TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2933 SUBJECT: Second-epoch VLA observations of SGR 1806-20 DATE: 05/01/04 15:15:00 GMT FROM: Chryssa Kouveliotou at MSFC B. M. Gaensler (CfA), C. Kouveliotou (NASA/MSFC, NSSTC), R. Wijers (U. Amsterdam), M. Garrett (JIVE), M. Finger, P. Woods, S. Patel (USRA, NSSTC), M. McLaughlin (Jodrell Bank), R. Fender (U. Southampton), T. Delaney (CfA) report on behalf of a larger team: We observed the soft gamma-ray repeater SGR 1806-20 with the Very Large Array (VLA) in the A-configuration for a second time on January 4, 2005 for one hour (14:15-15:15 UT). We detect the source at the same coordinates as reported by Gaensler et al. (GCN 2929). Preliminary flux levels, including improved estimates of the fluxes reported in GCN 2929, are as follows (the numbers in parentheses indicate 3 sigma errors in the last decimal points): Frequency Flux (2005 Jan 03.8) Flux (2005 Jan 04.6) (GHz) (mJy) (mJy) --------- -------------------- -------------------- 1.4 172(4) 135(10) 4.9 80(1) 66(2) 8.5 53(1) 38(5) These results indicate a decay in the flux at a rate of approximately 1.9 mJy/hour at 1.4 GHz. At the same time the spectrum has remained constant. On Jan 3, 2005 the spectral index (in the sense F_nu ~ nu^alpha) was alpha = -0.65+/-0.05, while on Jan 4, 2005 the spectral index was -0.61+/-0.06. A reanalysis of the VLA data from 3 Jan 2004 (GCN 2928, 2929) suggests that the radio source associated with SGR 1806-20 is slightly extended at both 4.9 and 8.5 GHz. Assuming that the underlying geometry is an optically thin, spherical thin shell, both data-sets independently suggest a source of approximate diameter 50-100 milliarcseconds. This implies a projected expansion speed of approx (0.3-0.6)c over the 7 days since the initial Giant SGR flare (assuming a source distance of 15.1 kpc; Eikenberry, S. et al., ApJ 616, 506, 2004). Further radio observations of this source with the Westerbork Synthesis Radio Telescope (WSRT), the MERLIN VLBI network and the Parkes Radio Observatory are currently underway or are being scheduled and will be reported as they are analyzed. We will continue monitoring the source with the VLA until it fades below detection level.