An Off-Axis Jet Model For GRB 980425 and Low-Energy Gamma-Ray Bursts

Using a simple off-axis jet model of gamma-ray bursts (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 51 ergs and the opening half-angle of 10° ≲ Δθ ≲ 30° is seen from the off-axis viewing angle θ v ~ Δθ + 10γ -1 , 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 E p (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 E γ .