TITLE:   GCN CIRCULAR
NUMBER:  16304
SUBJECT: GRB 140518A: Skynet GORT Detections of the Optical Afterglow
DATE:    14/05/18 18:24:41 GMT
FROM:    Adam S. Trotter at UNC-Chapel Hill/PROMPT/Skynet  <atrotter@physics.unc.edu>

A. Trotter, A. LaCluyze, J. Haislip, D. Reichart, K. McLin, L. Cominsky, 
H. T. Cromartie, A. Foster, N. Frank, K. Ivarsen, M. Maples, J. Moore, 
M. Nysewander, R. Beauchemin, T. Berger, A. Dow, M. Hinckle, A. 
Patterson, H. Pegues, J. Pozo, D. Waddell, and J. A. Crain report:

Skynet observed the Swift-XRT localization of GRB 140518A (Melandri et 
al., GCN 16298, Swift trigger 599287) with the 14" GLAST Optical Robotic 
Telescope (GORT) at the Hume observatory in CA, USA. Observations began 
at t=130s and continued until 2.4h post-trigger. Skynet took alternating 
exposures in the Rc and Ic bands, increasing from 10s to 160s. In early 
Ic and Rc images, and in later stacked Ic images, we detect an 
uncatalogued fading optical source at a position consistent with the 
afterglow candidate reported by Zheng et al. (GCN 16299).

The source faded from Ic=16.8 at t=130s to Ic > 18.7 at a mean time 
t~70m. In Rc band, we detect the source marginally at Rc~18.3 at t=4m, 
with Rc>20.1 at a mean time t~55m.

A preliminary light curve is at:
http://www.skynet.unc.edu/grb/grb140518a.png

Photometry is calibrated to six APASS-DR7 stars in the field, and has 
not been corrected for the Galactic foreground extinction corresponding 
to E(B-V)=0.016 (Schlegel et al. 1998).

No further Skynet observations are scheduled.