First Results from the XMM-Newton Reflection Grating Spectrometer

First Results from the XMM-NewtonReflection Grating Spectrometer

The XMM-Newton RGS Consortium

A.C. Brinkman*, A.J. den Boggende, L. Dubbeldam, J.W. den Herder, J.S. Kaastra,R. Mewe, and C.P. de Vries

Space Research Organization of the Netherlands

S.M. Kahn, E. Behar, J. Cottam, F.B.S. Paerels, J.R. Peterson, and A.P. RasmussenColumbia Astrophysics Laboratory, Columbia University

G. Branduardi-Raymont and I. SakelliouMullard Space Science Laboratory, University College London

M. Guedel, M. Audard, K. Thomsen, and A. ZehnderPaul Scherrer Institute

C. ErdSpace Science Department, European Space Agency

Overview of the RGS Experiment


* RGS incorporates an array of reflection gratings (RGA), which “picks off” ~ 40% of the light exiting the telescope and disperses it to a dedicated focal plane camera (RFC), consisting of 9 rectangular, back-illuminated CCDs arranged in a strip.

* There are two identical such units, RGS1 and RGS2, mounted behind the MM2 and MM3 telescopes, respectively. The remaining light from each of these two telescopes passes, undeflected through the RGAs to the EPIC-MOS cameras.

* RGS provides high sensitivity, high resolution, X-ray spectroscopy in the wavelength range 5 - 35 Angstroms, or E = 0.35 - 2.5 keV. This is a line-rich region of the spectrum, which contains the prominent K-shell transitions of low-Z abundant elements (C, N, O, Ne, Si) and the diagnostically-important L-shell transitions of Fe.

* The RGS operates simultaneously with EPIC. RGS spectra of sources in the field of view are obtained for every XMM observation.

Layout of the RGS on the XMM-NewtonSpacecraft


The Optical Design of RGS


* The RGS uses an “inverted Rowland circle” design. The gratings are mounted on a circle, which also includes the telescope focus and the RFC CCD strip.

* The gratings are all identical, and they are mounted at the same graze angle with respect to the incident ray passing through grating center.

* This configuration produces nearly stigmatic and aberration-free focussing at all wavelengths in the spectrum.

* The line spacing on the individual gratings is varied to correct for aberrations due to the converging bream.

The Optical Design of RGS


* The gratings are mounted in the “in-plane” configuration, where the light comes in at normal incidence to the grooves.

* The grooves are “blazed” to achieve maximum diffraction efficiency in first order at a wavelength of 15 Angstroms. The blaze angle is 0.7 degrees, and the groove density is 646 lines/mm.

The RGS Reflection Grating Array


The RGS Focal Camera


Comparison of the Grating Spectrometers on Chandra and XMM-Newton


Unique Capabilities of the RGS


* RGS provides an unparalleled combination of effective area and resolution at energies between 0.35 and ~ 1.5 keV.

* This band includes the important He-like lines of nitrogen, oxygen, and neon, as well as the n = 3 - 2 L-shell transitions of iron. The resolution is sufficient to unambiguously resolve these lines, which is essential for model-independent interpretations of the spectrum.

* Although it is a slitless spectrometer, RGS will also obtain reasonably high resolution spectra of moderately extended sources ( < few arcminutes). This is because the dispersion is very high, much higher, for example, than the transmission grating spectrometers on Chandra.

* RGS spectra are obtained in parallel with imaging studies performed with EPIC, and will be available for every observation conducted with XMM. This makes the experiment ripe for serendipitous discovery of unusual spectroscopic features in either the target source, or other sources in the field.

In-Flight Performance of the RGS


* The doors to the two RGS detectors were opened in late January (RGS2) and early February (RGS1).

* All subsystems have been operating nominally, with the exception of CCD4 on RGS2, which experienced a short in the clock driver, precluding its further use. This causes a gap in the spectral coverage for that instrument in the range 20 - 24 Angstroms. Fortunately, this band is still covered by RGS1, so the loss in science capability is minimal.

* The measured resolution agrees very well with raytrace predictions based on our prelaunch calibrations of the grating alignments and the telescope point spread function for both spectrometers.

* The effective area calibration has been harder to quantify because of the lack of an adequate “standard candle” source in the X-ray band. However, a variety of tests indicate that it is within ~ 5% of prelaunch predictions, except at the very shortest wavelengths.

* The measured background rates are ~ a factor two higher than initially expected.

In-Flight Performance of the RGS


RGS Observation of Capella


RGS Observation of Capella


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The XMM-Newton RGS team

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RGS Observation of Capella - Fe L Complexes


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RGS spectrum of Capella, Fe-L components

Fe21+ (0.05)

Fe20+ (0.06)

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Fe15+ (0.06)

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Capella (observed)

Fe17+ (0.25)

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RGS Observation of Capella - Comparison with the Chandra Grating Spectrometers


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Capella, comparison of XMM and Chandra

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The YY Gem/Castor Field


EXO 0748-67


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CAL 83


First Results from the XMM-Newton Reflection Grating Spectrometer

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