TITLE:   GCN CIRCULAR
NUMBER:  14608
SUBJECT: GRB 130427A: Ten nights of Skynet/PROMPT/GORT observations
DATE:    13/05/08 19:14:43 GMT
FROM:    Adam S. Trotter at UNC-Chapel Hill/PROMPT/Skynet  <atrotter@physics.unc.edu>

A. Trotter, D. Reichart, J. Haislip, A. LaCluyze, K. McLin, L. Cominsky, 
T. Berger, H. T. Cromartie, R. Egger, A. Foster, N. Frank, K. Ivarsen, 
M. Maples, J. Moore, M. Nysewander, E. Speckhard, and J. A. Crain report:

Skynet continued observing the Swift/XRT localization of GRB 130427A 
(Maselli et al., GCN 14448, Swift trigger #554620) with four 16" 
telescopes of the PROMPT array at CTIO, Chile (BVRI bands), and with the 
14" GLAST Optical Robotic Telescope (GORT) at the Hume Observatory in 
California (RcIc bands). Our observations span 10 nights, from t=0.65 to 
10.8 days post-trigger. Skynet has taken 2684 160-second exposures on 
the 4 PROMPT telescopes, and 360 160-second exposures on GORT, or a 
total of over 135 hours on source. We performed photometry on each 
exposure, calibrated to two SDSS stars in the field. We stacked 
exposures to improve sensitivity, in groups ranging from 3 exposures on 
night 1, to 60 exposures on night 10.

We detect a fading afterglow in BVRI at the position reported by Elenin 
et al. (GCN 14450), which is ~50" south of the initial XRT localization. 
From night 2 onwards, the light curves fade with an approximate power 
law index alpha=-1 (with no corrections for the known host galaxy flux). 
We see some evidence for flattening of the I-band light curve beginning 
at t~8 days, and of the R-band curve at t~10 days, though it is not 
clear whether this is due to host galaxy contamination or to an 
intrinsic re-brightening.

A preliminary light curve including all Skynet observations through 
t=10.8 days is at:
http://skynet.unc.edu/grb/grb130427a_10.png

See Trotter et al. (GCN 14497, GCN 14510) for descriptions and light 
curves of the first and second nights' observations. Further Skynet 
observations are ongoing.