{
  "bibcode": "2006GCN..5525....1C",
  "body": "B. E. Cobb and C. D. Bailyn (Yale), part of the larger SMARTS\nconsortium, report:\n\nUsing the ANDICAM instrument on the 1.3m telescope at CTIO, we\nobtained optical/IR imaging of the error region of GRB 060904b\n(GCN 5505, Grupe et al.) with a mid-exposure time of\n2006-09-04 06:28 UT (~4 hrs post-burst ) and again\nat 2006-09-04 08:28 UT (~6 hrs post-burst).  For each\nset of observations, total summed exposure times amounted\nto 15 minutes in I and V and 12 minutes in J and K.\n(Imaging was carried out in a symmetrical manner so that the mid-exposure\ntime is the same for all final combined images in a given\nset of observations.)\n\nThe afterglow of GRB 060904b (GRB 5505, Grupe et al.) is detected in\neach combined image.\n\nfilter  magnitude at            change in magnitude\n        4hrs post-burst         between 4 and 6hrs post-burst\n--------------------------------------------------------\nV       -                       0.56 +/- 0.06\nI       19.04 +/- 0.03          0.63 +/- 0.04\nJ       17.84 +/- 0.08          0.29 +/- 0.10\nK       15.99 +/- 0.06          0.46 +/- 0.07\n\nUnfortunately, imaging was done under non-photometric conditions so\nno Landolt or Persson standard stars are available with which\nto determine the offset between instrumental and apparent\nmagnitude.  Therefore, the above values are determined using \"on-chip\"\nstandards with USNO-B1.0 stars in the optical and 2MASS stars in the IR.\nThe errors on the photometric calibration are ~0.3 in I and ~0.1\nin J and K; these errors are in addition to the statistical errors listed\nabove. No USNO-B1.0 V-band values are available to calibrate\nour V-band observations, so we can only report the change in magnitude\nbetween epochs. These values have not been corrected for galactic \nextinction.\n\nThe average optical decay rate (afterglow flux proportional to t^-alpha)\nbetween 4 and 6 hours post-burst is alpha ~ 1.4, and the average IR\ndecay rate is alpha ~ 0.9.\n\nAfter correcting for a galactic extinction of E(B-V) = 0.173,\nthe spectral index of the afterglow at 4 hrs post burst is found to be\nbeta ~ -1.3 (flux proportional to wavelength^-beta).",
  "circularId": 5525,
  "createdOn": 1157431643000,
  "email": "cobb@astro.yale.edu",
  "subject": "GRB 060904b: SMARTS optical/IR afterglow observations",
  "submitter": "Bethany Cobb at Yale U  <cobb@astro.yale.edu>",
  "eventId": "GRB 060904B"
}