RADIO MONITORING AND HIGH-RESOLUTION IMAGING OF THE SOFT GAMMA-RAY REPEATER-1806-20

We report the results of radio flux-monitoring and high resolution obs ervations at 3.6 cm with the VLA of the central condensation in G10.0- 0.3, the radio nebula surrounding the soft gamma-ray repeater (SGR) 18 06-20. The quiescent X-ray source AX 1805.7-2025 is coincident with th e radio core suggesting that G10.0-0.3 is a plerionic supernova remnan t. The monitoring experiment was performed in 10 epochs spread over 5 months, starting just before the latest reactivation of SGR 1806-20 in gamma-rays. There is no apparent increase in the radio flux density f rom the central region of G10.0-0.3 on timescales of days to months fo llowing the gamma-ray bursts. At a resolution of 1'' the peak region o f G10.0-0.3 is seen to consist of a compact source with diffuse, one-s ided emission, reminiscent of core-jet geometry seen in AGNs and some accreting Galactic binaries. By analogy with these latter sources, we argue that the SGR 1806-20 is coincident with the core component. If s o, this is the first arcsecond localization of a high-energy transient . The lack of radio variability and the low brightness temperature of the central component distinguish SGR 1806-20 from other accreting bin aries. The structure of the high-resolution radio image also does not particularly resemble that seen in the vicinity of young pulsars. Thus there is no compelling observational evidence for either of the two m odels discussed for SGRs, the pulsar model or the accreting binary mod el.