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XMM-Newton Science Analysis System

epileupmask (ebkgmap-2.10.1) [xmmsas_20211130_0941-20.0.0]

What is the limitation of this task, if any?

First, this task works on the basis that pile-up rarely happens. If only a count rate in a pixel is given, even if the count rate is quite small, such as 0.001 cts/s, it is impossible to tell whether the pixel is piled up or not — it is either hardly piled up or very severely piled up (e.g., Ballet 1999). This task assumes always the former case, i.e., the count rate is small. In other words, the heavily piled-up pixels could be regarded as not being piled up. If there is a very bright and hence heavily piled-up point-like source, the centre of the source position could be regarded as non-piled-up, whereas an annular region centred at the source will be marked as piled-up.

Second, one does not know whether an event of interest is a piled-up or clean one. The task calculates on the basis that all the events are clean ones. For example, an event with the energy of 3-keV is calculated as such, whether it is caused by a real 3-keV photon or actually a combination of a 1-keV and a 2-keV photons.

Third, this task only takes into account an averaged count rate in the given duration, where the duration is either the whole observation period or that during which the attitude of the satellite is stable, depending on the command-line arguments. Even when a source becomes bright enough to cause a significant amount of pile-up at one stage (i.e., outburst) during a period, if the duration of the outburst phase is short enough, comparing with the whole period, the averaged count rate, which is the total count divided by the total exposure (of the period), could be below the threshold. This may happen not only for variable sources but also due to the moduration of the spacecraft attitude (again depending on the command-line arguments).

Fourth, the event file given should not be filtered based on the energy. Events in the whole energy band is essential to estimate piled-up pixels. Bad GTI events is allowed to be filtered, providing the given exposure extention/attiribute reflects the GTI information. Grade selection can also be made, if a user wishes — remember this task uses only the single-pixel events. The background events should be included in the event file, as long as it is a single-pixel event[*].

XMM-Newton SOC -- 2021-11-30