TITLE:   GCN CIRCULAR
NUMBER:  10061
SUBJECT: GRB 091018: Skynet/PROMPT Continued Observations of Fading
DATE:    09/10/23 14:36:01 GMT
FROM:    Aaron LaCluyze at U.North Carolina  <lacluyze@physics.unc.edu>

A. LaCluyze, D. Reichart, J. Haislip, K. Ivarsen, R. Egger, A. Foster, J. 
Moore, A. Oza, M. Schubel, J. Styblova, A. Trotter, J. A. Crain, and M. 
Nysewander report:

Skynet has continued to observed the afterglow (Stamatikos et al., GCN 
10034) of GRB 091018 (Stamatikos et al., GCN 10034) with four of the 16" 
PROMPT telescopes at CTIO in BVRI.

After fading with a power-law index of about -0.95 in BVR between 3 and 12 
hours after the trigger (LaCluyze et al., GCN 10046), the afterglow faded 
with a steeper power-law index of about -1.3 in BR between 12 hours and 1.5 
days after the trigger, which is consistent with the findings of Cano et 
al. (GCN 10044).

However, between 1.5 days and 3.4 days after the trigger, the afterglow 
faded with a shallower power-law index of about -0.9 in R.

The I - R color of the afterglow, which brightened by about 0.45 mag 
between 3 and 12 hours after the trigger (LaCluyze et al., GCN 10046), has 
faded by about 0.1 +/- 0.3 mag between 12 hours and 3.4 days after the 
trigger, which is consistent with no additional color change, but the 
uncertainty is large.

At 3.4 days after the trigger, the afterglow's magnitude was R = 22.27 
+0.28 -0.23 (statistical) +/- 0.56 (systematic; calibrated to 115 USNO B1 
stars).

Skynet's most recent BVRI light curve, calibrated to USNO B1 and NOMAD 
stars, can be found here:

http://skynet.unc.edu/grb/grb091018.png

Continued observations with larger telescopes are encouraged.