Submitted to ApJ, 16th of July 2010 Preprint typeset using LATEX style emulateapj v. 08/22/09 THE AFTERGLOWS OF SWIFT -ERA GAMMA-RAY BURSTS. I. COMPARING PRE-SWIFT AND SWIFT ERA LONG/SOFT (TYPE II) GRB OPTICAL AFTERGLOWS ∗ D. A. Kann,1 S. Klose,1 B. Zhang,2 D. Malesani,3 E. Nakar,4,5 A. Pozanenko,6 A. C. Wilson,7 N. R. Butler,8,9 P. Jakobsson,10,11 S. Schulze,1,11 M. Andreev,12,13 L. A. Antonelli,14 I. F. Bikmaev,15 V. Biryukov,16,17 M. Bottcher,¨ 18 R. A. Burenin,6 J. M. Castro Ceron,´ 3,19 A. J. Castro-Tirado,20 G. Chincarini,21,22 B. E. Cobb,23,24 S. Covino,21 P. D’Avanzo,22,25 V. D’Elia,14,26 M. Della Valle,27,28,29 A. de Ugarte Postigo,21 Yu. Efimov,16 P. Ferrero,1,30 D. Fugazza,21 J. P. U. Fynbo,3 M. G˚alfalk,31 F. Grundahl,32 J. Gorosabel,20 S. Gupta,18 S. Guziy,20 B. Hafizov,33 J. Hjorth,3 K. Holhjem,34,35 M. Ibrahimov,33 M. Im,36 G. L. Israel,14 M. Je´linek,20 B. L. Jensen,3 R. Karimov,33 I. M. Khamitov,37 U.¨ Kızıloglu,ˇ 38 E. Klunko,39 P. Kubanek,´ 19 A. S. Kutyrev,40 P. Laursen,3 A. J. Levan,41 F. Mannucci,42 C. M. Martin,43 A. Mescheryakov,6 N. Mirabal,44 J. P. Norris,45 J.-E. Ovaldsen,46 D. Paraficz,3 E. Pavlenko,17 S. Piranomonte,14 A. Rossi,1 V. Rumyantsev,17 R. Salinas,47 A. Sergeev,12,13 D. Sharapov,33 J. Sollerman,3,31 B. Stecklum,1 L. Stella,14 G. Tagliaferri,21 N. R. Tanvir,48 J. Telting,33 V. Testa,14 A. C. Updike,49 A. Volnova,50 D. Watson,3 K. Wiersema,48,51 D. Xu3 Submitted to ApJ, 16th of July 2010 ABSTRACT We have gathered optical photometry data from the literature on a large sample of Swift-era gamma- ray burst (GRB) afterglows including GRBs up to September 2009, for a total of 76 GRBs, and present an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 840 additional new photometry data points on a total of 42 GRB afterglows, including large data sets for GRBs 050319, 050408, 050802, 050820A, 050922C, 060418, 080413A and 080810. We analyzed the light curves of all GRBs in the sample and derived spectral energy distributions for the sample with the best data quality, allowing us to estimate the host galaxy extinction. We transformed the afterglow light curves into an extinction-corrected z = 1 system and compared their luminosities with a sample of pre-Swift afterglows. The results of a former study, which showed that GRB afterglows clustered and exhibited a bimodal distribution in luminosity space, is weakened by the larger sample. We found that the luminosity distribution of the two afterglow samples (Swift-era and pre-Swift) are very similar, and that a subsample for which we were not able to estimate the extinction, which is fainter than the main sample, can be explained by assuming a moderate amount of line-of-sight host extinction. We derived bolometric isotropic energies for all GRBs in our sample, and found only a tentative correlation between the prompt energy release and the optical afterglow luminosity at one day after the GRB in the z = 1 system. A comparative study of the optical luminosities of GRB afterglows with echelle spectra (which show a high number of foreground absorbing systems) and those without reveals no indication that the former are statistically significantly more luminous. Furthermore, we propose the existence of an upper ceiling on afterglow luminosities and study the luminosity distribution at early times, which was not accessible before the advent of the Swift satellite. Most GRBs feature afterglows that are dominated by the forward shock from early times on. Finally, we present the first indications of a class of long GRBs which form a bridge between the typical high- luminosity, high-redshift events and nearby low-luminosity events (which are also associated with spectroscopic SNe) in terms of energetics and observed redshift distribution, indicating a continuous distribution overall. Subject headings: gamma rays: bursts arXiv:0712.2186v4 [astro-ph] 16 Jul 2010 ∗ BASED IN PART ON OBSERVATIONS OBTAINED WITH 5 THE VERY LARGE TELESCOPE UNDER ESO PROGRAM Raymond and Beverly Sackler School of Physics & Astronomy, Tel Aviv University, Tel Aviv 69978, Israel 075.D-0787, PI TAGLIAFERRI. ALSO BASED PARTLY ON 6 OBSERVATIONS MADE WITH THE ITALIAN TELESCOPIO Space Research Institute (IKI), 84/32 Profsoyuznaya Str, NAZIONALE GALILEO (TNG) OPERATED ON THE ISLAND Moscow 117997, Russia 7 Department of Astronomy, University of Texas, Austin, TX ´ OF LA PALMA BY THE FUNDACION GALILEO GALILEI 78712, USA OF THE INAF (ISTITUTO NAZIONALE DI ASTROFISICA) 8 Townes Fellow, Space Sciences Laboratory, University of Cali- AT THE SPANISH OBSERVATORIO DEL ROQUE DE LOS fornia, Berkeley, CA, 94720-7450, USA MUCHACHOS OF THE INSTITUTO DE ASTROFISICA DE 9 Astronomy Department, University of California, 445 Camp- CANARIAS UNDER PROGRAM TAC 1238. bell Hall, Berkeley, CA 94720-3411, USA 1 Th¨uringer Landessternwarte Tautenburg, Sternwarte 5, D– 10 Centre for Astrophysics Research, University of Hertfordshire, 07778 Tautenburg, Germany College Lane, Hatfield, Herts, AL10 9AB, UK 2 Department of Physics and Astronomy, University of Nevada, 11 Centre for Astrophysics and Cosmology, Science Institute, Las Vegas, NV 89154 University of Iceland, Dunhagi 5, IS 107 Reykjavik, Iceland 3 Dark Cosmology Centre, Niels Bohr Institute, University of 12 Terskol Branch of Institute of Astronomy of RAS, Kabardino- Copenhagen, Juliane Maries Vej 30, DK-2100 København Ø, Den- Balkaria Republic 361605 Russian Federation mark 13 International Centre of Astronomical and Medico-Ecological 4 Division of Physics, Mathematics, and Astronomy, California Research of NASU, 27 Akademika Zabolotnoho St. 03680 Kyiv, Institute of Technology, Pasadena, CA 91125, USA Ukraine 2 Kann et al. 1. INTRODUCTION is that early afterglows are not as bright as expected, The study of the optical afterglows of gamma-ray and early optical faintness seems to be the norm rather bursts (GRBs), first discovered over a decade ago than the exception (Roming et al. 2006). Furthermore, (van Paradijs et al. 1997), has taken a great leap for- Swift-era optical afterglows are usually observed to have ward with the launch of the Swift satellite (Gehrels et al. fainter magnitudes than those of the pre-Swift era (e.g., 2004). Its high γ-ray sensitivity and rapid repointing ca- Berger et al. 2005a,b; Fiore et al. 2007) (mainly due to pabilities have ushered in an era of dense early afterglow even faint afterglows often being discovered thanks to observations. In the optical regime, one sobering result the rapid XRT positions), and also lie at higher red- shifts (Jakobsson et al. 2006a; Bagoly et al. 2006), with the most distant up to now at z =8.2 (Tanvir et al. 2009; 14 INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040, Monteporzio Catone (RM), Italy Salvaterra et al. 2009b). For recent reviews of the impact 15 Kazan State University and Academy of Sciences of Tatarstan, of Swift on GRB research, see Zhang (2007), M´esz´aros Kazan, Russia (2006) and Gehrels et al. (2009). 16 Crimean Laboratory of the Sternberg Astronomical Institute, The pre-Swift afterglows have been studied exten- Nauchny, Crimea, 98409, Ukraine 17 SRI “Crimean Astrophysical Observatory” (CrAO), Nauchny, sively, both in terms of their light curve behavior (e.g. Crimea, 98409, Ukraine Panaitescu & Kumar 2002; Zeh et al. 2006, and refer- 18 Astrophysical Institute, Department of Physics and Astron- ences therein), and via their spectral energy distributions omy, Clippinger 339, Ohio University, Athens, OH 45701, USA (e.g., Stratta et al. 2004; Kann et al. 2006; Starling et al. 19 European Space Agency (ESA), European Space Astronomy Centre (ESAC), P.O. Box - Apdo. de correos 78, 28691 Villanueva 2007a), which allow conclusions to be drawn concerning de la Ca˜nada, Madrid, Spain the rest frame line-of-sight extinction and even the dust 20 Instituto de Astrof´ısica de Andaluc´ıa (IAA-CSIC), Apartado type in some cases. In our previous study (Kann et al. de Correos, 3004, E-18080 Granada, Spain 2006, henceforth K06), we found that the afterglows that 21 INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy met our selection criteria typically had little line-of-sight 22 Universit`a degli studi di Milano-Bicocca, Dipartimento di extinction, and that the dust properties were best de- Fisica, piazza delle Scienze 3, 20126 Milano, Italy scribed by Small Magellanic Cloud (SMC) dust, which 23 Department of Astronomy, Yale University, P.O. Box 208101, shows no UV bump and strong FUV extinction (e.g., Pei New Haven, CT 06520, USA 24 Department of Astronomy, 601 Campbell Hall, University of 1992). These results were confirmed by Starling et al. California, Berkeley, CA 94720–3411 (2007a), who studied a smaller sample but also incor- 25 Dipartimento di Fisica e Matematica, Universit`adell’Insubria, porated X-ray afterglow data, as well as Schady et al. via Valleggio 11, 22100 Como, Italy (2007a, 2010), who also employed joint optical-to-X-ray 26 ASI-Science Data Centre, Via Galileo Galilei, I-00044 Fras- cati, Italy fits as well as UVOT (and ground-based) data. 27 INAF, Osservatorio Astronomico di Capodimonte, Salita Almost six years after the launch of Swift, the amount Moiariello, 16 80131, Napoli, Italy of published data on optical/NIR GRB afterglows have 28 European Southern Observatory, Karl Schwarschild Strasse 2, become sufficient to compile a large sample comparable D-85748 Garching bei M¨unchen, Germany 29 International Centre for Relativistic Astrophysics Network, to the pre-Swift sample studied by K06, and to deter- Piazzale della Republica 2, Pescara, Abruzzo, Italy mine if some of the afterglows detected by Swift are truly 30 Instituto de Astrof´ısica de Canarias, C/ V´ıa L´actea, s/n fundamentally different to those of the pre-Swift era.