Pile - up correction for the Swift - XRT observations in WT mode

— The detector at the focal plane of the Swift X-ray Telescope (XRT) supports four readout modes, automatically changed on board, to cover the dy-namical range of fluxes and rapid variability expected from GRB afterglows. The Windowed Timing (WT) mode is used for sources with flux higher than a few mCrab and is obtained by compressing 10 rows into a single row, and then reading out only the central 200 columns of the CCD. Point sources with a rate above ∼300 c s −1 produce severe pileup in the central region of the Point Spread Function. This paper presents three methods to correct the effects of the pileup in WT mode. On ground calibration results and data from the very bright GRB 060124 are used to define and test these methods. Pileup in Charge-Coupled Devices (CCD) is a phenomenon associated with the finite temporal and spatial resolution of the CCD. It is produced by the coincidence of two or more photons within a detection region in the same time interval. The detector is unable to temporally resolve the two or more photons and a pulse height that is roughly the sum of the pulse heights of the individual photon events results. The effect of pileup is a lowering of the source rate and a hardening of the observed spectrum. The detection area for Swift-XRT[1] CCD in WT mode is a 7 × 1 pixel string; an X-ray photon produces charges that are spread over this region with several possible shapes. A classification of these shapes is defined associating to each of them a grade number: low grades correspond to confined charge distributions and high grades to extended charge distributions. A further effect of the pileup is to change the event grade distribution with respect to the one expected from a faint source: groups of low grade events can be recorded as a single higher grade event producing a deficit of low grade events and an excess of high grade events.