Low frequency observations of the radio nebula produced by the giant flare from SGR 1806 - 20 Polarimetry and total intensity measurements

Context. The 2004 December 27 giant flare from SGR 1806-20 produced a radio nebula that was detectable for weeks. It was observed at a wide range of radio frequencies. Aims. To investigate the polarized signal from the radio nebula at low frequencies and to perform precise total intensity measurements. Methods. We made a total of 19 WSRT observations. Most of these were performed quasi simultaneously at either two or three frequencies, starting 2005 January 4 and ending 2005 January 29. We reobserved the field in 2005 April/May, which facilitated an accurate subtraction of background sources. Results. At 350 MHz, we find that the total intensity of the source is lower than expected from the GMRT 240 MHz and 610 MHz measurements and inconsistent with spectral indices published previously. Our 850 MHz flux densities, however, are consistent with earlier results. There is no compelling evidence for significant depolarization at any frequency. We do, however, find that polarization angles differ substantially from those at higher frequencies. Conclusions. Low frequency polarimetry and total intensity measurements provide a number of clues with regard to substructure in the radio nebula associated with SGR 1806-20. In general, for a more complete understanding of similar events, low frequency observations can provide new insights into the physics of the radio source.