TITLE:   GCN CIRCULAR
NUMBER:  15164
SUBJECT: GRB 130831A: Continued Skynet/PROMPT-SSO, PROMPT-CTIO and Yerkes-41 detections
DATE:    13/09/02 01:57:04 GMT
FROM:    Adam S. Trotter at UNC-Chapel Hill/PROMPT/Skynet  <atrotter@physics.unc.edu>

A. Trotter, J. Haislip, D. Reichart, A. LaCluyze, J. Moore, N. Frank, V. 
Hoette, D. Harper, R. Kron, K. Cudworth, E. Struble, R. Russell, T. 
Berger, H. T. Cromartie, R. Egger, A. Foster, K. Ivarsen, M. Maples, M. 
Nysewander, and J. A. Crain report:

Skynet continued observing the Swift-XRT localization of GRB 130831A 
(Hagen et al., GCN 15139, Swift trigger 568849) with the four 17" 
telescopes of the PROMPT array at Siding Spring Observatory, NSW, 
Australia, four 16" telescopes of the PROMPT array at Cerro Tololo, 
Chile, and the 41" telescope of Yerkes Observatory in Wisconsin, USA. 
Starting at 2013-08-31, 13:39 UT and continuing until 2013-09-01, 17:40 
UT (t=35.5m-28.2h post-trigger), Skynet took a total of 2,227 exposures 
of 60-160s in the BVRI (PROMPT) and g'i' bands (Yerkes). As we reported 
in Trotter et al. (GCN 15148), we detect a bright fading afterglow in 
all bands at RA 23:54:29.8, Dec 29:25:46.4 (J2000.0), within the error 
circle of the Enhanced Swift-XRT localization (Evans et al., GCN 15147).

We continue to detect a fading afterglow in stacked images, with R~20 
mag at t~15h, and R~20.6 mag at t~25h.

A preliminary light curve that includes our first-day PROMPT-CTIO and 
Yerkes data, and our second-day PROMPT-SSO data is at:
http://skynet.unc.edu/grb/grb130831a_2.png

BVRI magnitudes are in the Vega system, and g'i' magnitudes are in the 
AB system, calibrated to 3 APASS stars in the field. Photometry has not 
been corrected for the modest expected line-of-sight Milky Way dust 
extinction E(B-V)=0.046 (Schlegel et al. 1998).

Skynet observations are ongoing.