TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  179
SUBJECT: GRB981220 B, R and I band observations
DATE:    98/12/29 15:51:40 GMT
FROM:    Elena Pian at ITESRE-CNR,Bologna  <pian@tesre.bo.cnr.it>

GRB 981220 B, R and I band observations

N. Masetti, E. Palazzi, E. Pian, F. Frontera (ITESRE-CNR, Bologna), T. 
Galama, P. Vreeswijk, J. van Paradijs (Univ. of Amsterdam), O. Hainaut
(ESO), M. Fridlund (ESA/ESTEC), on behalf of a large collaboration, report:

We have imaged the error box of GRB981220 (GCN N. 159 and 160) with the ESO
New Technology Telescope equipped with EMMI plus B and I filters on 1998
Dec 23.04-23.07 UT, and with EMMI plus R filter on 1998 Dec 28.07-28.08 UT,
in seeing conditions of ~1.6 arcsec (FWHM). A preliminary analysis of the
900-seconds B-band exposure shows a marginal (3-sigma) detection of a
slightly extended source (~4 arcsec across) at a position (J2000) RA = 3h
42m 28.8s, Dec = +17o 09' 13.3" (3-sigma error of 1 arcsec), consistent
with the position of the radio source detected by Galama et al. (GCN 168) 
and Frail and Kulkarni (GCN 170), identified with the radio afterglow of
the GRB, and with the position of the faint source detected in the R-band
by Eichelberger et al. (GCN 176).  The detection of extended emission is
consistent with the suggestion that at least part of the observed optical
signal originates from a host galaxy.  The source has a total magnitude of
B ~ 24. Our 1200-seconds R-band exposure and 600-seconds I-band exposure
yield no source detection at the same position, down to 3-sigma limiting
magnitudes of R = 23.5 and I = 23, respectively. 

Our B measurement and the R magnitude reported in GCN 176 imply that the
detected emission has a very blue spectrum (alpha ~ 5, with f_nu =
k*nu^{alpha}), consistent with no detection in the I-band down to I = 25
(GCN 171). If this emission is dominated by the contribution of an
underlying galaxy, this result can be compared with the finding of Fruchter
et al. (1998, ApJ, in press, astro-ph/9807295) of remarkably blue colors
for the host galaxy of GRB970228. 
 
The B-band image is posted in the site 
http://tonno.tesre.bo.cnr.it/~masetti/grb981220.html

This GCN note can be cited.

[GCN OP NOTE:  Please see GCN 193 for a correction to this Circular.(13 Jan 99)]