XMM-Newton Science Analysis System: User Guide

6.3.3 Source detection & astrometry

• omdetect (step8). The task omdetect applies the source detection algorithm to the fast mode pseudo image to locate the existing sources. It builds a source list similar to the image data one (SWSRLI) which contains the source position in absolute detector pixel and the calculated source and background count rates (cts/s).

• omatt (step9). The task omatt performs astrometry conversions from pixel positions to astronomical coordinates in the source list and rotates the pseudo image to obtain a north aligned skyimage.

• omregion (step10). The task omregion produces the regions for the source and background needed later for evselect

• evselect (step11-12). The task evselect is run twice consecutively on the event-list file to extract the photon events for the source and the background in the specified time sampling. Intermediate rate files are created for the source and the background.

• omlcbuild (step13). The task omlcbuild generates coincidence loss and dead-time corrected source time series using the source and background rates produced by evselect. The background rates are subtracted and the photometry corrections are applied. If the source is offset from the centre, the knowledge of the PSF is used to account for possible missing photons. Time dependent sensitivity degradation is corrected as for image data. A light-curve is obtained as final result for each of the sources in the fast mode windows in the exposure.

  The parameter bkgfromimage=yes can be used to measure the background in the associated image mode data. This is the default option in SAS version 14. (See section § 6.3.4.)

• lcplot (step14). The task lcplot reads the light-curve file and makes some plots. Some statistics are also applied to assess possible source variability.