Stacked source detection
All data products are used in parallel by the source-detection tasks,
coupling images, exposure maps, and background maps for each observation,
instrument, and energy band, and detection masks for each observation and
instrument. Simultaneous source detection is performed by means of the usual
two-step process used for XMM-Newton data: sliding-box source detection
followed by maximum-likelihood fitting, described in the
2XMM User Guide. Deviating from standard source detection
- eboxdetect is run on each input observation separately and on mosaicked
images of all contributing observations, combined per EPIC instrument
and energy band by the task
emosaic. The resulting eboxdetect source lists are merged by the task
within a fixed matching radius of the pixel size times square root of
two (i.e. ∼11.3″ for the catalogue). The likelihood column of
the merged source list holds the maximum detection likelihood of a
source from all contributing lists.
- emldetect is called by
edetect_stack with a minimum detection likelihood of zero in order to
store the parameters of each box-detection source and each input image
in an intermediate source list without (de)selecting sources based on to
their detection likelihood.
Source parameters are derived from the simultaneous fit to all input data. The count rate is fit in each input image, while source position and extent are the same in all images. All-stack source parameters are calculated from all image-level values. Observation-level source parameters are calculated from the same fit by selecting the values from those images that were taken during the respetive observation only (see sketch below). A separate module of the task edetect_stack is dedicated to the calculation of the final source parameters, the quality assessment, and source filtering. In particular, the total equivalent likelihood over all observations and the likelihood for each individual observation are calculated for each detection and used for source selection. Sources are included in the final source list, if at least one of these equivalent likelihoods exceeds a user-defined minimum detection likelihood. As for the other Serendipitous Source Catalogues, a minimum likelihood of six is required.