GCN Circular 2933
Subject
Second-epoch VLA observations of SGR 1806-20
Date
2005-01-04T15:15:00Z (19 years ago)
From
Chryssa Kouveliotou at MSFC <chryssa.kouveliotou-1@nasa.gov>
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:
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). Preliminary flux
levels, including improved estimates of the fluxes reported in GCN 2929,
are as follows (the numbers in parentheses indicate 3 sigma errors in the
last decimal points):
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, 2929) 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, both data-sets 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.