Reflection Gratings XMMNewton RGS

ESA's XMM-Newton, launched in 1999, carries three X-ray telescopes with the largest effective area up to now. Each of the telescopes has about twice the area of

Capella: Oyn-Triplet

Procyon: OVII-Triplet

Capella: Oyn-Triplet

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Capella:

Ov-Triplet

Procyon: OVII-Triplet

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Procyon: Ov-Triplet

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Fig. 8.8 With Chandra-LETG measured spectrum (line-dotted) and fit (solid) for the O VII and C V triplets of Capella (left) and Procyon (right). The dotted lines represent the total background. The binsize is 0.02 A for O VII and 0.03A for C V [11]

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the Chandra Observatory telescope but, on the other hand, with an angular resolution 20 times worse. To achieve an appreciable spectral resolution, transmission gratings are needed to have an unfeasible ruling density, reflection gratings are, therefore, the first choice. Their ruling density can be much lower because they are operated under shallow angles.

The reflection grating spectrometer (RGS) consists of two identical instruments behind two of the three X-ray mirrors [3]. The design incorporates an array of 182 reflection gratings placed in the beam behind the mirror in the in-plane classical configuration. Since the beam is converging, the gratings are not parallel, and they all lie on a Rowland circle. The gratings pick off roughly half of the X-ray light and diffract it into an array of CCD detectors offset from the telescope focal plane (Fig. 8.9).

The spectral bandwidth is limited to 5-35 Aby the size of the detector and vignetting within the stacked gratings. The RGS provides a spectral resolution of X « 0.04 A or a resolving power X/AX between 150 and 800 in first order diffraction. With up to 250 cm2 effective area (both instruments including mirror and detector), the RGS is the most efficient spectrometer so far [6]. To give an example for the spectral capabilities of the RGS, absorption by oxygen in the Interstellar Medium has been studied in detail in the spectral region around 0.53 keV using the (Fig. 8.10). The complexity of this oxygen region led to the conclusion that the absorption feature is produced by atomic as well as compound oxygen.

Fig. 8.9 RGS instrument [3]

Fig. 8.10 Oxygen absorption due to the interstellar medium in the spectrum of Cyg X-2 [5]

Wavelength (A)

Fig. 8.10 Oxygen absorption due to the interstellar medium in the spectrum of Cyg X-2 [5]

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