TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2938 SUBJECT: SGR 1806-20: Temporal and Spectral behavior of radio transient DATE: 05/01/06 14:01:30 GMT FROM: Dale A. Frail at NRAO P. B. Cameron (Caltech) and S. R. Kulkarni (Caltech) report: "We have carried out a more thorough calibration of the three VLA epochs of the fading radio source toward SGR 1806-20 (GCN#s 2928, 2929, 2930, 2933, 2934, 2935). Combining this with the data from a fourth epoch observation made with the Australia Compact Array (GCN# 2937) we do a joint fit of the form: F_nu \propto t^\alpha nu^\beta. The best fit values are alpha= -1.62+/-0.06 and beta=-0.75+/-0.03. There is evidence of deviations from this simple power-law behavior. Fitting the decay rate alpha at each frequency we find a systematic steepening. The value of alpha is -1.0 at 1.4 GHz, -1.5 at 4.9 GHz, and -2.2 at 8.5 GHz. This behavior is reminiscent of the broadband afterglow of GRB 991216 (Frail et al. ApJ, 538, 129). A plot of the light curves is at: http://www.aoc.nrao.edu/~dfrail/sgr1806lc.eps. The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation. The Australia Telescope is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO."