TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  287
SUBJECT: GRB981220 VLBA observations
DATE:    99/03/30 22:18:31 GMT
FROM:    Greg Taylor at NRAO  <gtaylor@aoc.nrao.edu>

G. B. Taylor (NRAO), D. A. Frail (NRAO), and S. R. Kulkarni (Caltech) 
report:

"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 http://www.nrao.edu/~gtaylor/G981220.html.

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
G981220."

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