TITLE:   GCN CIRCULAR
NUMBER:  17210
SUBJECT: GRB 141221A: Skynet PROMPT-CTIO observations of the optical afterglow
DATE:    14/12/21 15:16:10 GMT
FROM:    Adam S. Trotter at UNC-Chapel Hill/PROMPT/Skynet  <atrotter@physics.unc.edu>

A. Trotter, J. Haislip, D. Reichart, A. Aji, R. Beauchemin, T. Berger, 
A. Dow, A. Foster, N. Frank, M. Hinckle, K. Ivarsen, A. LaCluyze, M. 
Maples, J. Moore, M. Nysewander, C. Salemi, L. Zbinden, and J. A. Crain 
report:

Skynet observed the Swift BAT/XRT localization of GRB 141221A (Sonbas et 
al., GCN 17206, Swift trigger=622006) with two 14" telescopes of the 
PROMPT array at Cerro Tololo, Chile. Starting at 2014-12-21 08:07:56 UT 
and continuing until 08:35 UT (t=45s-28m post-trigger), Skynet took a 
total of 44 exposures ranging from 5s to 160s in the V and I bands. We 
clearly detect the optical afterglow (Covino, GCN 17208, Guidorzi et 
al., GCN 17209) with a rising light curve prior to t=2min, where it 
peaks at I=14.8.

A preliminary light curve is at:

http://www.skynet.unc.edu/grb/grb141221a.png

Magnitudes are in the Vega System, calibrated to 3 APASS stars in the 
field.  Magnitudes have not been corrected for line-of-sight Milky Way 
dust extinction, with expected E(B-V)=0.024 (Schlafly & Finkbeiner 2011).

Skynet observations will continue.