XMM-Newton Users Handbook

3.4 REFLECTION GRATING SPECTROMETER (RGS)

Among the science instruments of XMM-Newton, the RGS is best suited for high spectral resolution (from 100 to 500, FWHM) X-ray spectroscopy in the energy range 0.33-2.5 keV or 5-38 Å⁴. The energy range covered by the RGS has a particularly high density of X-ray emission lines including the K-shell transitions and He-like triplets of light elements, such as C, N, O, Ne, Mg and Si; and the L-shell transitions of heavier elements like Fe and Ni; thus offering a large number of diagnostic tools to investigate the physical conditions and chemical composition of the emitting material.

Two of the three XMM-Newton X-ray telescopes are equipped with RGS units. These consist of Reflection Grating Assemblies (RGAs) and RGS Focal Cameras (RFCs), see Fig. 3. The RGAs are mounted in the light path of the two X-ray telescopes with EPIC MOS cameras at their primary focus. Each RGA intercepts about 58% of the total light focused by the mirror module. The grating plates in the RGAs have mean groove densities of about 645.6 lines mm$^{-1}$. The dispersion of the instrument is a slowly varying function of dispersion angle, and is equal to approximately 8.3 and 12.7 mm Å$^{-1}$ at 15 Å  in first and second order, respectively.

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 the soft energy response and aluminium-coated on the exposed side in order to suppress optical and UV light. Each has 1024x768 (27$\mu$)$^2$ pixels, half (1024x384) exposed to the sky and half used as a storage area. During readout, pixel on-chip binning (OCB) is performed in the default spectroscopy mode, leading to a bin size of (81$\mu$)$^2$, which is sufficient to fully sample the RGS line spread function (LSF). In the dispersion direction one bin corresponds to about 7, 10, and 14 mÅ in first order, and about 4, 6, and 10 mÅ in second order for wavelengths of 5, 15 and 38 Å, respectively. The size of one bin projected onto the sky is about 2”.5 in the cross-dispersion direction, and roughly 3, 5 and 7” and 4, 6, and 9” in the dispersion direction at 5, 15 and 38 Å in first and second order, respectively.

Both spectrometers cover the same field of view, with the dispersion direction along the spacecraft -Z axis. In the cross-dispersion direction, the size of the field of view is determined by the width of the CCDs (5$^\prime$), and the spatial resolution in this direction is largely determined by the imaging properties of the mirror. In the dispersion direction, the aperture of RGS covers the entire FOV of the mirrors, although the effective area decreases significantly for off-axis sources. For an on-axis source, the zero-order image of the gratings is not visible on the detector array.

After the first week of operations, an electronic component in the clock driver of CCD4 in RGS2 failed, affecting the wavelength range from 20.0 to 24.1 Å that includes, in particular, the O VII He-like triplet. A similar problem occurred in early September 2000 with CCD7 of RGS1 covering 10.6 to 13.8 Å, where important Ne lines are to be found. The total effective area is thus reduced by a factor of 2 in these wavelength bands. Observers should make any necessary adjustments to ensure the viability of measurements of any important spectral features that may fall in the bands covered by the two RGS CCD chain failures.

The performance of the RGS instruments is explained in detail in the following sections. Key performance parameters are summarised in Table 8.

Table 8: RGS In-orbit Performance

		RGS1			RGS2
		10 Å	15 Å	35 Å	10 Å	15 Å	35 Å
Effective area (cm$^2$)	1st order	51	61	21	53	68	25
	2nd order	29	15	-	31	19	-
Resolution (km s$^{-1}$)	1st order	1700	1200	600	1900	1400	700
	2nd order	1000	700	-	1200	800	-
Wavelength range	1st order	5 - 38 Å (0.35 - 2.5 keV)
	2nd order	5 - 20 Å (0.62 - 2.5 keV)
Wavelength accuracy	1st order	$\pm$5 mÅ			$\pm$6 mÅ
	2nd order	$\pm$5 mÅ			$\pm$5 mÅ
Bin size [3x3 (27$\mu$)$^2$ pixels]		2.5 arcsec (cross dispersion direction)
	7 - 14 mÅ (dispersion direction, first order)