TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1555 SUBJECT: Radio Observations of XRF 020903: The Missing Link DATE: 02/09/29 06:22:30 GMT FROM: Edo Berger at Caltech E. Berger, S. R. Kulkarni, D. A. Frail, A. Soderberg, P. A. Price, D. W. Fox, F. A. Harrison, and S. Yost report on behalf of the larger Caltech-NRAO-CARA GRB collaboration: Following the discovery of the optical transient of GRB 020903 (GCN #1530) by Soderberg et al. (GCN #1554), we observed the position of the OT with the VLA on Sep. 27.22 UT. We detect a bright radio source at the position of the OT, with a flux of 1.1 mJy at 8.46 GHz. The source is not detected in the NRAO VLA Sky Survey (NVSS; Condon et al. 1998, AJ, 115, 1693) above their flux limit of 1 mJy at 1.4 GHz. Observations undertaken today (Sep. 29.11 UT) show that the source is fading with a flux of 0.7 mJy at 8.46 GHz. The variable radio counterpart lends supports to the identification of the optical transient (OT) reported by Soderberg et al. as the afterglow of XRF 020903. Unlike the optical emission, the radio emission is dominated by the afterglow. The isotropic, prompt energy release of XRF 020903 (using the redshift of z=0.25, GCN #1554, and a rough estimate of the fluence detected by WXM; see the HETE-2 web page: http://space.mit.edu/HETE/Bursts/GRB020903/) is well below those of typical cosmological GRBs but higher than that of GRB 980425. Therefore, the energy in relativistic ejecta of XRF 020903 is intermediate between that of SN1998bw, about 10^49 erg, as inferred from the radio emission (Kulkarni et al. 1998, Nature, 395, 663), and that of typical cosmological GRBs. Finally, the ratio of energy in the relativistic ejecta to the energy in the SN of XRF 020903 is well below those of cosmological GRBs (typically 1-10), but higher than SN1998bw (~0.01). These arguments suggest that XRF 020903 is a "transition" object -- a link between GRB 980425 and comsological GRBs. Scaling the ratio of radio to X-ray luminosity of SN1998bw to XRF 020903, we predict detectable X-ray emission on this timescale, which may be an additional robust indicator of the relativistic energy in the ejecta. See http://www.astro.caltech.edu/~ams/XrF.html for a full discussion.