A&A 365, L1-L6 (2001)

XMM-Newton observatory[*]

I. The spacecraft and operations

F. Jansen1 - D. Lumb1 - B. Altieri2 - J. Clavel2 - M. Ehle2 - C. Erd1 - C. Gabriel2 - M. Guainazzi2 - P. Gondoin1 - R. Much1 - R. Munoz2 - M. Santos2 - N. Schartel2 - D. Texier2 - G. Vacanti1

Send offprint request: F. Jansen

Astrophysics Divn., Space Science Dept., ESTEC Postbus 299, Noordwijk 2200AG, The Netherlands - ESA Villafranca Satellite Tracking Station, Apartado 50727, 28080 Madrid, Spain

Received 2 October 2000 / Accepted 17 October 2000

The XMM-Newton Observatory is a cornerstone mission of the European Space Agency's Horizon 2000 programme, and is the largest scientific satellite it has launched to date. This paper summarises the principal characteristics of the Observatory which are pertinent to scientific operations. The scientific results appearing in this issue have been enabled by the unprecedentedly large effective area of the three mirror modules, which are briefly described. The in-orbit performance and preliminary calibrations of the observatory are briefly summarised. The observations from the XMM-Newton calibration and performance verification phase, which are public and from which most papers in this issue have been derived, are listed. The flow of data from the spacecraft, through the ground segment, to the production of preliminary science products supplied to users is also discussed.

Key words: instruments - X-ray astronomy

1 Introduction

On December 10, 1999 the XMM-Newton space observatory, was placed in a 48 hours orbit by the first commercial Ariane-V launch, V504. XMM-Newton is a cornerstone project in the ESA Horizon 2000 programme. Following an original proposal made to the agency in 1982, the primary objectives for the XMM-Newton mission were initially discussed by the scientific community at an ESA organised workshop, held in Lyngby, Denmark, in June 1985. High quality X-ray spectroscopy on faint sources was identified as the major next step following a series of X-ray missions flown in the 1990's. A key technology component to allow this advance was the development by ESA of the large area X-ray mirror modules. The measured in-orbit performance of these mirrors is described in Sect. 2.1. The early in-orbit calibration status of the instrumentation was confirmed by comparisons with extensive ground-based activities.

2 Spacecraft

The 4 tonne, 10 m long XMM-Newton spacecraft is the largest scientific satellite ever to be launched by the European Space Agency. It comprises two large payload modules connected by a long carbon fibre tube which forms the telescope optical bench (Fig. 1). On the Focal Plane Assembly are located the two Reflection Grating Spectrometer (Den Herder et al. 2001) readout cameras, an EPIC PN (Strüder et al. 2001) and two EPIC MOS imaging detectors (Turner et al. 2001). The spacecraft Service Module contains most of the spacecraft sub-systems, as well as the Optical Monitor instrument (Mason et al. 2001), star trackers and the three X-ray Mirror Modules.

2.1 The X-ray telescopes

Each of the three X-ray telescopes on board XMM-Newton consists of 58 Wolter I mirrors which are nested in a coaxial and confocal configuration (Aschenbach et al. 1987). This design provides a large collecting area over a wide energy band. The mirror grazing incidence angles range between 17 and 42 arcmin. Their focal length is 7.5 m and the diameter of the largest mirrors is 70 cm, to be compatible with the shroud of the launcher.

Table 1: Specifications of an XMM-Newton telescope
telescope focal length 7500 mm
number of mirrors per telescope 58
outer mirror radius 350 mm
inner mirror radius 153 mm
axial mirror length 600 mm
outer mirror thickness 1.07 mm
inner mirror thickness 0.47 mm
minimum packing distance 1 mm
mirror substrate material Nickel
reflective coating Gold

Mirrors were replicated from superpolished gold coated mandrels using a nickel electroforming technique (Gondoin et al. 1994). The 58 mirrors of each telescope are bonded on their entrance aperture to the 16 spokes of a single spider. An electron deflector is located in the exit aperture. It produces a circumferential magnetic field which prevents low energy electrons reflected by the mirrors reaching the focal plane detectors. X-ray baffles consisting of two sieve plates each with 58 annular apertures are located in front of the mirror systems. They act as collimators and considerably reduce the amount of straylight in the field of view of the focal plane cameras.

The point spread functions and effective areas of the telescopes were first characterized on-ground during an extensive calibration campaign (Gondoin et al. 1998a1998b). A comprehensive numerical model of the mirror system (Gondoin et al. 1996) was then used to generate an initial calibration database by extrapolating on-ground tests to in-orbit operation conditions and by interpolating between the finite number of measurement points. During the in-orbit calibration programme, appropriate celestial targets were observed to validate this initial database (Gondoin et al. 2000). Analysis results indicate that the telescopes' point responses measured in-orbit are identical to on-ground calibration measurements. In particular, extended sources in the center of the telescope field of view can be studied with a $\sim$5 arcsec spatial resolution (Table 2). Fitting residuals of the spectra of astrophysical plasmas broadly confirm the on-ground calibration of the relative effective area close to the Au M edges. No evidence for contamination is detected close to the C K and O K edges. Measurements of the in-orbit vignetting function match simulation results extrapolated from on-ground calibration. Pointings in the vicinity of the Crab nebula verify the high straylight rejection efficiency of the telescope baffles.

\includegraphics[width=8cm,clip]{XMM39_f1.eps}\end{figure} Figure 1: ``Open'' view of the XMM-Newton observatory. To the left the three mirror modules (with the RGA units mounted behind two of them) can be seen, while at the right the back-end of the instrument platform with all the radiators is visible
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Table 2: In-flight imaging quality of the XMM-Newton X-ray telescopes
  (1.5 keV)   (8 keV)  
FM2-pn 6.6'' 15.1'' 6.6'' 14.8''
FM3-MOS1 6.0'' 13.6'' 5.1'' 12.5''
FM4-MOS2 4.5'' 12.8'' 4.2 '' 12.2''

\end{figure} Figure 2: On-axis effective area of the XMM telescopes without (solid line) and with (dot-dashed line) Reflection Grating Assembly (RGA)
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\end{tabular}\end{figure} Figure 3: Specimen reported attitude history data. Relative RA & Dec reported during Revolution 81 observation of Lockman Hole (Right Ascension offset for clarity)
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...pt,bbury=64pt,bbllx=126pt,bblly=800pt,angle=270,clip=}\end{tabular}\end{figure*} Figure 4: Time throughout the 48 hour XMM-Newton orbit that the radiation levels are not exceeding critical limits. The red line traces the time after perigee, at which safe operations can begin. The blue line displays the equivalent period before perigee. The two large solar flares in summer 2000 can be clearly identified
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2.2 Attitude measurement

The spacecraft service module is equipped with redundant star trackers. The design specification was to make an absolute measurement accuracy of 4 arcsec half-cone angle (95% confidence). The actual resolution of the star tracker centroiding is 1-2 arcsec, therefore after early operations the system has been enabled to report any apparent attitude deviations. Preliminary analysis shows that over long periods the spacecraft is stable to within the star tracker measurement noise, except for occasional reported temporary excursions of several arcseconds when a guide star is momentarily lost (Fig. 3). As these excursions are recorded, the analysis software (Sect. 5) automatically corrects for such events.

With a compact mirror point spread function, the identification of serendipitous sources of fluxes as low as 10-15 ergs cm-2s-1 can be statistically limited to $\leq$1 arcsec, so the nominal attitude measurement limits this precision. Detailed trend analysis is under way to establish the systematic effects (tube thermo-elastic deformation, star tracker calibration etc.) which may reduce the 4 arcsec absolute location accuracy, down to the statistical limits. As shown in the paper by Hasinger et al. (this issue), knowledge of source locations in the field already would allow 1 arcsec astrometric accuracy.

\psfig{figure=XMM39_f5.ps,height=6.2cm,angle=270}\end{tabular}\end{figure} Figure 5: Comparison of background spectrum in PN camera (lower) for same spatial extraction scale as a typical (10-14 ergs cm-2 s-1 class field source (upper)
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2.3 Orbit

The orbit of the satellite is highly eccentric, with a $\sim$48 hour period. The perigee is 7000 km and apogee of 114000 km, while the inclination is $\sim$ $-40 ^{\rm o}$. This orbit was chosen to allow the 5 main X-ray cameras to be cooled to between $-80~^{\rm o}$C - $-100~^{\rm o}$C using only passive radiators. The lowest portion of this orbit is contained within the radiation belts, so operations are limited to altitudes $\geq 60\,000$ km, (Fig. 4). Depending on the season, the satellite is located in- or outside the magnetotail even at operational altitudes, and there is a modulation in the particle background fluxes experienced by the instruments. Limited experience to date shows that $\geq 65\%$ of observation time is executed in the lowest background periods.

The configuration of mirrors, optical bench tube, colimators and detector doors prevent a significant damaging proton flux reaching the instruments, even during the highest background periods.

Figure 5 shows the typical background in the PN camera during quiescent periods. Features to note are that for energies below 3 keV the diffuse background (mainly galactic) dominates. At higher energies a flatter hard spectrum results mainly from the un-rejected particle background of Compton, $\gamma$-ray and fluorescent X-ray events. In both EPIC cameras there is a fluorescent line feature at Al K (1.487 keV), and in the PN camera a signature of Cu K at 8.048 keV. For reference the plot also shows a spectrum of the brightest source in the Lockman Hole field (few 10-14 ergs cm-2 s-1). This comparison (both spectra were extracted from a 2 arcmin diameter) shows the power of XMM to perform spectral analysis on fainter sources than heretofore.

\end{figure*} Figure 6: Summary of the data flow in the XMM-Newton ground segment
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3 Ground station

Normal operations allow coverage of the observational periods of spacecraft orbit with only two ground stations (Perth and Kourou), however there is a seasonally varying gap close to spacecraft apogee of order 1 hour, during which no science operations can be conducted. This implies that uninterrupted exposures of about 65 ks can be implemented at the moment. It is anticipated that by the end of the year 2000 full orbit coverage can be achieved by using an additional operational antenna.

Packets of science data from the 6 instruments, are multiplexed with their housekeeping data and spacecraft telemetry, and transmitted to ground at a rate of 64 kbit/s maximum. Spacecraft data are merged with earth reception time references, and transmitted via ground telecommunications links to the Mission Operations Centre at Darmstadt. The MOC monitors real-time payload health, performs commanding of operations, and provides additional analysis of spacecraft data to provide (for example) the Attitude History Files for each science observation. The science data are further transmitted to the Science Operations Centre at Villafranca, where science Quick Look Analysis is performed. The SOC then processes the data into the Observation Data Files (ODF) for transmission to the science community.

4 Science data files and data flow

The instrument science data, housekeeping and auxillary data necessary to perform analysis are bundled into a data set known as the Observation Data File (ODFs).
Table 3: Sumary of calibration/performance verification observations
Revolution No. Targets Observed Revolution No. Targets Observed
47 3C 58 on & off axis, NGC 2516 76 N132D & Canopus off-axis
48 A496 on & off-axis 77 A S 1101
50 EXO 0748-67 78 RXJ 0720.4-3125
51 PKS 0537 79 Alph Pic & Canopus off-axis
52 V2416 SGR 81 BPM 16274 & Lockman Hole
53 Capella offaxis 82 Mkn 766 & MS 1229.2+6430
54 Capella and Crab off-axis 83 N132D
55 EXO 0748-67 84 PKS 0558-504 & Mkn 421
56 Crab Pulsar & GX 13+1 85 PSR 0540
57 PKS 0312-770 & GX 13+1 86 Coma
58 NGC 2516 & GX 13+1 87 PKS 2155-304
59 EXO 0748-67 88 Abell 2690 & BPM 16274
60 NGC 2516 & G21.5-09 89 NGC 253 & HZ43
61 Canopus on-axis, GX 13+1 off axis & G21.5-09 offset 90 G158-100 & GD153
62 Canopus, GX 13+1 and G21.5-09 all off-axis 91 AB Dor & Zeta Puppis
63 MS 0737.9+7441 92 LMC X-3 & NGC 2516
64 Canopus and G21.5-09 off-axis 93 Coma offset
65 1ES0102-72 and G21.5-09 off-axis 94 3C 273
66 LMC X-3 95 3C 273
67 Canopus off axis and EXO 0748-676 96 Akn 564
68 Canopus off axis and CAL 83 97 M 87 & IRAS 13349+2438
69 YY Gem 98 Coma offset
70 Lockman Hole 99 Coma offset
71 Lockman Hole 100 M 31 Core & A 1795
72 AB Dor 101 Coma offset & A 1835
73 Lockman Hole 102 Tycho & OY Car
74 Lockman Hole 104 BPM 16274
75 Mkn205    

This data set forms the basis of information provided to the Guest Observer community on CD-ROMs. Access to XMM-Newton data can also be achieved via http://xmm.vilspa.esa.es/, where eventually access to all XMM-Newton data will be available.

In order to process the ODFs, such that data are correctly calibrated for further scientific analysis, a data set named the Current Calibration File (CCF) is also supplied to the user. These data are offered as the best currently known calibration pertaining to the subject observation. The data not only allow the ODFs to be processed into calibrated datasets, but also provide all the necessary data to reduce the data scientifically (generate response matrices, make photometric corrections etc.). Improved calibration knowledge is provided from the SOC ftp site with sets of release notices detailing changes. An indexing mechanism ensures that the latest calibration data set for a particular observation date is always available.

The ODFs are first sent to the Survey Science Centre, where they are pipeline processed to produce a set of standard products (Watson et al. 2001). The Guest Observer receives these pipeline products which include cleaned and calibrated event lists, source detection lists and standard image, spectral and timing products. These form a starting point for the interactive analysis of the datasets. In addition to the standard XMM-Newton observation products, the SSC offers comprehensive catalogued cross-correlation analyses. All these are returned to the XMM-Newton Science Archive, and in the case of US Observers also to the NASA GSFC XMM-Newton Guest Observer Facility.

5 Science analysis software

The software used to generate the products has been produced by the SSC and the XMM-Newton SOC, with the extensive support by the instrument hardware teams. It is run in a fixed configuration for the pipeline generation. However the complete set of tasks is bundled together as the Science Analysis Sub-System to allow the Guest Observer to perform intercative analysis of the data.

A GUI aids the user to select task parameters and browse data sets, or point to extensive help documentation. Individual tasks have been chained together to make the calibrated lists, form image products, perform complex detection tasks etc. These metatasks can be re-run by the user with different activation parameters or updated calibration files. An interactive data display and filtering tool allows the user to visually inspect the event lists, filter the data with arbitrary selection expressions, extract images and spectra for later analysis with the SAS and other data analysis packages. The SAS contains an interactive calibration viewer which can be used to display each of the corrections applied to the data. This viewer uses the calibration data sets available to the user and links them with the calibration algorithms available in the SAS to display their combined effect.

As noted in the accompanying papers, the proto-type SAS tasks have been used extensively for basic processing. The public release of the collection of software tools will be made before the end of 2000.

6 Calibrations

The SAS has been widely used in analysis of in-flight calibration observations. An overview of the calibration status is supplied in the accompanying instrument descriptions. The data have been gathered to verify the on-ground measurements where the latter were not possible in a flight-representative mode. Due to these extensive ground calibration activities, the existing calibration knowledge was sufficient to enable the scientific analysis as described in the accompanying papers. Nevertheless, verification of the cross-calibration between XMM instruments, and between XMM and other observatories (CHANDRA, Beppo-SAX and RXTE) is on-going.

After public release of the SAS, the calibration and Performance Verification observation data sets will be made public through the XMM-Newton Archive (see Table 3).



© ESO 2001