TITLE: GCN CIRCULAR NUMBER: 17605 SUBJECT: GRB 150318A: Skynet PROMPT-CTIO Observations DATE: 15/03/18 18:23:19 GMT FROM: Adam S. Trotter at UNC-Chapel Hill/PROMPT/Skynet A. Trotter, D. Reichart, J. Haislip, A. Aji, R. Beauchemin, T. Berger, A. Dow, A. Foster, N. Frank, M. Hinckle, K. Ivarsen, A. LaCluyze, M. Maples, J. Moore, M. Nysewander, C. Salemi, L. Zbinden, and J. A. Crain report: Skynet observed the Swift BAT/XRT localization of GRB 150318A (Pagani et al., GCN 17599, Swift trigger=635289) with with one 24" telescope (PROMPT 8; I band) and four 16" telescopes (PROMPT 1,3,4,5; V,B,R,I bands) of the PROMPT array at CTIO, Chile. Starting at 2015-03-18 06:06:54 UT and continuing until 09:52 UT (t=2m-2.8h post-trigger), Skynet took a total of 376 exposures ranging from 10-160s. We stacked subsets of these images to maximize the S/N ratio, and detected no optical source in any band at the XRT position. Our 3-sigma limiting magnitudes are: ============================================== tmid scope band exposures lim mag ---------------------------------------------- 104m PROMPT8 I 69 x 80s >20.9 104m PROMPT5 I 43 x 160s >20.9 78m PROMPT4 R 37 x 80s >20.9 104m PROMPT1 V 42 x 160s >21.6 125m PROMPT3 B 18 x 160s >21.1 Limiting magnitudes are in the Vega System, calibrated to 3 APASS stars in the field. Magnitudes have not been corrected for line-of-sight Milky Way dust extinction, with expected E(B-V)=0.03 (Schlafly & Finkbeiner 2011). No further Skynet observations are scheduled.