Late-Time Radio Rebrightening of Gamma-Ray Burst Afterglows: Evidence for Double-Sided Jets

The central engine of gamma-ray bursts (GRBs) is believed to eject double-sided ultrarelativistic jets. For an observed GRB, one of the twin jets points toward us and is responsible for the prompt gamma-ray and subsequent afterglow emission. We consider in this Letter the other receding jet, which is expected to give rise to late-time radio rebrightening (RRB) when it becomes nonrelativistic (NR) and radiatively isotropic. The RRB peaks at a time 5 t NR = 2( E j , 51 / n ) 1/3 yr after the GRB (where t NR is the observed NR timescale for the preceding jet, E j is the jet energy, and n is the ambient medium density). The peak flux is comparable to the preceding jet emission at t NR . We expect the RRB of GRB 030329 1.7 yr after the burst, with a flux of ~0.6 mJy at 15 GHz. The cases of GRBs 970508 and 980703 have also been discussed. The detection of RRB, which requires a dense monitoring campaign as much as a few years after a GRB, will be the direct evidence for the existence of double-sided jets in GRBs and prove the black hole-disk system formation in the cores of progenitors.