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The X-Ray Telescope (XRT) on board Swift was mainly designed to providedetailed position, timing and spectroscopic information on Gamma-Ray Burst(GRB) afterglows. During the mission lifetime the fraction of observing timeallocated to other types of source has been steadily increased. In this paper,we report on the results of the in-flight calibration of the timingcapabilities of the XRT in Windowed Timing read-out mode. We use observationsof the Crab pulsar to evaluate the accuracy of the pulse period determinationby comparing the values obtained by the XRT timing analysis with the valuesderived from radio monitoring. We also check the absolute time reconstructionmeasuring the phase position of the main peak in the Crab profile and comparingit both with the value reported in literature and with the result that weobtain from a simultaneous Rossi X-Ray Timing Explorer (RXTE) observation. Wefind that the accuracy in period determination for the Crab pulsar is of theorder of a few picoseconds for the observation with the largest data time span.The absolute time reconstruction, measured using the position of the Crab mainpeak, shows that the main peak anticipates the phase of the position reportedin literature for RXTE by ~270 microseconds on average (~150 microseconds whendata are reduced with the attitude file corrected with the UVOT data). Theanalysis of the simultaneous Swift-XRT and RXTE Proportional Counter Array(PCA) observations confirms that the XRT Crab profile leads the PCA profile by~200 microseconds. The analysis of XRT Photodiode mode data and BAT event datashows a main peak position in good agreement with the RXTE, suggesting thediscrepancy observed in XRT data in Windowed Timing mode is likely due to asystematic offset in the time assignment for this XRT read out mode.

Timing accuracy of the Swift X-Ray Telescope in WT mode