  Recently, ground-based Imaging Atmospheric Cherenkov Telescopes have reported
the detection of very-high-energy (VHE) gamma-rays from some gamma-ray bursts
(GRBs). One of them, GRB~190829A, was triggered by the Swift satellite, and
about 20000 s after the burst onset the VHE gamma-ray emission was detected by
H.E.S.S. with ~ 5 sigma significance. This event had unusual features of having
much smaller isotropic equivalent gamma-ray energy than typical long GRBs and
achromatic peaks in X-ray and optical afterglow at about 1400 s. Here we
propose an off-axis jet scenario that explains these observational results. In
this model, the relativistic beaming effect is responsible for the apparently
small isotropic gamma-ray energy and spectral peak energy. Using a jetted
afterglow model, we find that the narrow jet, which has the initial Lorentz
factor of 350 and the initial jet opening half-angle of 0.015 rad, viewed
off-axis can describe the observed achromatic behavior in the X-ray and optical
afterglow. Another wide, baryon-loaded jet is necessary for the later-epoch
X-ray and radio emissions. According to our model, the VHE gamma rays observed
by H.E.S.S. at 20000 s may come from the narrow jet through the synchrotron
self-Compton process.
