Abstract
Aims.Long gamma-ray bursts (LGRBs) have been shown to be powerful probes of the Universe, in particular for studying the star formation rate up to very high redshift (z ∼ 9). Since LGRBs are produced by only a small fraction of massive stars, it is paramount to have a good understanding of their underlying intrinsic population in order to use them as cosmological probes without introducing any unwanted bias. The goal of this work is to constrain and characterise this intrinsic population.Methods.We developed a Monte Carlo model where each burst is described by its redshift and its properties at the peak of the light curve. We derived the best fit parameters by comparing our synthetic populations to carefully selected observational constraints based on the CGRO/BATSE,Fermi/GBM andSwift/BAT samples with appropriate flux thresholds. We explored different scenarios in terms of the cosmic evolution of the luminosity function and/or of the redshift distribution as well as including or not the presence of intrinsic spectral-energetics (Ep − L) correlations.Results.We find that the existence of an intrinsicEp − Lcorrelation is preferred but with a shallower slope than observed (αA ∼ 0.3) and a larger scatter (∼0.4 dex). We find a strong degeneracy between the cosmic evolution of the luminosity and of the LGRB rate, and show that a sample both larger and deeper than SHOALS by a factor of three is needed to lift this degeneracy.Conclusions.The observedEp − Lcorrelation cannot be explained only by selection effects although these do play a role in shaping the observed relation. The degeneracy between the cosmic evolution of the luminosity function and of the redshift distribution of LGRBs should be included in the uncertainties of star formation rate estimates; these amount to a factor of 10 atz = 6 and up to a factor of 50 atz = 9.
Highlights
Gamma-ray bursts (GRBs) are the most powerful electromagnetic phenomena and are associated with the relativistic ejection following the birth of a stellar mass compact object
The first result is that for a population with a non-evolving luminosity function and a constant Long gamma-ray bursts (LGRBs) efficiency, we cannot find a set of parameters that yield a good fit to the data
Authors usually assume a non-evolving luminosity function; as we have shown in Sect. 5.3, there is a strong degeneracy between the cosmic evolution of the LGRB efficiency and the LGRB luminosity function, which cannot be lifted with the current sample sizes
Summary
Gamma-ray bursts (GRBs) are the most powerful electromagnetic phenomena and are associated with the relativistic ejection following the birth of a stellar mass compact object (black hole or magnetar; see e.g., Piran 2005; Kumar & Zhang 2015). Among the various classes of GRBs, the case of long GRBs (LGRBs, which have a duration of the prompt emission longer than ∼2 s; Mazets et al 1981; Kouveliotou et al 1993) is the most promising for the study of the distant Universe These are the most frequent GRBs, and both theoretical progenitor models (‘collapsar’ model; Woosley 1993; Paczynski 1998) and observations have firmly associated them with the collapse of certain massive stars. They generally occur in faint, blue, low-mass star-forming galaxies (Le Floc’h et al 2003; Savaglio et al 2009; Palmerio et al 2019) and in the brighter regions of their hosts (Fruchter et al 2006; Svensson et al 2010; Blanchard et al 2016; Lyman et al 2017). Due to the short-lived nature of their massive star progenitors, LGRBs are expected to occur up to very high redshift, possibly in association with the first generation of stars (Bromm & Loeb 2006), and can be used as lighthouses to study galaxies (e.g., Le Floc’h et al 2006; Perley et al 2013; Vergani et al 2017) and the intergalactic medium at high redshift
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