Off-Axis Afterglow Emission from Jetted Gamma-Ray Bursts

We calculate Gamma-Ray Burst afterglow light-curves from a relativistic jet of initial opening angle theta_0, as seen by observers at a wide range of viewing angles, theta_obs, from the jet axis. We describe three increasingly more realistic models and compare the resulting light-curves. An observer at theta_obs < theta_0 should see a light curve very similar to that for an on-axis observer. An observer at theta_obs > theta_0 should see a rising light curve at early times, the flux peaking when the jet Lorentz factor sim 1/theta_obs. After this time the flux is not very different from that seen by an on-axis observer. A strong linear polarization (<40%) may occur near the peak in the light curve, and slowly decay with time. We show that if GRB jets have a universal energy, then orphan afterglows associated with off-axis jets should be seen up to a constant theta_obs, therefore the detection rate of orphan afterglows would be proportional to the true GRB rate. We also discuss the proposed connection between supernova 1998bw and GRB 980425.