TITLE: GCN CIRCULAR NUMBER: 20820 SUBJECT: GRB 170306A: LCO Cerro Tololo observations DATE: 17/03/06 09:02:26 GMT FROM: Cristiano Guidorzi at Ferrara U,Italy C. Guidorzi (U. Ferrara), S. Kobayashi, I.A. Steele (LJMU), A. Gomboc (U. Nova Gorica), C.G. Mundell (U. Bath) on behalf of a large collaboration report: We observed Swift GRB 170306A (Markwardt et al. GCN 20819) on March 6, 07:26:06 UT (~20 minutes since the GRB trigger time) with the 1-m LCO unit in Cerro Tololo with SDSS r and i filters. Within the Swift XRT error circle we detect an uncatalogued source at the following position: RA(J2000) = 17:32:16.76 Dec(J2000)= -44:44:53.7 with an error radius of 1". We find the following magnitude values as calibrated against nearby USNOB-1 objects (R2 and I values): Mid Time Exposure Filter Magnitude (AB) (min) (s) ------------------------------------------------------- 24 5x60 SDSS-I 18.5 +- 0.2 34 5x60 SDSS-R 19.0 +- 0.2 ------------------------------------------------------- At the moment we cannot make any statement on the temporal behaviour of the source. The expected Galactic dust extinction is A_r=1~mag, A_i=0.7 mag (from Schlafly et al. 2011). Given the crowded field, we caution that the source might be unrelated to the GRB.