GCN Circular 5525

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
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 , 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).