TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2513 SUBJECT: GRB 031220: Optical observations of Chandra X-ray sources DATE: 04/01/08 20:30:00 GMT FROM: Javier Gorosabel at LAEFF-INTA, Madrid J. Gorosabel (STScI/IAA-CSIC), J. Rhoads (STScI), A. Fruchter (STScI), D. Bersier (STScI), J.M. Castro Ceron (STScI), A. Levan (U. of Leicester), C. Smith (CTIO), report: We have acquired 5x480s R-band images of GRB 031220 (HETE trigger #2976) field with the CTIO 4-m Telescope (+Mosaic2) on 2003 Dec. 21.1787--21.2113 UT. The large field of view of Mosaic2 allowed us to simultaneously image the positions of the 7 X-ray sources detected by Chandra (GCN #2502). Fixing our zero point to the R-band magnitude of source #27 (based on the R-band images taken with the TNG, GCN #2503) we obtain the following magnitudes: Chandra R +/- ER R +/- ER ID # (CTIO, Dec. 21 ~04:30 UT) (TNG, Dec. 28 ~01:24 UT) ------------------------------------------------------------- 1 24.40 +/- 0.18 - 2 % - 6 20.61 +/- 0.01 20.61 +/- 0.05 7 23.03 +/- 0.04 23.48 +/- 0.11 17 23.06 +/- 0.04 23.10 +/- 0.07 27 19.42 +/- 0.01 19.42 +/- 0.05 37 25.15 +/- 0.24 * ------------------------------------------------------------- - No counterpart brighter than R = 24 (3 sigma). % No counterpart brighter than R = 25.3 (3 sigma). * Out of the FOV. ------------------------------------------------------------- Sources #6, #17 and #27, remain constant within photometric uncertainties. No comparison was possible for objects #1, #2 and #37, which are not present in the TNG images. Object #7 shows an R-band decay at a 3.8 sigma level, which makes it a suggestive afterglow candidate. However, the potential fading associated with object #7 would yield a power-law decay index Alpha = 0.21 +/- 0.05, quite unusual (but not unprecedented) for GRB afterglows. We encourage further deep R-band imaging to clarify the potential fading of source #7