Off-Axis Emission from the Beamed Afterglow of Gamma-Ray Bursts and a Possible Interpretation of the Slowly Declining X-Ray Afterglow of GRB 980425

Abstract

A formula is derived for off-axis emissions from the beamed afterglow of gamma-ray bursts in a synchrotron model. The formula is applied to inhomogeneous circumstellar matter obeying n~r -2, which is also suggested for SN 1998bw/GRB 980425. The slow decline rate as well as the X-ray flux, which is similar to the observed values, are obtained for an appropriate choice of parameters so that the association of GRB 980425 with SN 1998bw is further strengthened.