XMM-Newton Science Analysis System: User Guide

4.10 Detecting EPIC X-ray sources

The SAS metatask edetect_chain is a powerful tool to perform source detections on EPIC datasets. It allows searching for sources simultaneously in several energy bands, applying different source detection methods, creation of a final combined source list and the computation of sensitivity maps which contain info on point source detection upper limits. It performs the same kind of analysis as occurs in the SOC pipeline to make PPS products.

The several subtasks called from edetect_chain, their main purposes, needed input files and created output data sets are summarized in Table 5.

Table 5: Schematic overview of the EPIC source detection chain and related input and output data sets.

Task	Purpose	Input files	Output files
eexpmap	creation of exposure maps	attitude file, event list, image	exposure maps
emask	creation of detection masks	exposure map	detection mask
eboxdetect (local mode)	sliding box detection	images, exposure maps, detection mask	box detect source list
esplinemap	creation of background maps	image, exposure map, detection mask, local box list	background map
eboxdetect (map mode)	box detection using background maps	images, exposure maps, detection mask, background maps	map detect source list
emldetect	maximum likelihood fitting	images, exposure maps, background maps, map detect list	final source list
esensmap	creation of sensitivity maps	exposure map, detection mask, background map	sensitivity map

The final output of the EPIC source detection process is a merged source list FITS file which - for every detected source - lists among other parameters the source identification number, the instrument and energy band where it was detected, the source counts, source position and extent, source flux and count rate as well as hardness ratios. The user should read the task description of emldetect for a complete list of the output source list columns.

Another output file is a sensitivity map giving (rough) point source detection upper limits (vignetting corrected source count rate corresponding to the likelihood of detection as specified in the parameter file) for each image pixel.

In § 4.12.3 an example of how edetect_chain and related subtasks actually perform the EPIC source detection is shown.

The task ewavelet provides an alternative source detection method based on a Mexican hat wavelet algorithm which is especially well suited for the detection of extended sources. However, the user is cautioned that the determination of source parameters computed by ewavelet is not (yet) in general as reliable as that obtained from edetect_chain.

As of SAS v9.0, a set of two new tasks, called emosaic_prep and emosaicproc, allow the user to perform coherently source detection on several overlapping exposures, or "pseudo-exposures" as created by the task emosaic_prep, from the same or different observations, as well as from any combination of exposures from the three EPIC instruments. The source detection procedure follows the same steps as those described for the task edetect_chain for source detection on single exposures. This task has been designed specifically for the treatment of XMM-Newton observations taken in emosaic mode, where a single observation is made up of a number of different pointings.