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

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Published 2003 August 11 © 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A.
, , Citation Ryo Yamazaki et al 2003 ApJ 594 L79 DOI 10.1086/378736

1538-4357/594/2/L79

Abstract

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 ~1051 ergs and the opening half-angle of 10° ≲ Δθ ≲ 30° is seen from the off-axis viewing angle θv ~ 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 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.

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10.1086/378736