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GCN Circular 287

GRB981220 VLBA observations
1999-03-30T22:18:31Z (25 years ago)
Greg Taylor at NRAO <>
G. B. Taylor (NRAO), D. A. Frail (NRAO), and S. R. Kulkarni (Caltech) 

"On 1999 March 10.01 we carried out VLBA observations of the radio
source, J0342+1709, claimed to be a possible afterglow candidate for
GRB 981220 based on early observations at WSRT (GCN #168) and the VLA
(GCN #170).  Within the 0.9" x 0.9" error box set by VLA observations
at 8.4 GHz we detect an extended source with a peak flux density at 5
GHz of 404 +/- 60 microJy.  The position of the radio source is
ra=03h42m28.9611s dec=17d09'14.669" (equinox J2000) with an
uncertainty of 0.05 arcsec in each coordinate.  The integrated flux
density of this source is 528 +/- 90 microJy.  The flux density at 5
GHz measured by the VLA on March 8.96 was 510 +/- 26 microJy.  The
VLBA image can be viewed at

The standard models and redshift distribution of GRB afterglows
predict that the radio counterpart to G981220 should be unresolved by
our VLBA observations.  The "core-jet" morphology of the VLBA image
therefore makes it unlikely that J0342+1709 is associated with
G981220.  A more plausible explanation is that it is a highly variable
background intraday-variable (IDV) source.  Such sources are known to
vary on timescales of days to weeks, and consist of a strong core and
one-sided jet.  This explanation is supported by the location of
J0342+1709 outside of the refined IPN localization (GCN #270) for

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