GRB 981220 VLBA Observations

As reported in 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

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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Last Modified on 1999 March 30

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