Nearby SN-associated GRB 190829A: Environment, Jet Structure, and VHE Gamma-Ray Afterglows

We present a self-consistent paradigm to interpret the striking features of nearby low-luminosity GRB 190829A. Its prompt gamma-ray lightcurve has two separated pulses. We propose that the interaction of the hard prompt gamma-ray photons ( ${E}_{p}={624}_{-303}^{+2432}$ keV) of its initial pulse with the dusty medium (A_V = 2.33) not only results in the second soft gamma-ray pulse (E_p ~ 12 keV) but also makes a pre-accelerated e^± -rich medium shell via the γγ annihilation. In this paradigm, we show that the observed radio, optical, and X-ray afterglow lightcurves are well-fitted with the forward shock model. Its jet is almost isotropic (θ_j > 1.0 rad) with a Lorentz factor of ~35, and the electron density of the e^± -rich medium shell is ~15 cm^-3, which is about seven times higher than the electron density of its normal surrounding medium. The GRB ejecta catches up with and propagates into the e^± -rich medium shell at a region of R = (4.07-6.46) × 10¹⁶ cm. This results in a bright afterglow bump at ~10³ seconds after the GRB trigger. The predicted very high energy (VHE) gamma-ray emission from the synchrotron self-Compton process agrees with the H.E.S.S. observation. The derived broadband spectral energy distribution shows that GRB 190829A like nearby GRBs would be promising targets of the VHE gamma-ray telescopes, such as H.E.S.S., MAGIC, and CTA (Cerenkov Telescope Arrays).