Program RGS_fmat produces a response matrix for a fluxed spectrum created by the XMM-Newton SAS task rgsfluxer.
The user should provide an FITS file that is the output of the rgsfluxer program. That program must be run with 3600 equidistant wavelength bins between 4 and 40 Å. The program takes this fluxed spectrum, and creates a response matrix with effective area 1 for each wavelength, and a redistribution function normalised to unity for each wavelength.
This redistribution function is slightly different for RGS1 and RGS2, and also different for the second order spectra. Hence, the user must provide RGS_fmat the information about which RGS and spectral order is used.
In addition, it is also possible to make a single response matrix for combined RGS1 and RGS2 data, first and second order. In this case, first the program RGS_fluxcombine must have been run with option=2. That program then creates the rgsfluxer-compatible file, with the addition of four extra columns, containing for each wavelength the relative weight of the different instrument/order to the total spectrum. These weights are used to weigh the redistribution functions in the combined spectra.
The redistribution function has been parametrised as the sum of three (first order) or 4 (second order) Gaussians, with wavelength-dependent width and relative normalisations. These parametrisations have been obtained by fitting the “true” redistribution function as it is in SAS in 2014 after having re-investigated the RGS line-spread-function by the RGS consortium.
The model energy grid used for the matrix has a uniform spacing of 1/9th of the instrumental FWHM, approxinmated here simply by 0.06/, where is the spectral order. The matrix is fully -compatible, in the sense that it also uses the derivatives of the matrix with respect to energy to get the most accurate results. There is no way back from this to OGIP-typs spectra!
The redistribution function is limited to 1 Å around the central wavelength. For most continuum and absorption line spectra this is sufficient. In a few rare cases of very bright emission line sources, the user is advised to compare the resulting fit with a direct fit using the original SAS response matrices and spectral files.
The matrix is much smaller: the combined RGS matrix has a size of 8.2 Mbyte. This should be compared to the total size of the four matrices in OGIP format, which is over 200 Mbyte.