An Off-axis Jet Model for Multi-wavelength Afterglow Emission of GRB 190829A detected by H.E.S.S.

In the past three years, ground-based Imaging Atmospheric Cherenkov Telescopes, such as the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes and the High Energy Stereoscopic System (H.E.S.S.), have reported the detection of the very-high-energy (VHE) gamma-ray photons from four gamma-ray bursts. One of them, GRB 190829A, was detected by H.E.S.S. with ∼ 20 sigma significance. This event had more peculiar features. First, the prompt emission had much smaller isotropic equivalent gamma-ray energy than typical long gamma-ray bursts. Second, the early X-ray and optical light curves had achromatic peaks at 1.4 × 10³ s. We propose an off-axis jet model to explain those unusual observed properties. In this model, the relativistic beaming effect causes the apparently small isotropic gamma-ray energy and spectral peak energy. We find that a narrow fast jet with 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 peaks in the X-ray and optical light curves. Another slow-wide jet explains the late X-ray and radio fluxes. Our model parameters determined by X-ray, optical and radio afterglows may explain observed VHE gamma-ray flux by synchrotron self-Compton (SSC).