The Growth, Polarization, and Motion of the Radio Afterglow from the Giant Flare from SGR1806-20

The extraordinary giant flare (GF) of 2004 December 27 from the soft gamma repeater (SGR) 1806-20 was followed by a bright radio afterglow. We present an analysis of VLA observations of this radio afterglow from SGR1806-20, consisting of previously reported 8.5 GHz data covering days 7 to 20 after the GF, plus new observations at 8.5 and 22 GHz from day 24 to 81. We detect motion in the flux centroid of the afterglow, at an average velocity of 0.26 ± 0.03 c (assuming a distance of 15 kpc) at a position angle of −45◦. This motion, in combination with the growth and polarization measurements, suggests an asymmetric outflow, mainly from one side of the magnetar. We find a deceleration in the expansion, from ∼9 mas/day to <5 mas/day. The time of deceleration is roughly coincident with the rebrightening in the radio light curve, as expected to result when the ejecta from the GF sweeps up enough of the external medium, and transitions from a coasting phase to the Sedov-Taylor regime. The radio afterglow is elongated and maintains a 2:1 axis ratio with an average position angle of−40◦ (north through east), oriented perpendicular to the average intrinsic linear polarization angle. Kavli Institute of Particle Astrophysics and Cosmology, Menlo Park, CA 94025, USA National Radio Astronomy Observatory, Socorro, NM 87801, USA Harvard-Smithsonian Center for Astrophysics, 60 Garden Streen, Cambridge, MA 02138, USA NASA/MSFC, XD-12, NSSTC, 320 Sparkman Dr., Huntsville, AL 35805, USA School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK IAS, Einstein Drive, Princeton, NJ 08540, USA Dept. of Phys., BGU, P.O. Box 653, Beér Sheva 84105, Israel Joint Institute for VLBI in Europe, Postbus 2, 7990 AA, Dwingeloo, The Netherlands Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands