Radio emission of the magnetar SGR 1806-20 : Evolution of the magnetic field in the region of the radio afterglow

The results of radio observations of the afterglow produced by a giant gamma-ray flare from the magnetar SGR 1806-20 on December 27, 2004 are reported. The observations were carried out on the 32-m radio telescope of the Zelenchuk Observatory of the Institute of Applied Astronomy, Russian Academy of Sciences, at a wavelength of 3.5 cm. The observations confirm the enhanced radio brightness of the nebula around the magnetar in the period from 25 to 31 days after the flare. A comparison of the Zelenchuk observations with other data have enabled us to estimate the magnetic-field intensity in the radio-afterglow region using a model of synchrotron radiation with self-absorption in a relativistic plasma. The kinetic energy of the blast wave produced by the giant explosion of the magnetar is estimated.