GCN Circular 179

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
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) 
and Frail and Kulkarni (GCN 170
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), 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
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).  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
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 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
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). 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
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 for a correction to this Circular.(13 Jan 99)]