User's Guide – 1.1 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 srcmatch 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.

 

Sketch of stacked source detection, showing different PSF shapes in individual images, from which the observation-level and stacked source parameters are derived Sketch of a fit to the different PSF shapes in the three EPIC instruments at different off-axis and azimuth angles in overlapping observations. Position and extent of a source are the same in all oabservations, while the count rates are fit in each image. Observation-level and stacked parameters are derived from the same fit, using the parameters of the individual contributing images.


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