Abstract
Very-high-energy (VHE) emission is usually interpreted in the
synchrotron-self Compton (SSC) scenario, and expected from the low-redshift and
high-luminosity gamma-ray bursts (GRBs), as GRB 180720B and GRB 190114C.
Recently, VHE emission was detected by the H.E.S.S. telescopes from one of the
closest burst GRB 190829A which was associated with the supernova (SN) 2019oyw.
In this paper, we present a temporal and spectral analysis from optical bands
to Fermi-LAT energy range over multiple observational periods beginning just
after the BAT trigger time and extending for almost three months. We show that
the X-ray and optical observations are consistent with synchrotron
forward-shock emission evolving between the characteristic and cooling spectral
breaks during the early and late afterglow in a uniform-density medium.
Modeling the light curves together with its spectral energy distribution, it is
shown that the outflow expands with an initial bulk Lorentz factor of
$\Gamma\sim 30$, which is high for a low-luminosity GRBs and low for a
high-luminosity GRBs. The values of the initial bulk Lorentz factor and the
isotropic equivalent energy suggest that GRB 190829A is classified as an
intermediate-luminosity burst and consequently, it becomes the first burst of
this class in being detected in the VHE gamma-ray band by an imaging
atmospheric Cherenkov telescope, and, in turn, the first event without being
simultaneously observed by the Fermi-LAT instrument. Analyzing the
intermediate-luminosity bursts with $z\lesssim 0.2$ such as GRB 130702A, we
show that bursts with intermediate luminosity are potential candidates to be
detected in very-high energies.