm- ΩM = 0.3, ΩΛ = 0.7, and H0 = 71 km Mpc−1 s−1 have r been adopted throughout this Letter. t senetal. 2004;Malesanietal. 2004). SeveralshortGRBs s have been localized and observed by Swift and HETE- :a 2 recently, which are found to reside in nearby galaxies, 2. DATA v some of which are of early-type with little star formation We make a complete search from the literature for the i X (Gehrels et al. 2005; Fox et al. 2005; Villasenor et al. R-bandafterglowlightcurvesdetectedduringthetimepe- 2005; Hjorth et al. 2005a; Barthelmy et al. 2005; Berger riodfromFeb. 1997toAug. 2005. WeobtainaGRBsam- r a etal. 2005). Thisindicatesthattheyhaveadistinctorigin ple with 44 GRBs, which is tabulated in Table 11. These from the long species. Most of the well localized GRBs, light curves have at least four detections in the R-band. both long and short, are followed by long-lived, decaying Theredshiftsoftheburstsareavailable. Wecollectthefol- afterglows in longer wavelengths (Costa et al. 97; van lowingdatafortheseburstsfrompublishedpapersorfrom Paradijset al. 1997;Frailet al. 1997;Gehrels et al. 2005; GCN reports if the former are not available2, i.e. redshift Fox et al. 2005). Long GRBs have been themselves clas- (z),R-bandmagnitude,spectralindex(β), andextinction sified into two groups,optically bright andoptically dark, by the host galaxy (A ). For those bursts whose β and V basedonwhetherornotanopticaltransientisdetectedto A are not available, we take β = 0.75, the mean value V a given brightness limit at a given time delay (e.g. Groot of β in our sample, and A = 0. Galactic extinction cor- V et al. 1998; Fynbo et al. 2001; Berger et al. 2002; Jacob- rection is made by using a reddening map presented by sson et al. 2004; Rol et al. 2005). The origin of optically Schlegel et al. (1998). The extinction curve of the Milky dark GRBs is still unclear. Very early, tight upper limits Way3(Pei 1992) is adopted to calculate the extinction in madebytheSwiftUV-OpticalTelescopeindicatethatthe the local frame of the GRB host galaxy. The k-correction darkness is not caused by observational biases (Roming in magnitude is calculated by k = −2.5(β−1)log(1+z). et al. 2005). Based on X-ray afterglow data, a tentative Forlatetimedata,possiblefluxcontributionfromthehost bimodal distribution of X-ray luminosities has been also galaxy is subtracted. noticed (Bo¨er & Gendre 2000; Gendre & B¨oer 2005). Over more than 8 years of optical afterglow hunting, 3. THEBIMODALLUMINOSITYEVOLUTIONS 1AfullversionoftheGRBsamplewithreferencestotheobservationaldataareavailableintheelectronicversion 2Wecollecttheβ andtheextinctionAV ofeachburstfromthesameliteraturetoreducetheuncertaintiesintroducedbydifferentauthors. 3Wealsotriedothertypes ofextinction curves,andfoundthatourresultsareinsensitivetotheextinctionmodeladopted. 1 2 We convert the corrected magnitudes to fluxes (Fc) by vations were made around this epoch. This makes the using the photometric zero points given by Fukugita et luminosity derivations more reliable. Figure 2 shows the al. (1995). The luminosity at the cosmic proper time 2-dimensional distribution of the intrinsic R-band lumi- t′, LR(t′), is calculated by LR(t′) = 4πDL2(z)Fc, where nosity at 1 day5, LR,1d, versus Eγ,iso (panel a), and the D (z) is the luminosity distance at z. The luminosity distributions of the two quantities, respectively (panels b L error is calculated by ∆logL = {0.16(∆R2+∆A2 )+ and c). Flux thresholds in both the γ-ray and the op- R R′ tical bands introduce selection effects against low-energy, [∆βlog(1+z)]2}1/2,where∆Ristheobserveduncertainty low-luminosity bursts, and these are indicatively marked of the R band magnitude, ∆A ′ is the uncertainty of the R as the grey regions in Figure 2. There are three most hostgalaxyextinctionatthecosmicrestframewavelength prominent outliers whose light curves deviate from the λR′ = λR/(1+z), and [∆βlog(1+z)] is the error of the universal light curves, i.e. GRBs 970508, 030226, and k-correction. ′ ′ 050408. They are excluded in the statistical analyses (see The intrinsic R-band light curves [L (t) vs. t] are R more detailed discussionin §4). While the E distribu- displayed in Figure 1 for 42 bursts. The two nearby γ,iso tion displays a power-law with sharp cutoff around 1051.5 GRBs, 980425 and 031203 are not included, since their ergs (due to the selection effect), logL shows a well- light curves are significantly contaminated by the under- R,1d defined bimodal distribution, which is well fitted by a two lying supernova component (Galama et al. 1998; Thom- Gaussian model centered at logL / 1 erg s−1 = 44.66 sen et al. 2004). It is found that although the light c,1 ′ with σ1 = 0.41 and logLc,2/ 1 erg s−1 = 46.15 with curves at t < 0.1 days vary significantly, they are clus- σ = 0.77. The bimodality is at a confidence level of ′ 2 tered and follow two apparent universal tracks at t >0.1 3σ tested by a classification algorithm with the mini- days, indicating that within the optically bright GRBs mumEuclidiandistance discriminantandthe KMMalgo- there exist two well-separated sub-categories. The major- rithm (Ashman et al. 1994). A bootstrap test (105 boot- ity of the bursts (∼75%)comprises anoptically luminous strap samples) shows that the distributions of the means GRBgroup,whichincludesthewell-studiedGRBssuchas of logL of the two groups and their covariance (c) R,1d 030329,990123,and990510. Itisinterestingthatalthough are normal, which gives logL / 1 erg s−1 = 44.72+0.36, the isotropic gamma-ray energy (E ) of GRB 990123 c,1 −0.36 γ,iso logL / 1 erg s−1 = 46.15+0.14, and c = 0.11+0.16 at 3σ and GRB 030329 differ by almost 2 orders of magnitude, c,2 −0.20 −0.06 their late optical afterglow luminosities are similar4. The significancelevel. Theseresultsindicatethatthebimodal- ity is not due to statistical fluctuations. other∼25%GRBsinourGRBsamplecomprisesthe dim In order to further examine the bimodal distribution group, with the representative bursts being GRBs 021211 and 041006. We zoom in these light curves in the time at different epoches, we also derive the distributions at ′ regime from 0.1 days to 10 days in the inset of Figure 1. logt /1 day = −0.5 and 0.5, respectively. We find that ′ The bimodal lightcurve trajectories during this are more the distributionofthe luminosities atlogt /1 day=0.5is clearly visible. Based on the separation of the two groups bimodalwitha3σsignificancelevel. Thebimodalityofthe by the luminosity at 1 day (logLR,1d/erg cm−2 = 45.15, luminosity distribution at logt′/1 day=−0.5 has a lower see Figure 2) and adopting a typical temporal decay in- (i.e. 2σ) statistical significance. Nonetheless, the distri- dex ∼−1.2, we draw a division line for the two groups as bution still stands with a gap at logL /erg s−1 = 45.5. R ′ logL =45.15−1.2logt (the dashedlineinFigure1). It The lower significance is expected, because of the various R isfoundthat25(outof34)and7(outof10)lightcurvesin factors(e.g. reverseshock,earlyinjection,etc)concerning theluminousanddimgroups,respectively,coverthistime the early afterglows. regime and do not cross over the division line. They are themostrepresentative(withthesmallestscatter)onesin 4. CONCLUSIONSANDDISCUSSION both groups. The bursts in the luminous group are typi- We have derived the intrinsic R band afterglow cally brighter than those in the dim group by a factor of lightcurves within the cosmic proper rest frame with a ∼30. completed sample observedfrom Feb. 1997 to Aug. 2005. We read off or extrapolate/interpolate the luminosity These light curves follow two apparent universal tracks at a given epoch from the light curves, and perform rig- after 2 hours since the GRB triggers. The optical lumi- orous statistics to access the bimodality of our sample. nosity at 1 day clearly shows a bimodal distribution, with We first select the intrinsic luminosity at 1 day for our the peak luminosities being 1.4×1046 ergs s−1 for the lu- purpose. Our consideration is two folds. First, the early minous group and 5.3×1044 ergs s−1 for the dim group. optical light curves may have contributions from the re- One interesting feature for the dim group is that these verseshockcomponentoradditionalenergyinjectionfrom bursts all appear to have low redshifts. It has been pre- the central engine. The optical band may be below the viously speculated that nearby GRBs might be different cooling frequency or even below the typical synchrotron from their cosmological brethren (Norris 2002; Soderberg frequency so that the flux sensitively depends on many et al. 2004; Guetta et al. 2004). In our sample, the two unknown shock parameters. On the other hand, the late well-known nearby GRBs, 980425 and 031203, both be- emission is fainter and may contain luminosity contami- long to the dim group. Except GRB 980613 (z = 1.096) nation from the host galaxy. Second, most of the obser- and GRB 021211 (z = 1.006), other bursts in the dim 4WenoticethatNardinietal. (2005)independently obtained thesameresultduringtheprocesswhenourpaperwasbeingreviewed. 5InviewofthedifficultyofsubtractingthesupernovacontributionfromGRB980425(Galamaetal. 1998)andGRB031203(Thomsenetal. 2004),weusethefirsttwodatapoints(whicharearound1day)ineachburst’slightcurvetoderivetheupperlimitsoftheirluminositiesat1 day,bothgiving∼7×1043ergs−1. TheGalacticextinctioncorrectedluminositiesare8.3×1043ergs−1forGRB980425and9.2×1044ergs−1 forGRB031203. 3 group all have z < 1. Besides the low-z property, the effect is due to the sideways expansion of the jet, which bursts in the dim groupallhave an isotropicγ-rayenergy significantlyreducestheopticalluminosity(Rhoads1999). muchlowerthanthatofthe burstsintheluminousgroup. The two apparent universal lightcurve tracks at later They also have simple lightcurves. All the bursts in the times are intriguing. It is widely believed that after- dimgrouphaveasinglegamma-raypulse,exceptforGRB glows are synchrotron emission from shocked circumburst 990712whohastwowell-separatedpulses. We noticethat medium as the fireball is decelerated (M´esza´ros & Rees the observed R-band magnitudes for the dim GRBs are 1997; Sari et al. 1998; see also reviews by M´esza´ros generally ∼ (21−22.5) mag a few days after the trigger. 2002, Zhang & M´esza´ros 2004, Piran 2005). At a late Although a burst with log(L /erg s−1)= 44.72 (the typ- enough epoch, the optical band may be above both the R ical 1-day optical luminosity for the dim group) should typical synchrotron frequency and the synchrotron cool- be detected up to z = 2.4 for an observation threshold of ing frequency. In such a spectral regime and at a par- ′ R ∼ 22.5 mag, the efficiency to detect optical transients ticular epoch (e.g. t = 1 d), the optical luminosity fainter than R ∼ 21 is dramatically reduced. The obser- L ∝E(p+2)/4ǫp−1ǫ(p−2)/4, where E is the isotropic vational bias for the deficit of high-redshift, optical-dim R,1d k,iso e B k,iso kinetic energy of the fireball, ǫ and ǫ are shock energy e B GRBs thus cannot be ruled out. equipartition factors for electrons and magnetic fields, re- The extinction effects have been carefully taken into spectively, and p is the electron spectral index. We can account. The data indicate that the dim GRBs do not see that L is medium-density-independent, and only R,1d exhibit significantly higher extinction than the luminous weakly depends on ǫ . The universal afterglow luminos- B ones. It has been suggested that dust in the host galaxy ity thereforesuggeststhatbothE andǫ arestandard k,iso e may be destroyed by early radiation from γ-ray bursts values around 1 day for eachsubclass. A standardǫ sug- e and their afterglows (Waxman et al. 2000; Fruchter et gests universal properties of relativistic shocks. A stan- al. 2001). It is found that the optical extinctions are dard E , on the other hand, is intriguing, since E k,iso γ,iso 10−100timessmallerthanwhatareexpectedfromtheX- varyfor4ordersofmagnitudeamonglongdurationGRBs rayabsorption(Galamaetal. 2001),andthatthedimness and they generally follow a power-law distribution with a of GRB 021211, a representative burst in our dim group, cutoff at low luminosity end (Schmidt 2001,Norris 2002). could not be explained by the extinction effect (Holland They become standard only when jet beaming correction et al. 2004). The apparentbimodality thereforecouldnot is taken into account (Frail et al. 2001). Our results are be interpreted by the extinction effect. Our results then consistentwiththepicturethatGRBswithahigherE γ,iso suggest that there might be two types of progenitors or tends to have a higher γ-ray emission efficiency (Lloyd- two types of explosion mechanisms in operation. Ronning et al. 2004). The E derived using 10-hour k,iso Some GRBs show an initial shallow decay before land- X-ray data requires a jet beaming correction to achieve a ing onto the luminous branch. GRB 970508 is the most standard value (Berger et al. 2003). The early X-ray af- prominent one. The light curve is initially almost flat be- terglows in the cosmic proper frame for a group of GRBs fore re-brightening at about 0.5 days, peaks at 1 day, and observed with the Swift X-Ray Telescope indicate a large eventuallysettlesontotheluminousbranch,althoughwith scatterofE atearlytime(Chincarinietal. 2005). Our k,iso significantfluctuations (Pedersenetal. 1998). Thesefluc- resultsthereforesuggestapossibleevolutionofE with k,iso tuations are similar to those observed in GRBs 000301C, time. One scheme might be that GRB jets are initially 021004,and030329. Theinitialshallowdecayandfluctua- structured (Zhang & M´esza´ros 2002; Rossi et al. 2002), tionsarethoughttobeduetoadditionalenergyinjections and the early γ-ray and X-ray properties are sensitive to during the afterglow phase (Dai & Lu 2001; Bj¨ornsson the observer’s viewing angle. The jet structure tends to et al. 2004; Fox et al. 2003; Zhang et al. 2005). GRBs smear out with time, so that at later times, the outflow is 050408and050319havethesimilarbehavior. Wheninjec- moreisotropicandtheviewingangleeffectnolongerplays tion isessentially over,the totalafterglowkinetic energies an essential role. of these bursts are similar to those of the bursts in the We appreciate constructive comments from the refer- luminous group. Therefore they should be classified into ees during the reviewing process of this paper both in the luminous group. Another type of outliers are those ApJ Letters and Nature. This work is supported by lightcurveswithasharprapiddecayatearlytimes. GRB NASA under grants NNG05GB67G, NNG05GH92G, and 030226isthemostprominentoneinoursample. Thismay NNG05GH91G, as well as the National Natural Science be attributed by an early jet break, and the rapid decay Foundation of China (No. 10463001). 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ThedashedlineisadivisionofthetwogroupsofGRBs, logLR=45.15−1.2logt′. Theupperinsetzoomsinthelightcurvesinthetimeregimefrom0.1daysto10days. Thoseburstsmarkedwith bluecolorinthefigurelegendbelongtothedimgroup. 6 8 (b) 7 6 5 N 4 3 s 2 e l e 1 c t i 505 o n (a) (c) e 54 f f e c 53 t g r e /o 52 s γ,i E 51 g o L 50 031203 49 020903 selection effect 980425 48 47 43 44 45 46 47 0 2 4 6 8 10 12 L -1 N Log /erg s R,1d Fig. 2.—The2-dimensionaldistributionofLR,1d andEγ,iso (panel a), aswellasthedistributionsofbothquantities (panels bandc)for the bursts in our sample. The significant outliers, GRBs 030226, 970508, and 050408 have been excluded. The Eγ,iso has been corrected to the band pass 20−2000 keV in the rest frameaccording to the spectral parameters of prompt gamma-rayemission. The circled-crosses are the means of the two quantities for the two groups (excluding those bursts with limits). The grey area marks the parameter region in whichtheflux-thresholdselectioneffect playsadominantrole. Thedotted lineinpanel (b)isthebestfitusingatwoGaussianmodel. The perpendiculardotted-lineistheseparationbetween thedimandtheluminousgroupsinthetwoGaussianmodel. 7 Appended below is the full version of Table 1 with references to the observational data. It is available in the electronic version in ApJ Letters. GRBa z β(∆β) A (∆A ) Refb V,host V,host 970228 0.695 0.780(0.022) 0.5 1;2;2-3 970508 0.835 1.11 0 4;5;5-6 971214 3.42 0.87(0.13) 0.43 (0.08) 7;8;8-9 980326 1.0 0.8(0.4) 0 10;10;10-11 980425 0.0085 - - 12;-;13 980613 1.096 0.60 0.45 14;15;15 980703 0.966 1.013 (0.016) 1.50 (0.11) 16;17;17-20 990123 1.6004 0.750 (0.068) 0 21;22;22-24 990510 1.6187 0.55 0 25;26;26-28 990712 0.434 0.99 (0.02) 0 25;29;29-30 991208 0.706 0.75 0 31;32;32 991216 1.02 0.60 0 33;32;32,34 000131 4.5 0.70 0.18 35;35;35 000301C 2.03 0.70 0.09 36;37;37 000418 1.118 0.75 0.96 38;39;39 000911 1.058 0.724(0.006) 0.39 40;41;41-42 000926 2.066 1.00(0.18) 0.18(0.06) 43;44;44 010222 1.477 1.07 (0.09) 0 45;46;46 011121 0.36 0.80(0.15) 0 47;48;48 011211 2.14 0.56(0.19) 0.08(0.08) 49;50;51-54 020124 3.198 0.91 (0.14) 0 55;55;55-56 020405 0.69 1.43(0.08) 0 57;58;58-59 020813 1.25 0.85(0.07) 0.14(0.04) 60;61;61-62 020903 0.25 - - 63;-;63 021004 2.335 0.39 0.3 64;65;65-66 021211 1.01 0.69 0 67;68;68-70 030226 1.98 0.70(0.03) 0 71;72;72-73 030323 3.372 0.89(0.04) <0.5 74;74;74 030328 1.52 - - 75;-;76-83 030329 0.17 0.5 0.30(0.03) 84;85;85-87 030429 2.65 0.75 0.34 88;89;89 030723 2.10 1.0 0.4 90;90;90 031203 0.105 - - 91;-;92 040924 0.859 0.70 (0) 0.16 93;94;94-101 041006 0.716 0.55 0 102;102;102 050315 1.949 - - 104;-;105-107 050319 3.24 - - 108;-;109-115 050401 2.90 - - 116;-;117-121 050408 1.24 - - 122;-;123-128 050502 3.793 - - 129;-;130 050525 0.606 0.97(0.10) 0.25(0.16) 131;132;132-133 050730 3.97 - - 134;-; 134-141 050820 2.615 - - 142;-;143-148 Notes: a GRBs marked as bold font belong to the low-optical-luminosity group; others belong to the high-optical-luminosity group. b References: three groupsof references separatedby semicolonsare for z; β and host galaxyextinction; light curve data, respectively. 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