{
  "bibcode": "2009GCN.10244....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 091127 (GCN 10191, Troja\net al.) over seven epochs with mid-exposure times between ~2.2 and 100.8 \nhours post burst.\n\nThe optical afterglow of GRB 091127 (e.g. GCN 10192, Smith et al. & GCN \n10193, Immler et al., GCN 10194) is observed in our imaging.  In our first \nepoch, with a mid-exposure time of ~2.2 hours post-burst, the afterglow \nis clearly detected in our imaging with the following magnitudes:\n\nB = 17.21 +/- 0.02 +/- 0.20 (error in zeropoint)\nR = 16.70 +/- 0.02 +/- 0.23 (error in zeropoint)\nI = 16.61 +/- 0.02 +/- 0.30 (error in zeropoint)\nJ = 15.56 +/- 0.05\nH = 15.40 +/- 0.05\nK = 14.77 +/- 0.05\n\n(Optical photometry is calibrated against USNO-B1.0 stars in the field, so \nsuffers from a large photometric calibration error. IR photometry is \ncalibrated against 2MASS stars.)\n\nThe afterglow remains visible in our optical imaging over all our epochs \nof observation. Between 2.2 and 5.4 hours post burst, the afterglow decays \nwith a  power-law of alpha ~ -0.6  (where afterglow flux is proportional \nto t^alpha).  From 27.8 to 100.8 hours post-burst, the afterglow appears \nto have steepened its decay rate to alpha ~ -1.6. This agrees with the \nbehavior observed by Haislip et. al (GCNs 10219 & 10230).\n\nmid-exposure\ntime (hours\npost-burst) I mag\n--------------------------------------------------------\n   2.2  16.61 +/- 0.02  [+/- 0.30, zp error]\n   3.8  16.90 +/- 0.02  [+/- 0.30, zp error]\n   5.4  17.18 +/- 0.03  [+/- 0.30, zp error]\n  27.8  19.28 +/- 0.03  [+/- 0.30, zp error]\n  30.6  19.34 +/- 0.03  [+/- 0.30, zp error]\n  51.7  21.10 +/- 0.12  [+/- 0.30, zp error]\n100.8  21.38 +/- 0.07  [+/- 0.30, zp error]",
  "circularId": 10244,
  "createdOn": 1259884251000,
  "email": "bcobb@astro.berkeley.edu",
  "subject": "GRB 091127, SMARTS optical/IR afterglow observations",
  "submitter": "Bethany Cobb at UC Berkeley  <bcobb@astro.berkeley.edu>",
  "eventId": "GRB 091127"
}