Extremely Low Isotropic Gamma Ray Energy and the Low Peak Energy of GRB 980425 in the off - axis jet model with the intrinsically high peak energy

Using a simple off-axis jet model of GRBs, we can reproduce the observed unusual properties of the prompt emission of GRB 980425, such as the extremely low isotropic equivalent γ-ray energy, the low peak energy, the high fluence ratio, and the long spectral lag when the jet with the standard energy of ∼ 10 ergs and the opening half-angle of 10 . ∆θ . 30 is seen from the off-axis viewing angle θv ∼ ∆θ + 10γ , where γ is a Lorentz factor of the jet. For our adopted fiducial parameters, if the jet that caused GRB 980425 is viewed from the on-axis direction, the intrinsic peak energy Ep(1+z) is ∼2.0–4.0 MeV, which corresponds to those of GRB 990123 and GRB 021004. We also discuss the connection of GRB 980425 in our model with the X-ray flash, and the origin of a class of GRBs with small Eγ . Subject headings: gamma rays: bursts —gamma rays: theory