GCN Circular 1555

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
Not found
 
 
)
by Soderberg et al. (GCN #1554
Not found
 
 
), 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
Not found
 
 
, 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.