A&A 365, L1-L6 (2001)
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
|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|
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. 1998a, 1998b). 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 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.
|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|
|Open with DEXTER|
|(1.5 keV)||(8 keV)|
|Figure 2: On-axis effective area of the XMM telescopes without (solid line) and with (dot-dashed line) Reflection Grating Assembly (RGA)|
|Open with DEXTER|
|Figure 3: Specimen reported attitude history data. Relative RA & Dec reported during Revolution 81 observation of Lockman Hole (Right Ascension offset for clarity)|
|Open with DEXTER|
|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|
|Open with DEXTER|
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 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.
|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)|
|Open with DEXTER|
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, -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.
|Figure 6: Summary of the data flow in the XMM-Newton ground segment|
|Open with DEXTER|
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.
|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|
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.
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.
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).