TITLE: GCN CIRCULAR NUMBER: 20830 SUBJECT: GRB 170306B: Swift-XRT observations DATE: 17/03/07 11:32:04 GMT FROM: Andrea Melandri at INAF-OAB V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A. Cholden-Brown (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), B. Mingo (U. Leicester), A. Melandri (INAF-OAB) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Fermi/LAT-detected burst GRB 170306B in a series of observations tiled on the sky. The total exposure time is 4.5 ks, distributed over 8 tiles; the maximum exposure at a single sky location was 1.4 ks. The data were collected between T0+17.4 ks and T0+29.9 ks, and are entirely in Photon Counting (PC) mode. No uncatalogued X-ray sources have been detected. The 3-sigma upper limit in the field (not including the regions where the tiles overlap) ranges from ~0.007 to ~0.021 ct s^-1, corresponding to a 0.3-10 keV observed flux of 2.9e-13 to 8.5e-13 erg cm^-2 s^-1 (assuming a typical GRB spectrum). Four previously-catalogued X-ray sources have been detected, however their status as catalogued objects makes them unlikely to be the afterglow. The results of the XRT-team automatic analysis of the tiled XRT observations, including a position-specific upper limit calculator, are available at http://www.swift.ac.uk/xrt_products/TILED_GRB00065. This circular is an official product of the Swift-XRT team.