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January 29, 2014 1:25 WSPC/INSTRUCTION FILE ws-ijmpcs International JournalofModernPhysics:ConferenceSeries (cid:13)c WorldScientificPublishingCompany 4 1 0 2 n ON THE PUZZLE OF LONG AND SHORT GAMMA-RAY BURSTS a J 8 2 LANGSHAO1,2,3,FU-WENZHANG2,3,4,ZI-GAODAI5,6,ZHI-PINGJIN2,3,JING-ZHI YAN2,3,DA-MINGWEI2,3 E] 1Department of Physics, Hebei Normal University,Shijiazhang 050016, China;[email protected] 2Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China H 3KeyLaboratory of Dark Matterand Space Astronomy, Chinese Academy of Sciences, Nanjing . 210008, China h 4College of Science, Guilin Universityof Technology, Guilin 541004, China p 5School of Astronomy and Space Sciences, Nanjing University,Nanjing 210093, China - o 6Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University),Ministry of r Education, Nanjing 210093, China t s a ReceivedDayMonthYear [ RevisedDayMonthYear 1 v Inthispaperwegiveabriefreviewofourrecentstudiesonthelongandshortgamma-ray 3 bursts(GRBs)detectedSwift,inanefforttounderstandthepuzzleofclassifyingGRBs. 6 We consider that itis stillan appealing conjecture that both longand shortGRBs are 1 drawnfromthesameparentsamplebyobservational biases. 7 . Keywords: gamma-rayburst 1 0 PACSnumbers:98.70.Rz 4 1 : 1. Introduction v Xi Before the Swift mission, it was generally accepted that there are two types of gamma-ray bursts (GRBs; long versus short) based on the bimodal distribution r a of their observed prompt duration1. Even though this dichotomous classification scheme used to be consistent with observationaland theoretical consequences, it is 2 found controversialin the Swift era . Here we present the results of our latest studies of both long and short GRBs detected by Swift and address several issues that may help solve the puzzle of classifying GRBs. 2. Distribution of duration First of all, the prompt duration could not be well defined given that (1) the ra- diative distinction between the prompt and afterglow emission could not be well defined, let alone the fact that both the prompt and afterglow emission are very 3 diverse in their observational properties and (2) the duration is strongly depen- dent on the sensitivity and bandpass ofthe detector and, in particular,the prompt 1 January 29, 2014 1:25 WSPC/INSTRUCTION FILE ws-ijmpcs 2 Lang Shao, Fu-WenZhang, Zi-Gao Dai, Zhi-Ping Jin, Jing-Zhi Yan, Da-Ming Wei 4 duration of GRBs with extended emission is debatable . A strong selection effect has been revealed regarding the distribution of the prompt duration by different detectors5,6,7.Thebimodaldistributionisquestionableanditisstillanopenques- tion as to whether there are two types of GRBs. 3. Hardness ratio Thehardnessratiowasconsideredasthespectralindicatorofthedifferencebetween 1 long and short GRBs . However, it has been found that the spectral evolution of the promptemissionshouldbe animportantfactorthat determines the correlation 7 between the hardness ratio and prompt duration , i.e., shorter GRBs tend to be harder and longer ones tend to be softer because of the universality of the spectral evolution of both long and short GRBs3,8. 4. Redshift distribution The redshift distribution has been found different for long and short GRBs. How- ever, there are many obstacles to obtaining a reshift, making it a rare event. The distribution of all GRBs is very close to the so-called Erlang distribution k−1 F(x;k,λ)=1−Xe−λx(λx)n/n!, (1) n=0 withshape paramenterk =2andrateλ=1,knownasthe probabilitydistribution of the waiting time until the second “arrival” in a one-dimensionalPoissonprocess 7 with a given rate (see Fig. 1) . Thepreviouslyconsidereddiscrepancyofredshiftdistributionbetweenlongand shortGRBsmaybecausedbytheobservationalbiases.Fortheredshiftsof14short GRBsinoursample,12ofthemaremeasuredfromthepresumedhostgalaxiesand only 2 of them are measured from their afterglows. For the redshifts of 137 long GRBsinoursample,only10ofthemaremeasuredfromthepresumedhostgalaxies andthe restaremeasurefromtheir afterglows.With asame measurementmethod, 7 long and short GRBs appear to have a similar redshift distribution (see Fig. 1) . 5. Light curve Techniquely, short GRBs are those bursts that have a prompt duration less than 2 seconds as monitored in the Burst Alert Telescope (BAT). However,the composite X-ray light curves including both the prompt and afterglow emission suggest that mostoftheshortGRBsmightalsohaveanintrinsicallylongpromptduration3,7.If a GRB indeed have a short (.2 s) promt duration, the transition from the promt to the afterglow emission in 2 seconds should be recognizable on the composite lightcurve.Unfortunately,this casehasneverbeenclearlyestablishedforthe short GRBs detected so far due to the lack of sufficient data. January 29, 2014 1:25 WSPC/INSTRUCTION FILE ws-ijmpcs Long and Short Gamma-Ray Bursts 3 6. Conclusion We therefore consider that it is still an appealing conjecture that both long and short GRBs detected by Swift are drawn from the same parent sample purely by observationalbiases.Itisyetprematuretohaveadichotomousclassificationscheme based on the prompt duration or any other related observational properties. Acknowledgments This work was supported in part by the National Natural Science Foundation of China(grants11103083,10973041,10921063and11163003)andtheNationalBasic ResearchProgramofChina(No.2007CB815404).F.-W.Z.acknowledgesthesuport by the China Postdoctoral Science Foundation funded project (No. 20110490139), GuangxiNationalScience Foundation (No.2010GXNSFB013050)andthe doctoral researchfoundation of Guilin Univsersity of Technology. References 1. C. Kouveliotou, et al., Astrophy. J. 413 (1993) L101. 2. B. Zhang, Comptes Rendus Physique 12 (2011) 206. 3. L.Shao, Y.-Z.Fan and D.-M. Wei,Astrophy. J. 719 (2010) L172. 4. B. Zhang, et al., Astrophy. J. 655 (2007) L25. 5. T. Sakamoto, et al., Astrophy. J. Suppl. 175 (2008) 179. 6. F. Frontera, et al., Astrophy. J. Suppl. 180 (2009) 192. 7. L.Shao,Z.-G.Dai,Y.-Z.Fan,F.-W.Zhang,Z.-P.JinandD.-M.Wei,Astrophy. J.738 (2011) 19. 8. G. Ghirlanda, G. Ghisellini, L. Nava and D. Burlon, Mon. Not. R. Astron. Soc. 410 (2011) L47. 1.0 0.8 Long Host 1-H1+zLe-z N0.6 Short <L(cid:144)z Long+Short HN 0.4 Long 0.2 0.0 0 2 4 6 8 z Fig.1. Redshiftdistributionof137longand14shortGRBs.Theredshiftdistributionof10long GRBs measured from their host galaxies is shown in gray dashed steps. The solid curve is the Erlangdistribution(seeRef.7).