TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1385 SUBJECT: GRB020321 : Optical observations. DATE: 02/05/06 10:43:33 GMT FROM: Isabel Salamanca at U. of Amsterdam Isabel Salamanca, Evert Rol, L. Kaper (Univ. of Amsterdam), A. J. Castro-Tirado (IAA-CSIC Granada), A. Fruchter (STScI, Baltimore), J. Greiner (MPE Garching and AI Potsdam), J. Hjorth, (Univ. of Copenhagen), E. Pian (INAF, OA Trieste), E. van den Heuvel (Univ. of Amsterdam), on behalf of GRACE report: Deep broad-band images of the NFI error box (GCN 1285 & 1348) and WFC error box (GCN 1281 & 1284) of GRB 020321 were obtained with EFOSC at the 3.6m telescope at ESO, La Silla. The log of observations follows below: Date (2000) exposure Filter lim mag (3 sigma) ========================================================== March 21/22 900 sec/each UBVR U=22.4 B=24.1 V=23.6 R=23.8 March 23/24 900 sec/each VR V=23.7 R=23.6 April 18/19 1200 sec R R=23.6 Magnitudes were calibrated by using the standard star WOLF 629 of the Landolt catalogue, observed during the first night, which was photometric. Very close to the position of the X-ray counterpart reported by in 't Zand et al. (GCN 1348) we found two objects which are not present in the DSS, with coordinates: Source RA(J2000) DEC(J000) error A 16h 12m 44.1s, -83d 43' 09.2" 0.5 arcsec B 16h 12m 44.4s, -83d 43' 15.5" 2 arcsec X-ray 16h 12m 43.7s, -83d 43' 13.9" 4 arcsec Their magnitudes at the three epochs are: (March 21.18102 BURST TRIGGER in BeppoSAX) Epoch UT(2002) Filter Source A Source B =========================================================== March 22.2814 R 20.81 +/-0.03 22.84 +/- 0.11 March 22.3179 V 21.43 +/-0.03 22.98 +/- 0.14 March 22.4260 B 22.79 +/-0.08 23.84 +/- 0.21 March 22.4378 U > 23.6 > 23.6 March 24.3958 R 20.78 +/-0.04 23.30 +/- 0.22 March 24.4076 V 21.47 +/-0.05 23.54 +/- 0.30 April 19.2908 R 20.93 +/-0.04 23.68 +/- 0.26 ------- Note: UTs are mid-exposure. Both sources are extended. Source "A" does not appear in the DSS 2 (red), although it has a magnitude similar to other stars that are barely visible in the DSS image. However, it does not show any clear fading behaviour as would be expected from a GRB optical afterglow. Source "B" consists of two 'blobs', one of which disappears at epochs 2 and 3. The decrease in magnitude is too shallow for the usual power-law behaviour of an afterglow: the index alpha would be ~0.2, while for gamma-ray afterglows alpha is usually around 1. However, this could be consistent with an OT superposed on its host-galaxy. Images in V and R for the three epochs, as well as a light curve can be found at http://zon.wins.uva.nl/~isabel/GRBs/GRB020321/grb020321.html This message can be cited.