Broad-band modelling of the GRB 230812B afterglow: Implications for very-high-energy

A significant fraction of the energy from the gamma-ray burst (GRB) jets, after powering the keV–MeV emission, forms an ultra-relativistic shock propagating into the circum-burst medium. The particles in the medium accelerate through the shock and produce the afterglow emission. Recently, a number of GRB afterglows were observed in TeV γ-rays by Cherenkov Telescopes. This new observational window provides access to the broad-band spectra of GRB afterglows, which contain rich information on the microphysics of relativistic shocks and the profile of the circum-burst medium. Since the transition from synchrotron to inverse Compton regime in afterglow spectra occurs between hard X-rays and the very-high-energy (VHE) γ-rays, it is necessary to have a detection in one of these bands to identify the two spectral components. The early afterglow data in hard X-rays, along with the GeV emission, could help to accurately constrain the spectral shape and capture the spectral turnover to distinguish the two components. We present a multi-wavelength spectral and temporal study, focussed on the keV-VHE domain, of GRB 230812B, one of the brightest GRBs detected by Fermi Gamma-ray Burst Monitor (Fermi/GBM). We also include the detection of a 72 GeV photon by Large Area Telescope (Fermi/LAT) during the early afterglow phase. Through detailed modelling of the emission within the afterglow external forward shock in a wind-like scenario, we predict the multi-wavelength afterglow observations from optical (up to approximately one day) to high-energy band. We emphasise the importance of following up poorly localised GRBs by demonstrating that even in cases without prompt localisation, such as GRB 230812B, it is possible to recover the emission using imaging atmospheric Cherenkov telescopes (IACTs) thanks to their relatively wide field of view. The low energy threshold of the Large-Sized Telescope is essential in discovering the VHE component at the much higher redshifts typical for long GRBs.