XMM-Newton Science Analysis System


omlcbuild (omlcbuild-1.41) [xmmsas_20230412_1735-21.0.0]

Description

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:

• 1. Get the photometry aperture radius from CAL
• 2. Get CAL aperture radius
• 3. Compute the correction factor in the case the photometry aperture radius is greater than the CAL aperture radius in order to later scale the counts to the CAL aperture radius.
• 4. Compute the count rate corresponding to the OM detection limit
• 5. If the use of the Imaging-mode window is requested for computing the background level, then read the image corresponding to this window, remove all the sources from this image, and calculate the average value of the Imaging-mode background.
• 6. Get the list of sources seen in the science window
• 7. For each of these sources:
• 7.1. Compute the source and background regions corresponding to the radii given in the source list (the background region is an annulus; the window size is not yet taken into account). These regions have been used by "Evselect" for extracting the events corresponding to the source and its background.
• 7.2. Check for the neighbouring sources which may contribute to the counts obtained by Evselect. These regions corresponding to these sources are avoided when computing the background.
• 7.3. Calculate the median value of the background (this value is used for representing the error bars in cases when the source and background rate columns are not present in the data set).
• 7.4. Check whether the MOD8FLAG is set to true
• 7.5. Compute the PSF map corresponding to the source
• 7.6. Compute the real source and background extraction areas (the neighbouring sources are avoided when computing the background area), as well as the PSF-fractions corresponding to these areas.
• 7.7. Compute the PSF-fractions within and outside of the OM science window
• 7.8. Compute the ratios (scaling factors) between the PSF-fractions corresponding to the extraction, CAL, and photometric radii (the extraction radius is not necessarily the same as the CAL radius, and the latter is not necessarily the same as the photometry radius).
• 7.9. Build the light-curves (loop through all of the time bins).
• 7.9.1. If the option bkgfromimage is set to “ yes” then use the Imaging-mode background value, otherwise compute the background by solving the system of two equations with two unknowns, using the source and background extraction areas and PSF-fractions, and, of course, the counts from the source and background columns of the input data table. If the source is very bright and the count rates are badly affected by the coincidence losses and modulo-8 noise then nullify the background, assuming that neglecting the background in such cases is a more robust procedure than computing a wrong background value due to the modulo-8 noise pattern.
• 7.9.2. Subtract the background value from the data corresponding to the source column (initially, in this column we have the source+background counts).
• 7.9.3. Correct the source counts for the PSF-fraction outside of the science window. These will be the counts extracted from the region having the source extraction radius.
• 7.9.4. Convert the source and background counts to count rates by dividing them by the time bin value.
• 7.9.5. Scale the background counts to the area corresponding to the source extraction radius.
• 7.9.6. Scale the source and background count rates to the CAL aperture radius.
• 7.9.7. Compute the sum of the source and background count rates.
• 7.9.8. Correct the background value for the coincidence losses by using the CAL routine with the frame time and dead fraction as input parameters.
• 7.9.9. Correct the background value for the time-dependent sensitivity of the photodetector.
• 7.9.10. Correct the source+background value for the coincidence losses by using the CAL routine with the frame time and dead fraction as input parameters.
• 7.9.11. Correct the source+background value for the time-dependent sensitivity of the photodetector.
• 7.9.12. If the ratio of the coincidence-loss corrected count rate to the initial count rate exceeds 2.0 then flag this time bin as having high-coincidence losses.
• 7.9.13. Subtract the background
• 7.9.14. Compute the source and background magnitudes.
• 8.0. Compute average count rates and magnitudes for the entire light curve.
• 9.0. Perform the source variability tests for the entire light curve.