THE CHANGING STRUCTURE OF THE RADIO NEBULA AROUND THE SOFT GAMMA-RAY REPEATER SGR-1806-20

From earlier Very Large Array (VLA) observations, we have shown that t he soft gamma-ray repeater (SGR) 1806-20 is coincident with the radio nebula G10.0-0.3. Furthermore, we have shown that the peak of the quie scent X-ray emission coincides with a compact radio source that has a ''core-jet'' morphology. We present new results from a VLA monitoring campaign of the compact source over 1.8 yr at arcsecond resolution. In agreement with earlier low-resolution data, we see no evidence for st rong flaring either before or after a gamma-ray burst. Amazingly, howe ver, between the first epoch and the second, which are separated by 1. 4 yr, we observe an apparent angular displacement of the jetlike featu re by 50 degrees. We argue that these jets are created episodically an d then fade away on timescales of about 1 yr. We compare these jets wi th those seen in accreting Galactic sources, such as GRS 1915+10 and S S 433, and find qualitative differences between the jets of the SGR an d the accreting sources. We suggest that the jets in the SGR are power ed by sudden deposition of particles and fields from an underlying com pact object and are delineated by nonthermal radio emission from the e jected material. Thus far, the relationship of the creation of the jet s to gamma-ray bursts is unclear. Energy in the jet plasma is found to be E similar or equal to 10(2)E(gamma), E-gamma being the energy radi ated in the most luminous gamma-ray events. In particular, we draw an analogy to bursts in magnetoactive stars and the Sun; in this case, ho wever, they could represent dramatic events on the surface of a hypoth etical highly magnetized (10(14)-10(15) G) neutron star.