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) × 10 16 cm. This results in a bright afterglow bump at ∼10 3 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).