XMM-Newton Science Analysis System: User Guide

4.8.2 Creating response matrices

Analysis of EPIC data products is generally performed by specialised software packages including Xspec[28], Ximage [16] or Xronos[15] (http://heasarc.gsfc.nasa.gov/docs/xanadu/xanadu.html). In addition to the calibrated products, some of these packages require the generation of specific files. In particular, the spectral fitting technique used by Xspec requires a characterization of the EPIC detector response to simulate an output spectrum observed by EPIC. The response function gives the probability that an incoming photon of energy E will be detected in a channel I. This discrete function can be calculated as a product of a Redistribution Matrix File (RMF) by an Auxiliary Response File (ARF). These response files shield the user from the complexity of the EPIC instrument response which varies across the field of view.

There are currently two approaches to obtain RMF redistribution matrix files:

1. The user can make use of ready made (canned) response matrices made available by the EPIC team and accessible through the EPIC Response Files page at
   

   http://www.cosmos.esa.int/web/xmm-newton/epic-response-files
   

   They are virtually identical to the files produced by the SAS task rmfgen.

   Special care must be taken in choosing the appropriate canned RMFs as they depend on the readout mode, the pattern selection, the observation date and the position of the source.

2. The user can also create the RMF using the rmfgen task (rmfgen might take some time to complete, depending on the hardware). The input spectrum file contains the necessary ancillary information to allow the correct response to be made. It corrects for instrumental effects specific to the spectrum and writes the result to a specified dataset.

The RMFs are spatially dependent for both MOS and pn. If source extraction regions are large, e.g. for extended sources, timing mode data or complex regions containing many excluded sources, it is important to specify an appropriate number of detector map bins to allow the SAS to calculate an average response matrix (also see § 4.8.6). It is recommended to use 160 bins in each dimension by:

rmfgen spectrumset=<spectrum_file> rmfset=<rmf_file> detxbins=160 detybins=160

The ARF response file of the EPIC camera shall then be generated by the task arfgen. This task calculates an effective area curve as a function of energy, to be used in conjunction with the RMF file generated before. For each row of the RMF there is a corresponding element in the 1-D ARF. This is normally adjusted by specifying the previously generated response matrix as an input file to the arfgen task.

The arfgen task generates an ARF file taking into account the following effects:

1. Telescope effective area including vignetting by the RGA structure for the MOS cameras,
2. EPIC filter transmission,
3. EPIC CCD quantum efficiency,
4. Region and pattern selections,
5. Fraction of the PSF in the accumulation region (including chip gap, bad pixel and out of observing window effects),
6. Out-of-time events smearing (pn).

The above effects generally depend on the source position in the EPIC field of view. Spatial response variation over an extended source is also taken into account (see § 4.8.6).

As of SAS v12, a full 2-D parameterisation of the PSF as a function of camera, energy and off-axis angle covering the whole field of view is available as the default mode. A full description of the PSF modeling is available at [40]. The 2-D PSF parameters are included in the ELLBETA extension of the PSF CCFs. ELLBETA supersedes all previous PSF models used so far. Users can still access the parametrization of old models through calview, which reads the corresponding extensions of the PSF CCF constituent, or by selecting the appropriate value in the input parameters of those tasks (e.g., arfgen through its input parameter psfmodel), which make use of the PSF.

As of SAS v20.0 a new parameter for arfgen is available, applyfluxcorr which, when set to yes, will correct the EPIC effective areas to be in better agreement with the absolute flux measurements provided by NuSTAR. Using this correction will result in a reduction of the EPIC effective area by about $\sim$18% over the whole energy range. However, as of the release of this guide, the corresponding CCF files are not yet available and the parameter is non-functional. Please watch for an upcoming CCF release of the XRT3_XAREAEF file, which will include the corresponding information.