A complete sample of bright Swift Long Gamma-Ray Bursts: Sample presentation, Luminosity Function and evolution

📝 Abstract

We present a carefully selected sub-sample of Swift Long Gamma-ray Bursts (GRBs), that is complete in redshift. The sample is constructed by considering only bursts with favorable observing conditions for ground-based follow-up searches, that are bright in the 15-150 keV Swift/BAT band, i.e. with 1-s peak photon fluxes in excess to 2.6 ph s^-1 cm^-2. The sample is composed by 58 bursts, 52 of them with redshift for a completeness level of 90%, while another two have a redshift constraint, reaching a completeness level of 95%. For only three bursts we have no constraint on the redshift. The high level of redshift completeness allows us for the first time to constrain the GRB luminosity function and its evolution with cosmic times in a unbiased way. We find that strong evolution in luminosity (d_l=2.3\pm 0.6) or in density (d_d=1.7\pm 0.5) is required in order to account for the observations. The derived redshift distribution in the two scenarios are consistent with each other, in spite of their different intrinsic redshift distribution. This calls for other indicators to distinguish among different evolution models. Complete samples are at the base of any population studies. In future works we will use this unique sample of Swift bright GRBs to study the properties of the population of long GRBs.

💡 Analysis

We present a carefully selected sub-sample of Swift Long Gamma-ray Bursts (GRBs), that is complete in redshift. The sample is constructed by considering only bursts with favorable observing conditions for ground-based follow-up searches, that are bright in the 15-150 keV Swift/BAT band, i.e. with 1-s peak photon fluxes in excess to 2.6 ph s^-1 cm^-2. The sample is composed by 58 bursts, 52 of them with redshift for a completeness level of 90%, while another two have a redshift constraint, reaching a completeness level of 95%. For only three bursts we have no constraint on the redshift. The high level of redshift completeness allows us for the first time to constrain the GRB luminosity function and its evolution with cosmic times in a unbiased way. We find that strong evolution in luminosity (d_l=2.3\pm 0.6) or in density (d_d=1.7\pm 0.5) is required in order to account for the observations. The derived redshift distribution in the two scenarios are consistent with each other, in spite of their different intrinsic redshift distribution. This calls for other indicators to distinguish among different evolution models. Complete samples are at the base of any population studies. In future works we will use this unique sample of Swift bright GRBs to study the properties of the population of long GRBs.

📄 Content

arXiv:1112.1700v2 [astro-ph.CO] 14 Feb 2012 Draft version March 25, 2022 Preprint typeset using LATEX style emulateapj v. 5/2/11 A COMPLETE SAMPLE OF BRIGHT SWIFT LONG GAMMA-RAY BURSTS I: SAMPLE PRESENTATION, LUMINOSITY FUNCTION AND EVOLUTION R. Salvaterra,1 S. Campana2, S.D. Vergani2,3 S. Covino2 P. D’Avanzo2 D. Fugazza2 G. Ghirlanda2 G, Ghisellini2 A. Melandri2 L. Nava4 B. Sbarufatti2 H. Flores3 S. Piranomonte5 G. Tagliaferri2 Draft version March 25, 2022 ABSTRACT We present a carefully selected sub-sample of Swift Long Gamma-ray Bursts (GRBs), that is com- plete in redshift. The sample is constructed by considering only bursts with favorable observing conditions for ground-based follow-up searches, that are bright in the 15-150 keV Swift/BAT band, i.e. with 1-s peak photon fluxes in excess to 2.6 ph s−1 cm−2. The sample is composed by 58 bursts, 52 of them with redshift for a completeness level of 90%, while another two have a redshift constraint, reaching a completeness level of 95%. For only three bursts we have no constraint on the redshift. The high level of redshift completeness allows us for the first time to constrain the GRB luminosity function and its evolution with cosmic times in a unbiased way. We find that strong evolution in luminosity (δl = 2.3 ± 0.6) or in density (δd = 1.7 ± 0.5) is required in order to account for the obser- vations. The derived redshift distribution in the two scenarios are consistent with each other, in spite of their different intrinsic redshift distribution. This calls for other indicators to distinguish among different evolution models. Complete samples are at the base of any population studies. In future works we will use this unique sample of Swift bright GRBs to study the properties of the population of long GRBs. Subject headings: gamma–ray: burst – stars: formation – cosmology: observations.

1. INTRODUCTION Gamma-ray bursts are powerful flashes of high–energy photons occurring at an average rate of a few per day throughout the Universe. They are detected at all red- shifts, from the local Universe up to the extreme high redshifts (Salvaterra et al. 2009; Tanvir et al. 2009; Cuc- chiara et al. 2011a). Our knowledge of the distribution of long GRBs through cosmic times is still hampered by the fact that most of the observed Swift GRBs are with- out redshift. Indeed, the measure of the distance has been secured for only ∼1/3 of the cases. Given the low completeness level in redshift determination, the effect of possible observational biases could be important in shap- ing their redshift distribution (Fiore et al. 2007). This fact strongly limit the possibility of well grounded sta- tistical studies of the rest-frame properties of long GRBs and their evolution with cosmic time. Therefore, it is of paramount importance to obtain an unbiased complete sample of GRBs, capable to fully represent this class of object. To this end, we present in this paper a well selected sub-sample of the full Swift database. We select bursts that have favorable observing conditions for redshift de- termination from ground and that are bright in the 15- 150 keV Swift/BAT band. We find 58 bursts matching our selection criteria with a completeness level in redshift 1 INAF, IASF Milano, via E. Bassini 15, I-20133 Milano, Italy, ruben@lambrate.inaf.it 2 INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, I-23807 Merate (LC), Italy 3 Laboratoire GEPI, Observatoire de Paris, CNRS-UMR8111, Univ. Paris-Diderot 5 place Jules Janssen, 92195 Meudon, France 4 SISSA, via Bonomea 265, I-34136 Trieste, Italy 5 INAF, Osservatorio Astronomico di Roma, Via Frascati 33, 00040, Monte Porzio Catone, Rome, Italy determination of 90%. The completeness level increases up to ∼95% by considering the redshift constraints im- posed by the detection of the afterglow or host galaxy in some optical filters. Therefore, our selection criteria allow us to construct a sizable sample of long bursts that is (almost) complete in redshift, providing the solid basis for the study of the long GRB population in an unbiased way. In particular, since our selection is based on the brightness in the Swift/BAT band, our sample is not bi- ased against the detection of dark bursts, thus providing a complete description of the whole long GRB popula- tion. In the present paper, we will take advantage of the high completeness level of our sample to constrain the GRB luminosity function (LF) and its evolution with cosmic time. In the last few years, this problem has been faced by many different authors ((e.g. Porciani & Madau 2001, Firmani et al. 2004, Guetta et al. 2005, Natarajan et al. 2005, Daigne et al. 2006, Salvaterra & Chincarini 2007, Salvaterra et al. 2009b, Butler et al. 2010, Wanderman & Piran 2010, Campisi et al. 2010, Qin et al. 2010, Virgili et al. 2011, Robertson & Ellis 2012). There is a general agreement about the fact that GRBs must have experienced some sort of evolution through cosmic time, whereas the nature and the

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