TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5525 SUBJECT: GRB 060904b: SMARTS optical/IR afterglow observations DATE: 06/09/05 04:47:23 GMT FROM: Bethany Cobb at Yale U B. E. Cobb and C. D. Bailyn (Yale), part of the larger SMARTS consortium, report: Using the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained optical/IR imaging of the error region of GRB 060904b (GCN 5505, Grupe et al.) with a mid-exposure time of 2006-09-04 06:28 UT (~4 hrs post-burst ) and again at 2006-09-04 08:28 UT (~6 hrs post-burst). For each set of observations, total summed exposure times amounted to 15 minutes in I and V and 12 minutes in J and K. (Imaging was carried out in a symmetrical manner so that the mid-exposure time is the same for all final combined images in a given set of observations.) The afterglow of GRB 060904b (GRB 5505, Grupe et al.) is detected in each combined image. filter magnitude at change in magnitude 4hrs post-burst between 4 and 6hrs post-burst -------------------------------------------------------- V - 0.56 +/- 0.06 I 19.04 +/- 0.03 0.63 +/- 0.04 J 17.84 +/- 0.08 0.29 +/- 0.10 K 15.99 +/- 0.06 0.46 +/- 0.07 Unfortunately, imaging was done under non-photometric conditions so no Landolt or Persson standard stars are available with which to determine the offset between instrumental and apparent magnitude. Therefore, the above values are determined using "on-chip" standards with USNO-B1.0 stars in the optical and 2MASS stars in the IR. The errors on the photometric calibration are ~0.3 in I and ~0.1 in J and K; these errors are in addition to the statistical errors listed above. No USNO-B1.0 V-band values are available to calibrate our V-band observations, so we can only report the change in magnitude between epochs. These values have not been corrected for galactic extinction. The average optical decay rate (afterglow flux proportional to t^-alpha) between 4 and 6 hours post-burst is alpha ~ 1.4, and the average IR decay rate is alpha ~ 0.9. After correcting for a galactic extinction of E(B-V) = 0.173, the spectral index of the afterglow at 4 hrs post burst is found to be beta ~ -1.3 (flux proportional to wavelength^-beta).