TITLE:   GCN CIRCULAR
NUMBER:  10046
SUBJECT: GRB 091018:  Skynet/PROMPT Observations of Fading
DATE:    09/10/19 23:59:19 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 observed the Swift/BAT localization of GRB 091018 (Stamatikos et 
al., GCN 10034) with four of the 16" PROMPT telescopes at CTIO beginning 
3.0 hours after the the trigger in BVRI.

We detect the afterglow (Stamatikos et al., GCN 10034) in all filters.

Between 3.0 and 12.3 hours after the trigger, the afterglow faded with 
periods of rebrightening (1) beginning around 5 hours and peaking around 6 
hours, and (2) beginning around 8 hours and peaking around 9 hours.

Overall, if fitted with a power law, the afterglow faded with a power-law 
index of about -0.95 in BVR but only about -0.65 in I.  I - R brightened by 
about 0.45 mag over the course of 12 hours.

At 12.3 hours after the trigger, the afterglow's magnitude was R = 19.82 
+0.14 -0.12 (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