GRB 981220 - NTT observation

GRB 981220

Image taken at La Silla with NTT (B filter)

December 24.05 UT: NTT + EMMI + B filter, 15 min exposure

North is at top, East is to the left; the field size is 1'.5 x 1'.5. The possible host galaxy (B ~ 24) of GRB981220 is indicated by the horizontal dashes.

GCN Circular no. 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) 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.


ERRATUM

GCN Circular no. 193


GRB 981220, correction to GCN 179

N. Masetti, E. Palazzi, E. Pian (ITESRE-CNR, Bologna), by apologizing with the GCN readers, would like to notify them about a change in GCN N. 179 (optical ESO-NTT observations of GRB981220): the observation time of the B and I observations is incorrect, and should be modified from 'Dec 23.04-23.07 UT' to 'Dec 24.04-24.07 UT'.

Therefore, the upper limit of B > 25.4 given in GCN 191 implies a variation of more than 1 magnitude in ~3.5 hours, namely a much steeper lower limit for the power-law decay index than that suggested in GCN 191.



The BeppoSAX-GRB team in Bologna

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