A refined position catalogue of the Swift XRT afterglows

Received ; accepted Abstract. We present a catalogue of refined positions of 68 gamma ray bur st (GRB) afterglows observed by the Swift X-ray Telescope (XRT) from the launch up to 2005 Oct 16. This is a result of the refinement of the XRT boresight calibration. We tested this correction by means of a systematic study of a large sample of X-ray sources observed by XRT with well established optical counterparts. We found that we can reduce the systematic error radius of the measurements by a factor of two, from 6.5 '' to 3.2 '' (90% of confidence). We corrected all the positions of the aft erglows observed by XRT in the first 11 months of the Swift mission. This is particularly important for the 37 X-ray aft erglows without optical counterpart. Optical follow-up of dark GRBs, in fact, will be more effi cient with the use of the more accurate XRT positions.