Off-Axis Afterglow Emission from Jetted Gamma-Ray Bursts

We calculate Gamma-Ray Burst afterglow light-curves from a relativistic jetof initial opening angle theta_0, as seen by observers at a wide range ofviewing angles, theta_obs, from the jet axis. We describe three increasinglymore realistic models and compare the resulting light-curves. An observer attheta_obs < theta_0 should see a light curve very similar to that for anon-axis observer. An observer at theta_obs > theta_0 should see a rising lightcurve at early times, the flux peaking when the jet Lorentz factor sim1/theta_obs. After this time the flux is not very different from that seen byan on-axis observer. A strong linear polarization (<40%) may occur near thepeak in the light curve, and slowly decay with time. We show that if GRB jetshave a universal energy, then orphan afterglows associated with off-axis jetsshould be seen up to a constant theta_obs, therefore the detection rate oforphan afterglows would be proportional to the true GRB rate. We also discussthe proposed connection between supernova 1998bw and GRB 980425.Comment: 5 pages, 3 figures, to appear in ApJL. Section on GRB orphan afterglows added, section on GRB 980425/SN 1998bw association expande