Resolving the excess of long GRB’s at low redshift in the Swift era

Abstract

Utilizing more than 100 long gamma-ray bursts (LGRBs) in the Swift -Ryan-2012 sample that includes the observed redshifts and jet angles, Le & Mehta performed a timely study of the rate density of LGRBs with an assumed broken power-law GRB spectrum and obtained a GRB-burst-rate functional form that gives acceptable fits to the preSwift and Swift redshift, and jet angle distributions. The results indicated an excess of LGRBs at redshift below z ∼ 2 in the Swift sample. In this work, we are investigating if the excess is caused by the cosmological Hubble constant H 0 , the gamma-ray energy released E * γ , the low-and high-energy indices (α, β) of the Band function, the minimum and maximum jet angles θ j,min and θ j,max , or that the excess is due to a bias in the Swift -Ryan-2012 sample. Our analyses indicate that none of the above physical parameters resolved the excess problem, but suggesting that the Swift -Ryan-2012 sample is biased with possible afterglow selection effect. The following model physical parameter values provide the best fit to the Swift -Ryan-2012 and preSwift samples: the Hubble constant H 0 = 72 kms −1 Mpc −1 , the energy released E * γ ∼ 4.47 × 10 51 erg, the energy indices α ∼ 0.9 and β ∼ −2.13, the jet angles of θ j,max ∼ 0.8 rad, and θ j,min ∼ 0.065 and ∼ 0.04 rad for preSwift and Swift, respectively, s ∼ −1.55 the jet angle power-law index, and a GRB formation rate that is similar to the Hopkins & Beacom observed star formation history and as extended by Li. Using the Swift Gamma-Ray Burst Host Galaxy Legacy Survey (SHOALS) Swift -Perley LGRB sample and applying the same physical parameter values as above, however, our model provides consistent results with this data set and indicating no excess of LGRBs at any redshift.

How unbiased statistical methods lead to biased scientific discoveries: A case study of the Efron-Petrosian statistic applied to the luminosity-redshift evolution of Gamma-Ray Bursts

“…However, such a modeling approach is impossible with the original method of Efron-Petrosian and requires parametric modeling of the luminosity/energetics and redshift distribution as well as the detection threshold. Le, Ratke & Mehta (2020) uses purely parametric methods to determine the GRB formation rate ρ(z). They find that there is no deviation from the SFR at any redshift for the complete unbiased LGRB Swift-Perley and Swift-Ryan-b samples.…”

The Cosmic History of Long Gamma-Ray Bursts

“…The general trend of the GRB cosmic rate at low redshifts shows a decrease in the efficiency of GRBs being produced with decreasing redshift (Figure 3). This result argues against the claims for an increase in the LGRB efficiency toward lower redshifts (the so-called low-redshift excess- Yu et al 2015;Petrosian et al 2015;Tsvetkova et al 2017;Lloyd-Ronning et al 2019; but see also Pescalli et al 2016;Le et al 2020;Bryant et al 2021) obtained through nonparametric methods. The low-redshift GRB uptick could be partially absorbed if the jet opening angle evolves with redshift (Lloyd-Ronning et al 2020).…”

Does the Redshift Distribution of <i>Swift</i> Long GRBs Trace the Star-Formation Rate?

“…They also created a Monte Carlo simulation, which they found to agree with the Monte Carlo simulations conducted by [23]; in other words, excess GRBs resulted from incomplete datasets. The investigation by [25] also found no excess GRBs at any redshift.…”