{
  "bibcode": "2004GCN..2513....1G",
  "body": "J. Gorosabel (STScI/IAA-CSIC), J. Rhoads (STScI), A. Fruchter\n(STScI), D. Bersier (STScI), J.M. Castro Ceron (STScI),\nA. Levan (U. of Leicester), C. Smith (CTIO), report:\n\nWe have acquired 5x480s R-band images of GRB 031220 (HETE trigger\n#2976) field with the CTIO 4-m Telescope (+Mosaic2) on 2003 Dec.\n21.1787--21.2113 UT. The large field of view of Mosaic2 allowed\nus to simultaneously image the positions of the 7 X-ray sources\ndetected by Chandra (GCN #2502).\n\nFixing our zero point to the R-band magnitude of source #27 (based\non the R-band images taken with the TNG, GCN #2503) we obtain the\nfollowing magnitudes:\n\n    Chandra          R +/- ER                   R +/- ER\n    ID #    (CTIO, Dec. 21 ~04:30 UT)   (TNG, Dec. 28 ~01:24 UT)\n    -------------------------------------------------------------\n     1           24.40 +/- 0.18                    -\n     2                  %                          -\n     6           20.61 +/- 0.01             20.61 +/- 0.05\n     7           23.03 +/- 0.04             23.48 +/- 0.11\n     17          23.06 +/- 0.04             23.10 +/- 0.07\n     27          19.42 +/- 0.01             19.42 +/- 0.05\n     37          25.15 +/- 0.24                    *\n    -------------------------------------------------------------\n     - No counterpart brighter than R = 24 (3 sigma).\n     % No counterpart brighter than R = 25.3 (3 sigma).\n     * Out of the FOV.\n    -------------------------------------------------------------\n\nSources #6, #17 and #27, remain constant within photometric\nuncertainties. No comparison was possible for objects #1, #2 and\n#37, which are not present in the TNG images. Object #7 shows an\nR-band decay at a 3.8 sigma level, which makes it a suggestive\nafterglow candidate.\n\nHowever, the potential fading associated with object #7 would\nyield a power-law decay index Alpha = 0.21 +/-  0.05, quite\nunusual (but not unprecedented) for GRB afterglows.\n\nWe encourage further deep R-band imaging to clarify the potential\nfading of source #7",
  "circularId": 2513,
  "createdOn": 1073593800000,
  "email": "jgu@laeff.esa.es",
  "subject": "GRB 031220: Optical observations of Chandra X-ray sources",
  "submitter": "Javier Gorosabel at LAEFF-INTA, Madrid  <jgu@laeff.esa.es>",
  "eventId": "GRB 031220"
}