TITLE: GCN GRB OBSERVATION REPORT NUMBER: 191 SUBJECT: GRB 981220 Optical observations DATE: 99/01/09 00:35:46 GMT FROM: Mark R. Metzger at CIT GRB 981220 Optical Observations, BVRI M. R. Metzger, C. Martin, and B. Kern (Caltech) report: Images of the GRB 981220 joint localization errorbox (GCN 159,160) were obtained on UT Dec 23.15 (VRI, GCN 163) , UT Dec 24.2 (BVRI), and UT Dec 27.2 (VRI) at the Palomar 5m telescope using COSMIC. The seeing varied between 1.4 and 2.0 arcsec (FWHM). Photometric calibration was perfomed on UT Dec 24 using Landolt standards, and astrometry was computed using a solution of 70 nearby stars from USNO A2.0. For reference, on this system we find the following magnitudes for calibrating objects near the Galama et al (GCN 168) radio source: mag(+-) Pos (J2000) B V R Ic 03 42 28.69 +17 09 26.1 23.81(14) 22.98(13) 22.34(09) 22.00(15) 03 42 28.90 +17 09 33.0 21.61(03) 20.75(04) 20.12(03) 19.56(04) 03 42 30.51 +17 09 20.9 22.12(06) 20.58(06) 19.65(04) 18.61(05) 03 42 28.33 +17 09 42.8 19.46(02) 18.47(04) 17.97(02) 17.36(02) At the radio position of Frail and Kulkarni (GCN 170), no source was detected on Dec 23.15 to the limits (2-sigma) I > 23.0, R > 23.8, V > 24.2. The object marked "1" on the APO image referred to by Vrba et al on GCN 171 has I = 19.6 +- 0.1 on Dec 23.15 and Dec 24.2 at rough position 03h42m31.7 +17:10:04; it is fairly red, with (V-I) ~= 2. This disagrees with the photometry reported on GCN 171, and could be due to a difference in photometric zero point or different object. We can also place B-band upper limits at the radio position. On Dec 24.2, there is no significant detection (though possibly a very faint peak), which puts a limit of B >= 25.4 (1-sigma). Any host is less blue and fainter than implied on GCN 179 given the Echelberger et al. (GCN 176) magnitude. The detection of flux by Masetti et al. (GCN 179) with B ~= 24 on Dec 23 would imply variability, and thus may be the optical afterglow of GRB 981220. The V limit on Dec 23 is consistent with a blue spectrum, and the power law time decay in B would be steeper than alpha = -1.7 (though uncertainties are large). No significant variability of objects (other than asteroids) is detected over the Dec 23 to Dec 27 interval in V, R, or I, consistent with other reports. A B-band image from Dec 24 may be found at http://astro.caltech.edu/~mrm/grb981220.html. This report may be cited in publications.