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eimsimprep (eimsim-2.4.2) [xmmsas_20211130_0941-20.0.0]


Make maps of expected background counts

This function may be performed alone by calling the script with entrystage and finalstage=`makebkgmap'. Processing is performed by the task bkgtemplategen. Note that background maps are expected to be supplied by the user if parameter bkgstyle is set to `products'; In this case, of course, the present function is skipped.

A realistic image simulation includes background counts. The XMM EPIC background has not yet been fully characterised but it seems clear that it consists primarily of one component from cosmological x-ray background, which is vignetted to the same degree as x-rays from discrete sources, plus a second component which is a combination of instrumental and particle background and which exhibits, in comparison, only a small amount of spatial variation. It ought therefore to be possible to produce quite a good background model by adding some scaled combination of normal (ie, vignetted) and non-vignetted exposure maps. This is what is done by the present function of eimsimprep.

One background map is produced per band, per exposure, per observation. There are at present two ways of supplying the background rates, controlled by the parameter bkgstyle.

If bkgstyle is set to `srcspecset', the total (ie, vignetted plus non-vignetted) background rates (in counts per second per square arcsec) are read for the appropriate band, instrument and filter from the source template set (name as supplied to parameter srcspecset). At present the fraction of these rates which is to be considered non-vignetted is hard-wired at 0.2. This will no doubt become more sophisticated in the future.

If bkgstyle is set to `user' on the other hand, the respective amounts of vignetted vs. non-vignetted component are supplied via list parameters bkgrates and vigfractions. Each of these parameters should be supplied with a list with $N$ elements, 1 per each of $N$ energy bands listed in the FLUX_SCALES table of the source template set, in the same order as the band IDs occur in that table. The maximum total background rates (in counts arcsec$^{-2}$ sec$^{-1}$) should be supplied via bkgrates; the fractions of these which are to be considered vignetted are supplied via vigfractions. The units of the output image are counts pixel$^{-1}$.

As mentioned above, if parameter bkgstyle is set to `products', the user is expected to supply maps for all necessary instruments and bands. It is up to the user to make sure that such maps are consistent with the remainder of the product templates (exposure maps in particular). At present there is no parameter by which the user can point eimsimprep to these files; instead one just names them correctly and places them in the correct directory. (This rather rudimentary interface may change in future.) The easiest way to determine the correct file names for these is to run eimsimprep with astest=`yes' and bkgstyle=`user' or `srcspecset'. In `astest' mode, the command lines are not sent to the shell, just printed to standard output; the appropriate file names will be found in the invocations of task bkgtemplategen.

Note that the list of simulated sources should contain sources which are too faint to stand any significant chance of being detected - otherwise it would not be possible to assess the sensitivity of the detection scheme. These faint sources will therefore contribute to the vignetted component of the background. This is in accordance with the actual x-ray sky: on statistical and other grounds it is presently believed that a significant fraction of the cosmological x-ray background is comprised of unresolved point sources (ref????*****). It may therefore be desirable to adjust the faint-end cutoff in the list of simulated sources so as to generate a background which mimics as nearly as possible the (somewhat lumpy) statistical qualities of the real x-ray background. A disadvantage in doing this is that the spectrum of the cosmological background is known to be somewhat harder than that of populations of (so far) detectable sources (ref []). Representing it in part by sources and in part by a smooth vignetted map hopefully will allow us to overcome this difficulty.

**** Use Ueda et al, Ap J 518, 656-671 (1999) for the ref?

XMM-Newton SOC -- 2021-11-30