TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4972
SUBJECT: GRB 060418, SMARTS continuing afterglow observations
DATE:    06/04/18 07:22:10 GMT
FROM:    Bethany Cobb at Yale U  <cobb@astro.yale.edu>

B. E. Cobb (Yale), part of the larger SMARTS consortium, reports:

Continuing ANDICAM observations (see GCN 4970 for observing details) of 
the afterglow of GRB 060418 (Falcone et al. GCN 4966) record the steady 
fading behavior of the afterglow between all 45-second individual I-band 
exposures, as shown below. These preliminary magnitudes were 
calibrated using several USNO-B1.0 stars and the error in the photometric 
calibration is ~0.2 mag.  The relative error is ~0.05 magnitudes.

time
post-burst      I-magnitude
1.09            16.50
1.17            16.66
1.41            16.79
1.48            16.85
2.25            17.44
2.32            17.49
2.57            17.61
2.64            17.71
3.17            17.79   (225-second exposure)
3.34            17.99   (225-second exposure)

The decay rate is, therefore, approximately alpha = -1.2 between about 1 
and 3 hours post-burst.  This is similar to the power law index 
calculated by Nysewander et al. (GCN 4971) during the first hour 
post-burst and suggests that no jet break has yet been observed.