XMM-Newton Science Analysis System: User Guide

4.9 EPIC-pn Out-of-time events

For the EPIC imaging observing modes, photons are not only registered during the actual integration interval but also during the readout of the CCD (shift of charges along a column toward the readout node). These so called Out-of-Time (OoT) events get a wrong RAWY value assigned and thus finally a wrong energy correction (the correction for charge transfer inefficiency (CTI) depends on the distance from the readout node).

The effect of OoT events broadens spectral features and can be seen in EPIC images as a strip of wrongly reconstructed event positions in RAWY. The fraction of OoT events scales with the mode-dependent ratio of integration and readout time and is highest for the pn full frame (6.3 %) and extended full frame (2.3 %) mode (see the XMM-Newton User Handbook [3] for further details).

If OoT events are a problem for the analysis (if highest spectral resolution is required or if a clean image of e.g. extended emission surrounding a bright source is needed), the tasks epproc (§ 4.3.2) and epchain offer the possibility to simulate OoT events based on the original event list. Note that OoT events do not need to be removed for the purpose of source detection as they are dealt with in the background characterisation stage there (§ 4.12.3 and task description of esplinemap). The pipeline tasks need to be run twice, first creating an OoT event list and then the "normal" calibrated event list. The OoT event list is produced by calling the subtask epevents with the non-default parameter setting withoutoftime=yes. The OoT event list is created treating all events as out-of-time events: after the pattern recognition for the same TIME, PHA, and RAWX a new RAWY value is simulated by randomly shifting the pattern along the RAWY axis and performing the gain and CTI correction afterwards.

epchain resembles largely the epproc task but can speed up the process to remove OoT events as raw intermediate files from the first run (to create the OoT event list) can be kept for the second run (to create the "normal" calibrated event list) allowing the user to skip some of the already performed processing steps.

An OoT event list from a pn imaging mode ODF can be created to derive output products like images and spectra. The effect of OoT events in images and spectra is highlighted in the next two sections. Details of the necessary procedure to treat OoT events are given in the corresponding pages of the SAS Analysis Threads.