TITLE:   GCN CIRCULAR
NUMBER:  14909
SUBJECT: GRB 130615A: Continued Skynet/PROMPT Observations
DATE:    13/06/16 22:03:11 GMT
FROM:    Nathan Frank at U.North Carolina  <nrfrank@live.unc.edu>

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

Skynet continued observing the field of GRB 130615A (Sonbas et al., GCN 14897, Swift trigger #558271) with four 16" telescopes of the PROMPT array at CTIO, Chile, starting at 2013-06-16, 04:10 UT, and continuing until 10:11 UT (t=18.4h-24.4h post-trigger). In Frank et al. (GCN 14901) we reported detection of an uncatalogued optical source at the position reported by Elliott et al. (GCN 14898) and Evans et al. (GCN 14900).  We could not determine from our data whether the source was fading.

We took ~300 60s exposures in each of the B, V, R and I bands, with a mean time t~0.9d.  We do not detect the optical source in the B, R or I bands in stacks of all the exposures; there is a questionable (3-sigma) detection in the V-band stack. The 3-sigma limiting magnitudes of our stacked images are:
Filter       B       V       R       I
Mag       >21.83  >21.95  >22.09  >21.17

A light curve of both the first and second nights' data is at:
http://skynet.unc.edu/grb/grb130615a_2.png

Photometry is calibrated to 6 APASS stars in the field and magnitudes are in the Vega system. No correction has been applied for the expected line-of-sight Milky Way extinction of E(B-V)=0.12 (Schlegel et al. 1998). These observations are consistent with a fading afterglow with alpha <~ -0.4.

No further Skynet observations are scheduled.