The redistribution function of the EPIC-MOS and EPIC-pn cameras is spatially dependent. In the pn the spectral resolution of the detector improves for positions further away from the CAM-EX, such that the worst resolution is obtained at the RAW-Y=199 position. The MOS redistribution is uniform over the camera except for a small patch in each camera, coincident with the boresight, which shows a broadened response below 1 keV that has evolved with time. This spatial and temporal dependence is coded into CCFs and is treated automatically by rmfgen.
The task calculates an average RMF over the spatial region used to extract the source spectrum. For a correct treatment the task does need to know how the source flux is distributed within the extraction region. A detector map mechanism, successfully employed in arfgen, has been adopted for this purpose. Essentially a grid is placed over the source region and the redistribution parameters calculated at each point. These values are then averaged in some way. By means of command line parameters the averaging may be performed assuming that the flux distribution is that of the point spread function (PSF), a flat uniform distribution or it may be defined explicitly by supplying the task with an image containing the source region.
For point sources the PSF option should be chosen by setting detmaptype=psf. The PSF for photons of energy 2 keV is used. For a single circular, annular or rectangular (BOX) region the EXTENDED mode PSF is used which is based upon a King profile and is the most efficient parameterisation. A source region with a different shape, for instance a region with circles excluded to remove other point sources, will use the HIGH mode implementation.
Large, non-pointlike, sources may be either approximated by a uniform distribution, by setting detmaptype=flat or modelled accurately by creating an image of the source region, using evselect or xmmselect, and supplying this to the task by:
rmfgen detmaptype=array detmaparray=<image_name>
The image MUST be created in detector coordinates (not X/Y coords) and should entirely cover the region used for extracting the spectrum. If the image does not cover the whole region then a warning (detmapXBoundsExceeded or detmapYBoundsExceeded) is issued. It should be noted that this process averages the parameters used to model the redistribution function rather than averaging the function itself. Hence there is little computational overhead associated with this step. Images should be created with sufficient resolution to sample the variation in flux distribution; a 100 by 100 pixel image may be sufficient.
For a more in-depth discussion of detector maps please see the arfgen user guide, section 7.3.