The performance of out-of-time events of the follow-up X-ray telescope onboard Einstein probe

Purpose: The purpose of this study is to investigate the performance of out-of-time events (OoT events) in the Follow-up X-ray Telescope (FXT) onboard the Einstein Probe. The OoT events, registered on the PN-CCD during its readout time, can provide information on the sources, e.g., spectral indices, and diagnose the behavior of pile-up.
Methods: In order to analyze the characteristics of OoT events, we simulated sources, each of which had been modeled using an absorbed power law combined with Gaussians, in different FXT modes and different luminosities. This allowed us to determine the OoT spectral indices and pile-up effect.
Results: In the timing mode of FXT, the OoT events occupy a large proportion and the OoT correction must be done with the assumption that all photons originate from the source position on the CCD. While for some bright sources, the OoT events may be sufficient to derivate the source spectra, especially when pile-up occurs, particularly in the full frame mode and the partial window mode of FXT.
Conclusion: We conclude that the OoT events can provide additional information about the celestial sources and can be used to estimate whether the pile-up occurs or not and the scope of pile-up. Furthermore, they can be also used to determine the new sources and improve the detection significance of a source.