GCN Circular 2513

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