Understanding the nature of the optical emission in gamma-ray bursts: analysis from TAROT, COATLI, and RATIR observations

We collected the optical light-curve data of 227 gamma-ray bursts (GRBs) observed with the TAROT, COATLI, and RATIR telescopes. These consist of 133 detections and 94 upper limits. We constructed average light curves in the observer and rest frames in both X-rays (from Swift/X-Ray Telescope) and the optical. Our analysis focused on investigating the observational and intrinsic properties of GRBs. Specifically, we examined observational properties, such as the optical brightness function of the GRBs at T = 1000 s after the trigger, as well as the temporal slope of the afterglow. We also estimated the redshift distribution for the GRBs within our sample. Of the 227 GRBs analysed, we found that 116 had a measured redshift. Based on these data, we calculated a local rate of ρ₀ = 0.2 Gpc^-3 yr^-1 for these events with z < 1. To explore the intrinsic properties of GRBs, we examined the average X-ray and optical light curves in the rest frame. We use the AFTERGLOWPY LIBRARY to generate synthetic curves to constrain the parameters typical of the bright GRB jet, such as energy (⟨E₀⟩ ~ 10^53.6 erg), opening angle (⟨θ_core⟩ ~ 0.2 rad), and density (⟨n₀⟩ ~ 10^-2.1 cm^-3). Furthermore, we analyse microphysical parameters, including the fraction of thermal energy in accelerated electrons (⟨ϵ_e⟩ ~ 10^-1.37) and in the magnetic field (⟨ϵ_B⟩ ~ 10^-2.26), and the power-law index of the population of non-thermal electrons (⟨p⟩ ~ 2.2).