GCN Circular 10244

B. E. Cobb (UC Berkeley) reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained
optical/IR imaging of the error region of GRB 091127 (GCN 10191
Not found
 
 
, Troja
et al.) over seven epochs with mid-exposure times between ~2.2 and 100.8 
hours post burst.

The optical afterglow of GRB 091127 (e.g. GCN 10192
Not found
 
 
, Smith et al. & GCN 
10193
Not found
 
 
, Immler et al., GCN 10194
Not found
 
 
) is observed in our imaging.  In our first 
epoch, with a mid-exposure time of ~2.2 hours post-burst, the afterglow 
is clearly detected in our imaging with the following magnitudes:

B = 17.21 +/- 0.02 +/- 0.20 (error in zeropoint)
R = 16.70 +/- 0.02 +/- 0.23 (error in zeropoint)
I = 16.61 +/- 0.02 +/- 0.30 (error in zeropoint)
J = 15.56 +/- 0.05
H = 15.40 +/- 0.05
K = 14.77 +/- 0.05

(Optical photometry is calibrated against USNO-B1.0 stars in the field, so 
suffers from a large photometric calibration error. IR photometry is 
calibrated against 2MASS stars.)

The afterglow remains visible in our optical imaging over all our epochs 
of observation. Between 2.2 and 5.4 hours post burst, the afterglow decays 
with a  power-law of alpha ~ -0.6  (where afterglow flux is proportional 
to t^alpha).  From 27.8 to 100.8 hours post-burst, the afterglow appears 
to have steepened its decay rate to alpha ~ -1.6. This agrees with the 
behavior observed by Haislip et. al (GCNs 10219
Not found
 
 
 & 10230).

mid-exposure
time (hours
post-burst) I mag
--------------------------------------------------------
   2.2  16.61 +/- 0.02  [+/- 0.30, zp error]
   3.8  16.90 +/- 0.02  [+/- 0.30, zp error]
   5.4  17.18 +/- 0.03  [+/- 0.30, zp error]
  27.8  19.28 +/- 0.03  [+/- 0.30, zp error]
  30.6  19.34 +/- 0.03  [+/- 0.30, zp error]
  51.7  21.10 +/- 0.12  [+/- 0.30, zp error]
100.8  21.38 +/- 0.07  [+/- 0.30, zp error]