Low-luminosity gamma-ray bursts as a distinct GRB population: a firmer case from multiple criteria constraints

Abstract

The intriguing observations of the Swift/Burst Alert Telescope (BAT) X-ray flash (XRF) 060218 and the BATSE-BeppoSAX gamma-ray burst GRB 980425, both with much lower luminosity and redshift compared to other observed bursts, naturally lead to the question of how these low-luminosity (LL) bursts are related to high-luminosity (HL) bursts. Incorporating the constraints from both the flux-limited samples observed with Compton Gamma-ray Observatory (CGRO)/BATSE and Swift/BAT and the redshift-known gamma-ray burst (GRB) sample, we investigate the luminosity function for both LL and HL GRBs through simulations. Our multiple criteria, including the log N − log P distributions from the flux-limited GRB sample, the redshift and luminosity distributions of the redshift-known sample and the detection ratio of HL and LL GRBs with Swift/BAT, provide a set of stringent constraints to the luminosity function. Assuming that the GRB rate follows the star formation rate (SFR), our simulations show that a simple power law (PL) or a broken power-law model of luminosity function fails to reproduce the observations and a new component is required. This component can be modelled with a broken power, which is characterized by a sharp increase in the burst number at around L < 10 47 erg s −1 . The lack of detection of moderate-luminosity GRBs at redshift ∼0.3 indicates that this feature is not due to the observational biases. The inferred local rate, ρ 0 , of LL GRBs from our model is ∼200 Gpc −3 yr −1 at ∼10 47 erg s −1 , much larger than that of HL GRBs. These results imply that LL GRBs could be a separate GRB population from HL GRBs. The recent discovery of a local X-ray transient 080109/SN 2008D would strengthen our conclusion if the observed non-thermal emission has a similar origin as the prompt emission of most GRBs and XRFs.

ON THE CLASSIFICATION OF GRBs AND THEIR OCCURRENCE RATES

“…After describing the observational properties of the above sub-classes, their interpretation within the IGC paradigm, the NS-NS merger scenario and the fireshell scenario, we present some prototypes (see Sections 4,5,6,7,8,and 9,respectively). We then proceed in Section 10 to estimate their observed occurrence rates and to compare and contrast our results with those outlined in the literature (see, e.g., Soderberg et al 2006;Guetta & Della Valle 2007;Liang et al 2007;Virgili et al 2009Virgili et al , 2011Rangel Lemos et al 2010;Wanderman & Piran 2010Kovacevic et al 2014;Sun et al 2015). We then draw some general conclusions in Section 11.…”

“…In Section 10, we introduce the procedure outlined in Sun et al (2015) for estimating the local rates and their evolution with the redshift of the above sub-classes of long and short bursts, assuming no beaming (note: the recent observation of the absence of GeV emission associated to a BdHN may limit this assumption). By ignoring possible redshift-evolution of the GRB sub-classes luminosity functions and assuming that the GRB cosmic event rate density is redshift-independent (e.g., f (z) = 1), the above method duly takes into account observational constraints, i.e., the detector solid angle coverage of the sky Ω and sensitivities which in turn define a maximum volume of observation depending on the intrinsic luminosity of the sources (see Section 10 and Soderberg et al 2006;Guetta & Della Valle 2007;Liang et al 2007;Virgili et al 2009Virgili et al , 2011Rangel Lemos et al 2010;Wanderman & Piran 2010Kovacevic et al 2014;Sun et al 2015 for details). We obtain:…”

THE AFTERGLOWS OFSWIFT-ERA GAMMA-RAY BURSTS. I. COMPARING PRE-SWIFTANDSWIFT-ERA LONG/SOFT (TYPE II) GRB OPTICAL AFTERGLOWS

“…On the other hand, the almost continuous distribution of energy releases may indicate that all events are part of a single population that can be described by a single power-law luminosity function. A deeper analysis is beyond the scope of this paper, but it would require an estimation of the true rate of these events, similar to what has been done for the local universe SN events (Pian et al 2006;Soderberg et al 2006b;Liang et al 2007a;Guetta & Della Valle 2007;Virgili et al 2009). …”

Short versus long gamma-ray bursts: a comprehensive study of energetics and prompt gamma-ray correlations

“…Early hints to the existence of distinct populations of gamma-ray transients (e.g., Mazets et al 1981;Norris et al 1984), and at least two classes of short-hard (Type-I) & long-soft (Type-II) GRBs (Dezalay et al 1992) have now been extensively corroborated and confirmed by the prompt-emission data E-mail: amir@physics.utexas.edu (AS); nemiroff@mtu.edu (RJN) from independent gamma-ray detector missions (e.g., Kouveliotou et al 1993a;Gehrels, Ramirez-Ruiz & Fox 2009; or follow-up observations of the afterglows or host galaxies (e.g., Zhang et al 2009;Berger 2011Berger , 2014. Although the possibility of more than two classes of GRBs with distinct progenitors has been extensively discussed and considered (e.g., Horvath 1998;Mukherjee et al 1998;Hakkila et al 2001;Balastegui, Ruiz-Lapuente & Canal 2001;Hakkila et al 2004b;Horvath et al 2006;Gehrels et al 2006;Chattopadhyay et al 2007;Horvath et al 2008;Virgili, Liang & Zhang 2009;Gao, Lu & Zhang 2010;Horvath et al 2012;Levan et al 2014;Kbori et al 2014), it has remained a matter of debate and speculation to this date (e.g., Hakkila et al 2000cHakkila et al ,a,b, 2003Hakkila et al , 2004aShahmoradi 2013b;Zhang et al 2014;Levan et al 2014).…”