TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1156
SUBJECT: GRB 011121: Radio Observations
DATE:    01/11/24 00:09:06 GMT
FROM:    Shri Kulkarni at Caltech  <srk@astro.caltech.edu>

R. Subrahmanyan, Australia Telescope National Facility,
S. R. Kulkarni & E. Berger, California Institute of Technology, and 
D. A. Frail, National Radio Astronomy Observatory report 
on behalf of a larger collaboration:

"We imaged the error circle of GRB 011121 (GCN 1147, 1148) with the
Australia Telescope Compact Array (ATCA), Narrabri, Australia. The
observations were conducted in the 3-cm band.  The observations started
at UT 1600, November 22 2001 and lasted 12 hours.

The rms of the combined 8640 & 8768 MHz image is 35 microJy and the
beam is 1.37x1.21 arcsec (PA=-27 deg).  Only one source is detected in
the general vicinity of the optical transient (OT) reported by
Wyrzkyowski et al. (GCN 1150).  This source, hereafter
ATCA 113429.6-760141, is is located at 11h 34m 29.63s -76d 01m 41.5s
(J2000, uncertainty of about 0.2 arcsec).  It has a peak flux of 0.2
mJy and is embedded in some radio nebulosity; the integrated flux is
0.25 mJy. 

The discrepancy between the position of ATCA 113429.6-760141 and the
position of the optical transient (GCN 1150) motivated us to
reinvestigate the optical/IR astrometry. As reported in GCN 1155, the
coordinates of the OT as given in GCN 1150 are likely to be in error
and that the true position of the OT/NIRT is, within errors, the same
as that of ATCA 113429.6-760141.  We conclude that ATCA 113429.6-760141
is the radio afterglow of GRB 011121. 

Separately, we note that the radio frame does not suffer from zonal
errors. In contrast, GSC suffers from such errors (and USNO less so).
Thus the position of ATCA 113429.6-760141 is quite secure and
with additional observations the source will be located to an accuracy
better than the usual radio-optical frame tie.

The 2-arcsec nebulosity is likely the radio host galaxy of GRB 011121.
If so, assuming that the nebulosity is 0.05 mJy (a lower limit; we
could be missing some extended flux) and a typical spectral index of
-0.5 (between 1.4 and 8.5 GHz) we obtain Star Formation Rate (SFR, M>5
solar masses; Salpeter IMF) of 13 solar masses per year or SFR(M > 0.1
Msun) of 70 solar masses per year. 

We encourage deep optical/IR imaging to fully reveal the extent
of the presumed host galaxy."