{
  "bibcode": "2004GCN..2728....1B",
  "body": "GRB040912(=H3557): Candidate Optical Afterglow\n\nN. Butler, G. Ricker, R. Vanderspek, J. Villasenor, P. Csatorday,\nP. Schechter, S. Burles (MIT); S. Malhotra (STSCI), and D. Osip (LCO),\non behalf of a larger collaboration report:\n\nWe observed the HETE WXM error region from Butler et al. (GCN 2701)\nin two epochs with the 27'x27' FOV IMACS camera on the Magellan 6.5m \nBaade Telescope at Las Campanas Observatory.  Epoch1 consisted of\n3x180s exposures in R band, with a mean observing time of Sep 13.15716 UT \n(t_burst + 13.57 hours).  Epoch2 consisted of 2x300s exposures in R\nband, with a mean observing time of Sep 14.20236 UT (t_burst + 38.65 hours).\nThe 3-sigma limiting magnitudes are R=25.3 and R=25.6 for Epoch1 and \nEpoch2, respectively. \n\nWe calibrate our astrometry and photometry against the USNO stars of\nHenden et al. (GCN 2720).  We detect sources within 1\" of the Chandra\npositions for 8 of the 22 Chandra sources reported in Butler et al. \n(GCN 2716):\n\nCX0#\tR-magE1\t\t\tR-magE2\n1\t.\t\t\t.\n2\t23.95+/-0.07\t\t24.10+/-0.07\n3\t.\t\t\t.\n4\t24.29+/-0.01\t\t24.02+/-0.08\n5\t.\t\t\t.\n6\t.\t\t\t.\n7\t22.55+/-0.04\t\t22.53+/-0.04\n8\t.\t\t\t.\n9\t.\t\t\t24.4+/-0.1\n10\t.\t\t\t.\n11\t.\t\t\t.\n12\t25.00+/-0.13\t\t24.67+/-0.09\n13\t22.25+/-0.04\t\t23.27+/-0.05\n14\t.\t\t\t.\n15\t.\t\t\t.\n16\t.\t\t\t.\n17\t.\t\t\t.\n18\t22.38+/-0.04\t\t22.31+/-0.03\n19\t.\t\t\t.\n20\t.\t\t\t.\n21\t25.12+/-0.14\t\t24.56+/-0.10\n22\t.\t\t\t.\n\nOf the optical sources we detect, the source corresponding to \nCXO#13 has declined in brightness the most (by 1.02 pm 0.09 mag),\nand we tentatively suggest this source to be the optical afterglow\nof GRB040912.  The flux decay between our 2 epochs corresponds to \na fade with temporal index alpha=-0.9+/-0.1.\n\nThis message may be cited.",
  "circularId": 2728,
  "createdOn": 1095700150000,
  "email": "nrbutler@space.mit.edu",
  "subject": "GRB040912(=H3557): Candidate Optical Afterglow",
  "submitter": "Nat Butler at MIT/CSR  <nrbutler@space.mit.edu>",
  "eventId": "GRB 040912"
}