XMM-Newton SAS Home Page
XMM-Newton Science Analysis System

omlcbuild (omlcbuild-1.41) [xmmsas_20230412_1735-21.0.0]


This task constructs the PPS product OM OSW FITS source timeseries from two rate files (source and background) produced by separate runs of EVSELECT on a tracking-shifted, QE-corrected OM fast-mode event-list. The output timeseries has 4 columns (background subtracted source rate & error, background rate & error). All rates are in counts/s.

In addition to the two rate files, the two region files (generated by OMREGION) are read to deduce the extraction areas. The following procedure is performed for each time bin. The counts measured in the source and `background` regions, together with the PSF function evaluated over the corresponding regions, are used to set up equations which are then solved to yield source and background rates evaluated for the nominal coincidence-loss region (r=12 pixels). If the parameter bkgfromimage was set to `yes' then the task would read the Imaging-mode data file (image), remove all the sources from this image and calculate the average value of the background level. This value will be used in further processing, so the output background light-curve will be constant. Note that allowance is made for proximity of the source to the edge of the OSW - the fraction of PSF which falls out of the window is restored. These rates are separately corrected for coincidence losses. Subsequent background subtraction, followed by the corrections for dead-time and any PSF beyond 12 pixels, leads to the final source rate. The code allows for cases where the time-binning of the source and `background` rate files differ a little - linear interpolation is employed and the binning and timing alignment of the emerging source light curve matches that of the input source rate file.

Errors are computed assuming Poisson statistics though a systematic error of 2% is later included as a measure of non-Poissonian effects at high count rates.

The basic procedure implemented in the task is as follows:

XMM-Newton SOC -- 2023-04-16