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Second-epoch VLA observations of SGR 1806-20

ATel #375; B. M. Gaensler (CfA), C. Kouveliotou (NASA/MSFC, NSSTC), R. Wijers (U. Amsterdam), M. Garrett (JIVE), M. Finger, P. Woods, S. Patel (USRA, NSSTC), M. McLaughlin (Jodrell Bank), R. Fender (U. Southampton), T. Delaney (CfA) report on behalf of a larger team:
on 4 Jan 2005; 15:31 UT
Credential Certification: Bryan Gaensler (bgaensler@cfa.harvard.edu)

Subjects: Radio, Neutron Star, Soft Gamma-ray Repeater, Transient, Variables, Pulsar

Referred to by ATel #: 380

We observed the soft gamma-ray repeater SGR 1806-20 with the Very Large Array (VLA) in the A-configuration for a second time on January 4, 2005 for one hour (14:15-15:15 UT). We detect the source at the same coordinates as reported by Gaensler et al. (GCN #2929; ATEL #373). Preliminary flux levels, including improved estimates of the fluxes reported in ATEL #373, are as follows (the numbers in parentheses indicate approximate uncertainties in the last significant figures):

  
Frequency   Flux (2005 Jan 03.8)   Flux (2005 Jan 04.6)    
(GHz) (mJy) (mJy)
--------- -------------------- --------------------
1.4 172(4) 135(10)
4.9 80(1) 66(2)
8.5 53(1) 38(5)
These results indicate a decay in the flux at a rate of approximately 1.9 mJy/hour at 1.4 GHz. At the same time the spectrum has remained constant. On Jan 3, 2005 the spectral index (in the sense F_nu ~ nu^alpha) was alpha = -0.65+/-0.05, while on Jan 4, 2005 the spectral index was -0.61+/-0.06.

A reanalysis of the VLA data from 3 Jan 2004 (GCN #2928, GCN #2929; ATEL #373) suggests that the radio source associated with SGR 1806-20 is slightly extended at both 4.9 and 8.5 GHz. Assuming that the underlying geometry is an optically thin, spherical thin shell, data at both frequencies independently suggest a source of approximate diameter 50-100 milliarcseconds. This implies a projected expansion speed of approx (0.3-0.6)c over the 7 days since the initial giant SGR flare (assuming a source distance of 15.1 kpc; Eikenberry, S. et al., ApJ 616, 506, 2004).

Further radio observations of this source with the Westerbork Synthesis Radio Telescope (WSRT), the MERLIN VLBI network and the Parkes Radio Observatory are currently underway or are being scheduled, and will be reported as they are analyzed. We will continue monitoring the source with the VLA until it fades below detection level.