XMM-Newton Science Analysis System: User Guide

6.2.4 Source detection, astrometry and photometry

• omqualitymap (step8). With this task image quality information is stored for later use. Pixel quality information is carried on and assigned to detected sources through quality flags. It will be used for additional source detection in mosaiced or stacked images.

• omdetect (step9). The task omdetect applies a source detection algorithm to find the sources present in the image. First the background is modelled. Then the algorithm looks for sources matching the criteria defined by the task parameters. Aperture photometry is applied to the detected sources. The point spread function (PSF) of OM is taken into account. The output is a source list (SWSRLI) which contains the source positions in absolute detector pixel and the calculated sources and background count rates (cts/sec). Coincidence loss correction is also applied to the count rates of the sources and the background. An optional output file, called the region file, can be generated with a name defined by the parameter regionfile. The region file describes the area associated with each identified source in the image. It is used by the image display tool.

  Extended sources are also identified if the parameter detectextended is set. The photometry of an extended source is made by adding all the counts received in an elliptical area and the background is measured in an annulus, as for point sources. Coincidence loss correction is applied by considering that every pixel is a point like source.

  Since SAS v9.0, omdetect can be used on final mosaiced or stacked images in order to find fainter, previously undetected sources.

• omqualitymap (step10). In this second call the task uses the stored pixel quality information to assign quality flags to the previously detected sources.

• ommag (step11). The task ommag converts count rates into the instrumental OM photometric system. Count rates are corrected for detector deadtime. Time sensitivity degradation of the detector is computed for the epoch of the observation and the count rates are corrected for this effect. For the UV filters, the PSF is extrapolated to account for the larger aperture of 35 pixels radius. The corrected count rates are converted into instrumental magnitudes. The task requires input from the source list file SWSRLI. The output is added as extra column to the source list file (SWSRLI). The sensitivity limit of the specific exposure and the time sensitivity degradation are written as keywords in the source list.

• omatt (step12). The task omatt reads the intermediate image generated by ommodmap and reads the source list file generated by omdetect and ommag. It converts detector position from pixel coordinates into RA/Dec positions based on information from the attitude history file, the instrument boresight file and the OM filter distortion map. Since SAS 12, the attitude is obtained by default from the so-called Raw Attitude File (*RAS.ASC) instead of the Attitude History File (*ATS.FIT). This new method provides a better attitude reconstruction. The celestial positions are added as two new columns to the SWSRLI.FIT file. A subset of USNO catalogue can be provided optionally. Then source cross correlation will be performed and the computed offsets will be added to the derived coordinates of the sources. The most accurate astrometry can be achieved in this way. If the catalogue file is not present, then omichain will attemp to run scat to produce the catalogue file. This requires that this utility is installed into the system, otherwise source cross correlation will not be performed.

  A skyimage (SIMAGE.FIT) is generated from the input image, by resampling the image, applying the distortion correction and north aligning the image. No other correction is applied to this image, i.e. coincidence losses and deadtime are not corrected. The skyimage provides the most accurate positional information of an image in the OM image data processing chain. Note that IMAGE_ has no distortion correction and therefore its positional information is less accurate than in the SIMAGE.