TITLE:   GCN CIRCULAR
NUMBER:  14815
SUBJECT: GRB 130606A: Skynet/PROMPT detection of the optical afterglow
DATE:    13/06/07 20:46:16 GMT
FROM:    Adam S. Trotter at UNC-Chapel Hill/PROMPT/Skynet  <atrotter@physics.unc.edu>

A. Trotter, A. LaCluyze, D. Reichart, J. Haislip, T. Berger, M. Carroll,
H. T. Cromartie, R. Egger, A. Foster, C. Foster, N. Frank, K. Ivarsen,
D. James, M. Maples, J. Moore, M. Nysewander, E. Speckhard, P. Taylor
and J. A. Crain report:

Skynet observed field of GRB 130606A (Ukwatta et al., GCN 14781, Swift
trigger #557589), using the optical localization of Xu et al. (GCN
14783).  It took ~150 160-second exposures in each of the g', R and z'
bands with three 16" telescopes of the PROMPT array at CTIO, Chile,
starting at 2013-06-07, 00:47 UT (t=3.75h post-trigger), and continuing
until t=9.7h.

We detect a fading afterglow in the z' band in most individual
exposures, with z~18.5 at t~6.3h, and an approximate temporal index
alpha~-1.4.   We do not detect the afterglow in the R or g' bands in
individual exposures, consistent with the spectroscopic redshift z=5.91
reported by Castro-Tirado et al. (GCN 14796).  Stacks of ~70 160-second
exposures at a mean time t~5.7h yield a questionable R~21.9 detection
and a 3-sigma upper limit g'>23.7.  Similar stacks at t~8.5h yield
3-sigma upper limits R>22.7 and g'>23.6.  A preliminary light curve is at:

http://www.skynet.unc.edu/grb/grb130606a.png

Photometry is calibrated to 10 SDSS stars in the field; g' and z'  
magnitudes are in the AB system; R-band magnitudes are in the Vega
system, with the SDSS calibration stars transformed according to Jester
(2005).  No correction has been applied for the expected line-of-sight
Milky Way extinction of E(B-V)=0.02 (Schlegel et al. 1998).

Further Skynet observations are scheduled.