TITLE:   GCN CIRCULAR
NUMBER:  10110
SUBJECT: GRB 091018, SMARTS optical/IR afterglow observations
DATE:    09/10/29 21:47:42 GMT
FROM:    Bethany Cobb at UC Berkeley  <bcobb@astro.berkeley.edu>

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 091018 (GCN 10034, 
Stamatikos et al.) over several epochs, starting ~3.9 hours post-burst. 
For the first epoch, several dithered images were obtained in each filter,
with total summed exposure times of 180s in each of BRIJK and 120s in each 
of H and V.  For later epochs, total summed exposure times amounted to 15 
minutes in I and V and 12 minutes in J and K.

At a mid-exposure time of 2009-10-19 00:57 UT (4.1 hrs post-burst), the 
GRB afterglow (e.g. GCN 10034, Stamatikos et al., GCN 10036, Schaefer et 
al., GCN 10039, Filgas et al.) is detected with the following magnitudes:

B = 19.59 +/- 0.04
V = 19.27 +/- 0.05
R = 18.88 +/- 0.04
I = 18.44 +/- 0.04
J = 18.0 +/- 0.2
H = 17.1 +/- 0.2
K = 16.1 +/- 0.2

Between 4.1 hrs and 33.0 hrs post-burst, the GRB afterglow fades with a 
decay rate of approximately alpha = -1.3 (where afterglow flux is 
proportional to t^alpha).

time
post-burst      I-band magnitude
4.1 hrs         18.44 +/- 0.04
5.5 hrs         18.82 +/- 0.04
8.0 hrs         19.26 +/- 0.04
9.7 hrs         19.49 +/- 0.05
28.6 hrs        21.04 +/- 0.06
33.0 hrs        21.27 +/- 0.08

(Optical photometry is calibrated against Landolt standard stars
and IR photometry is calibrated against 2MASS stars in the field.)