{
  "bibcode": "2009GCN.10110....1C",
  "body": "B. E. Cobb (UC Berkeley) reports:\n\nUsing the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained \noptical/IR imaging of the error region of GRB 091018 (GCN 10034, \nStamatikos et al.) over several epochs, starting ~3.9 hours post-burst. \nFor the first epoch, several dithered images were obtained in each filter,\nwith total summed exposure times of 180s in each of BRIJK and 120s in each \nof H and V.  For later epochs, total summed exposure times amounted to 15 \nminutes in I and V and 12 minutes in J and K.\n\nAt a mid-exposure time of 2009-10-19 00:57 UT (4.1 hrs post-burst), the \nGRB afterglow (e.g. GCN 10034, Stamatikos et al., GCN 10036, Schaefer et \nal., GCN 10039, Filgas et al.) is detected with the following magnitudes:\n\nB = 19.59 +/- 0.04\nV = 19.27 +/- 0.05\nR = 18.88 +/- 0.04\nI = 18.44 +/- 0.04\nJ = 18.0 +/- 0.2\nH = 17.1 +/- 0.2\nK = 16.1 +/- 0.2\n\nBetween 4.1 hrs and 33.0 hrs post-burst, the GRB afterglow fades with a \ndecay rate of approximately alpha = -1.3 (where afterglow flux is \nproportional to t^alpha).\n\ntime\npost-burst      I-band magnitude\n4.1 hrs         18.44 +/- 0.04\n5.5 hrs         18.82 +/- 0.04\n8.0 hrs         19.26 +/- 0.04\n9.7 hrs         19.49 +/- 0.05\n28.6 hrs        21.04 +/- 0.06\n33.0 hrs        21.27 +/- 0.08\n\n(Optical photometry is calibrated against Landolt standard stars\nand IR photometry is calibrated against 2MASS stars in the field.)",
  "circularId": 10110,
  "createdOn": 1256852862000,
  "email": "bcobb@astro.berkeley.edu",
  "subject": "GRB 091018, SMARTS optical/IR afterglow observations",
  "submitter": "Bethany Cobb at UC Berkeley  <bcobb@astro.berkeley.edu>",
  "eventId": "GRB 091018"
}