Low frequency observations of the radio nebula produced by the giant flare from SGR 1806-20

TL;DR

This study presents low-frequency radio observations of the nebula from the 2004 giant flare of SGR 1806-20, revealing spectral and polarization properties that shed light on the nebula's substructure and physics.

Contribution

It provides new low-frequency radio measurements of the nebula, highlighting spectral inconsistencies and polarization angle differences that improve understanding of the source's substructure.

Findings

Total intensity at 350 MHz is lower than expected from higher frequency data.

No significant depolarization observed at any frequency.

Polarization angles differ substantially from higher frequency measurements.

Abstract

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. 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. 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. 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.