XMM Users' Handbook

   
REFLECTION GRATING SPECTROMETER (RGS)

Principal Investigator: Dr. A. C. Brinkman, Space Research Organization of the Netherlands

Two of the three XMM X-ray telescopes are equipped with RGS units. These consist of Reflection Grating Assemblies (RGAs) and RGS Focal Cameras (RFCs), see Fig. 3. Among the science instruments of XMM, RGS is best suited for high spectral resolution ( $E/\Delta E$ from 200 to 800) X-ray spectroscopy in the energy range 0.35-2.5 keV (5-35 Å).² The energy range covered by the RGS has a particularly high density of X-ray emission lines, thus offering a large number of diagnostic tools to investigate the composition of the emitting material, e.g., the L shell transitions of heavy elements like Fe and Ni, and K shell transitions of lighter elements, such as N, O, Ne, Mg, and Si. In its high time resolution mode, the RGS offers the possibility to conduct fast soft X-ray spectro-photometry of individual lines.

The grating plates in the RGAs have mean groove densities of ca. 645.6 lines mm^-1. The dispersion of the instrument is 7.39 and 11.144 mm/Å for orders -1 and -2, respectively. The RGAs are mounted in the light path of the two X-ray telescopes with EPIC MOS cameras in their primary focus. Each RGA intercepts about 58% of the total light focused by the mirror module.

The RFCs consist of linear arrays of 9 MOS CCD chips (similar to those in the EPIC MOS cameras), which are located along the dispersion direction of the RGAs. The RGS MOS chips are back-illuminated in order to maximise their soft energy response and aluminium-coated on the exposed side in order to suppress optical/UV light. They have $1024\times768\ (27 \mu {\rm m})^2$ pixels, half of which ( $1024\times384$) are exposed to the sky, while the other half is used as a storage area. During readout, $3\times3$ pixel on-chip binning (OCB) is performed in the default spectroscopy mode, leading to a pixel size for RGS observations of $(81\ \mu$m)², which is sufficient to fully sample the RGS line-spread function (LSF). For an on-axis source the zeroth order image of the gratings is not visible on the detector array.

There are small, but measurable differences between the two RGS units. While these do not lead to a noticeable effect on the effective areas, it is expected that the energy resolution of RGS-2 will be lower than than that of RGS-1 (as shown in Figs. 48-49).