TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2483 SUBJECT: GRB 031203: Variable Radio Source DATE: 03/12/08 06:47:38 GMT FROM: Alicia Soderberg at Caltech A. M. Soderberg, S. R. Kulkarni (Caltech) and D. A. Frail (NRAO) report on behalf of a larger collaboration: "On 2004 Dec. 7.52 UT we carried out further VLA observations at 8.5 GHz of the radio source reported earlier (GCN 2473) and which lies within the 6-arcsecond error circle of the XMM source S1 (GCN 2464). The source flux density has decreased by a factor of two since our last report. Given that S1 is also fading (GCN 2477) we conclude that likely the radio source is the afterglow of GRB 0312103 (GCN 2459). As noted by Bloom et al. (GCN 2481) and Prochaska et al. (GCN 2482) a galaxy at a redshift of 0.105 is coincident with the radio source. Owing to the large zenith angle of the source the reported radio source position may suffer from larger than normal astrometric error (which will be improved by future observations). The radio source and the putative host galaxy can be considered to coincident within the true astrometric error. The isotropic energy release (assuming redshift of 0.105) in the 20-200 keV band is about 9E49 erg (using the data from GCN 2460). The isotropic X-ray luminosity extrapolated to 10 hr (using data from GCN 2477) is about 5E43 erg/s. These are lower than normal releases and luminosities (see Berger, Kulkarni and Frail 2003) and the apparent early fading in the radio are consistent with GRB 031203 being an abnormally low luminosity GRB. This GRB may bridge GRB 980425 (at z=0.0085) associated with SN 1998bw and cosmological GRBs (typically z of 0.3 to 1). Further low frequency observations will settle the issue of total calorimetry. The search for an underlying SN is worth trying, though obscuration within and outside our Galaxy may prove to be vexing."