GCN Circular 2513
Subject
GRB 031220: Optical observations of Chandra X-ray sources
Date
2004-01-08T20:30:00Z (21 years ago)
From
Javier Gorosabel at LAEFF-INTA, Madrid <jgu@laeff.esa.es>
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