GCN Circular 287
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." This message is citeable.