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Energetic GeV photons expected from the closest and the most energetic Gamma-ray bursts (GRBs) provide a unique opportunity to study the very-high-energy emission as well as the possible correlations with lower energy bands in realistic GRB afterglow models. In the standard GRB afterglow model, the relativistic homogeneous shock is usually considered to be fully adiabatic, however, it could be partially radiative. Based on the external forward-shock scenario in both stellar wind and constant-density medium, we present a radiative-adiabatic analytical model of the synchrotron self-Compton (SSC) and synchrotron processes considering an electron energy distribution with a power-law index of |$1\lt p\lt 2$| and |$2\le p$|⁠. We show that the SSC scenario plays a relevant role in the radiative parameter |$\epsilon$|⁠ , leading to a prolonged evolution during the slow cooling regime. In a particular case, we derive the Fermi /LAT light curves together with the photons with energies |$\ge 100$|  MeV in a sample of nine bursts from the second Fermi /LAT GRB catalogue that exhibited temporal and spectral indices with |$\gtrsim 1.5$| and |$\approx 2$|⁠ , respectively. These events can hardly be described with closure relations of the standard synchrotron afterglow model, and also exhibit energetic photons above the synchrotron limit. We have modelled the multiwavelength observations of our sample to constrain the microphysical parameters, the circumburst density, the bulk Lorentz factor, and the mechanism responsible for explaining the energetic GeV photons. [ABSTRACT FROM AUTHOR]

GRB 190829A has been widely studied due to its nature and the high-energy emission presented. Due to the detection of a very high energy component by the High Energy Stereoscopic System and the event's atypically middling luminosity, it has been categorized in a select, limited group of bursts bordering classic gamma-ray bursts (GRBs) and nearby sub-energetic events. Given the range of models utilized to adequately characterize the afterglow of this burst, it has proven challenging to identify the most probable explanation. Nevertheless, the detection of polarization data provided by the MASTER (Mobile Astronomical System of TElescope Robots) collaboration has added a new aspect to GRB 190829A that permits us to attempt to explore this degeneracy. In this paper, we present a polarization model coupled with a thin-shell synchrotron forward-shock model – a component in all models used to describe GRB 190829A's afterglow – in order to fit the polarization's temporal evolution with the existing upper limits (⁠|$\Pi < 6{{\ \rm per\, cent}}$|⁠). We find that the polarization generated from an on-axis emission is favoured for strongly anisotropic magnetic field ratios, while an off-axis scenario cannot be fully ruled out when a more isotropic framework is taken into account. [ABSTRACT FROM AUTHOR]

We present the largest optical photometry compilation of gamma-ray bursts (GRBs) with redshifts (z). We include 64813 observations of 535 events (including upper limits) from 28 February 1997 to 18 August 2023. We also present a user-friendly web tool grbLC which allows users to visualize photometry, coordinates, redshift, host galaxy extinction, and spectral indices for each event in our database. Furthermore, we have added a Gamma-ray Coordinate Network (GCN) scraper that can be used to collect data by gathering magnitudes from the GCNs. The web tool also includes a package for uniformly investigating colour evolution. We compute the optical spectral indices for 138 GRBs, for which we have at least four filters at the same epoch in our sample, and craft a procedure to distinguish between GRBs with and without colour evolution. By providing a uniform format and repository for the optical catalogue, this web-based archive is the first step towards unifying several community efforts to gather the photometric information for all GRBs with known redshifts. This catalogue will enable population studies by providing light curves (LCs) with better coverage since we have gathered data from different ground-based locations. Consequently, these LCs can be used to train future LC reconstructions for an extended inference of the redshift. The data gathering also allows us to fill some of the orbital gaps from Swift in crucial points of the LCs, e.g. at the end of the plateau emission or where a jet break is identified. [ABSTRACT FROM AUTHOR]

Cosmological models and their corresponding parameters are widely debated because of the current discrepancy between the results of the Hubble constant, H 0, obtained by SNe Ia, and the Planck data from the cosmic microwave background radiation. Thus, considering high redshift probes like gamma-ray bursts (GRBs) is a necessary step. However, using GRB correlations between their physical features to infer cosmological parameters is difficult because GRB luminosities span several orders of magnitude. In our work, we use a three-dimensional relation between the peak prompt luminosity, the rest-frame time at the end of the X-ray plateau, and its corresponding luminosity in X-rays: the so-called 3D Dainotti fundamental plane relation. We correct this relation by considering the selection and evolutionary effects with a reliable statistical method, obtaining a lower central value for the intrinsic scatter, σ int = 0.18 ± 0.07 (47.1 per cent) compared to previous results, when we adopt a particular set of GRBs with well-defined morphological features, called the platinum sample. We have used the GRB fundamental plane relation alone with both Gaussian and uniform priors on cosmological parameters and in combination with SNe Ia and BAO measurements to infer cosmological parameters like H 0, the matter density in the universe (ΩM), and the dark energy parameter w for a w CDM model. Our results are consistent with the parameters given by the Lambda cold dark matter model but with the advantage of using cosmological probes detected up to z  = 5, much larger than the one observed for the furthest SNe Ia. [ABSTRACT FROM AUTHOR]