The Growth, Polarization, and Motion of the Radio Afterglow from the Giant Flare from SGR 1806–20

The extraordinary giant flare (GF) of 2004 December 27 from the soft gamma repeater SGR 180620 was followed by a bright radio afterglow. We present an analysis of VLA observations of this radio afterglow from SGR 180620, consisting of previously reported 8.5 GHz data covering days 7–20 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 ∼ 9t o!5 mas day 1 . 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 SedovTaylor 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. Subject headings: pulsars: individual (SGR 180620) — radio continuum: general — stars: flare — stars: neutron — stars: winds, outflows

[1]  W. Hajdas,et al.  An exceptionally bright flare from SGR 1806–20 and the origins of short-duration γ-ray bursts , 2005, Nature.

[2]  S. R. Kulkarni,et al.  An outburst of relativistic particles from the soft γ-ray repeater SGR1900+14 , 1999, Nature.

[3]  B. R. G. Aensler,et al.  A RE-BRIGHTENING OF THE RADIO NEBULA ASSOCIATED WITH THE 2004 DECEMBER 27 GIANT FLARE FROM SGR 1806–20 , 1992 .

[4]  D. Frail,et al.  A Relativistic Particle Outburst From The Soft Gamma-Ray Repeater SGR1900+14 , 1998, astro-ph/9812457.

[5]  S. Eikenberry,et al.  The connection between W31, SGR 1806-20, and LBV 1806-20: Distance, extinction, and structure , 2003, astro-ph/0311313.

[6]  An expanding radio nebula produced by a giant flare from the magnetar SGR 1806–20 , 2005, Nature.

[7]  Detection of a radio counterpart to the 27 December 2004 giant flare from SGR 1806–20 , 2005, Nature.

[8]  D. Eichler.  Waiting for the Big One: a new class of soft gamma-ray repeater outbursts? , 2002, astro-ph/0204512.

[9]  T. Sakamoto,et al.  A giant γ-ray flare from the magnetar SGR 1806–20 , 2005, Nature.

[10]  D. Frail,et al.  The Changing Structure of the Radio Nebula around the Soft Gamma-Ray Repeater SGR 1806–20 , 1997 .

[11]  Abraham Loeb,et al.  Generation of Magnetic Fields in the Relativistic Shock of Gamma-Ray Burst Sources , 1999, astro-ph/9904363.

[12]  N. McClure–Griffiths,et al.  Constraints on the Distance to SGR 1806–20 from H I Absorption , 2005, astro-ph/0503171.

[13]  The Angular Size and Proper Motion of the Afterglow of GRB 030329 , 2004, astro-ph/0405300.

[14]  C. Carilli,et al.  Synthesis Imaging in Radio Astronomy II , 1999 .