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 observations at 3.6 cm with the VLA, of the central condensation in G 10.0−0.3, the radio nebula surrounding the soft gamma ray repeater (SGR) 1806−20. The quiescent X–ray source AX 1805.7−2025 is coincident with the radio core suggesting that G 10.0−0.3 is a plerionic supernova remnant. The monitoring experiment was performed in 10 epochs spread over five months, starting just before the latest reactivation of SGR 1806−20 in γ-rays. There is no apparent increase in the radio flux density from the central region of G 10.0−0.3 on timescales of days to months following the γ-ray bursts. At a resolution of 1 arcsec the peak region of G 10.0−0.3 is seen to consist of a compact source with diffuse, one-sided 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 so, 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 binaries. 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 models discussed for SGRs, the pulsar model or the accreting binary model.