[vsnet-grb-info 8603] GRB 091029, SMARTS optical/IR afterglow observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  10111
SUBJECT: GRB 091029, SMARTS optical/IR afterglow observations
DATE:    09/10/29 23:30:19 GMT
FROM:    Bethany Cobb at UC Berkeley  <bcobb at 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 091029 (GCN 10097, Grupe et 
al.) with a mid-exposure time of 50 minutes post-burst (2009-10-29 04:43 
UT). Total summed exposure times amounted to 36 minutes in I and 30 
minutes in J. Imaging was done under poor (2.5") seeing conditions.

The afterglow of GRB 091029 (e.g. GCN 10097, Grupe et al.; GCN 10098, 
Filgas et al.; GCN 10099, LaCluyze et al.) is detected in our images. 
Preliminary comparison to USNO-B1.0 stars in I and 2MASS stars in J 
indicates the following afterglow magnitudes:

time post-burst         I mag           J mag
50 minutes              17.9+/-0.1      17.6+/-0.1

In individual I-band images, the afterglow fades by ~0.4 magnitudes 
between 30 and 69 minutes post-burst. During this time, the afterglow 
exhibits a shallow decay rate of alpha ~ -0.4+/-0.1 (where afterglow flux 
is proportional to t^alpha), which is consistent with the decay rate 
measured by LaCluyze et al. (GCN 10099).