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

摘要

From earlier Very Large Array (VLA) observations, we have shown that the 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 quiescent 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 strong flaring either before or after a γ-ray burst. Amazingly, however, between the first epoch and the second, which are separated by 1.4 yr, we observe an apparent angular displacement of the jetlike feature by 50°. We argue that these jets are created episodically and then fade away on timescales of about 1 yr. We compare these jets with those seen in accreting Galactic sources, such as GRS 1915 + 10 and SS 433, and find qualitative differences between the jets of the SGR and the accreting sources. We suggest that the jets in the SGR are powered by sudden deposition of particles and fields from an underlying compact object and are delineated by nonthermal radio emission from the ejected material. Thus far, the relationship of the creation of the jets to γ-ray bursts is unclear. Energy in the jet plasma is found to be E approx= 10~2E_γ, E_γ being the energy radiated in the most luminous γ-ray events. In particular, we draw an analogy to bursts in magnetoactive stars and the Sun; in this case, however, they could represent dramatic events on the surface of a hypothetical highly magnetized (10~(14)-10~(15) G) neutron star.