Pile-up fraction on the follow-up X-ray telescope onboard Einstein Probe

Purpose In this work, we present the pile-up fractions on the follow-up X-ray telescope (FXT) onboard Einstein Probe using its different read-out modes and filters for various source fluxes. This investigation could help understand the pile-up effect on FXT and choose the filter and readout mode when observing dedicated sources.
Methods We use the Simulation of X-ray Telescopes package to conduct the simulation from incidence of photons from celestial sources to the detection on the FXT pnCCD. Based on the reconstructed signals, we make a statistic on the counts of incident photons and detected events to obtain the pile-up fraction for each set of read-out mode and filter configuration.
Results and conclusion From the perspective of incident photons among all detected events, the pile-up fraction increases monotonically with the brightness of the source. From the viewpoints of valid detected events, the fraction values exhibit an initial nonlinear increase, followed by a decrease, and a subsequent increase as the source fluxes increase, attributed to the presence of pile-up effects and the selection criteria for valid events. For some certain fluxes, the fraction value is larger in faster readout modes than lower modes. From the viewpoint of incident photon in valid detected events, a 10 % pile-up fraction corresponds to 5 mCrab in full frame mode (FF), 120 mCrab in partial windowed mode (PW), 3 Crab in Timing mode (TM) under the medium filter type. At these fluxes, the deviation of the fitted photon indices is within 10 % from that of the input source model.