ESA Science & Technology - Publication Archive

Scaling the XMM data-handling system to the specific needs of the mission has resulted in a flexible and relatively uncomplicated On-Board Data-Handling (OBDH) Subsystem. The XMM spacecraft is operated in real-time and is fully compliant with the ESA packet data-handling standards. Packets are of variable length and presented to the OBDH when available. The OBDH telemetry/telecommand cycles are asynchronous to the instrument and subsystem cycles. The telemetry rates can easily be reallocated to cope with unforeseen spacecraft configurations, e.g. lost telemetry sources. The absence of requirements for automatic reconfiguration has contributed to the reduced subsystem complexity.

The XMM Electrical Power Subsystem (EPS) is 'conventional' in design in that it follows the ESA Power Standard. This article describes the subsystem's main features and performances, and the steps that have been taken in implementing the lessons learnt from the highly successful SOHO spacecraft recovery efforts, which were described in detail in ESA Bulletin No. 97.

The pointing and alignment performance of the XMM spacecraft will have a very strong influence on the quality of the scientific results obtainable. The pre-launch unit and subsystem tests and subsequent analyses have shown that the scientific requirements will indeed be met with comfortable margins and the performance goals will be met for more than 90% of all anticipated observations.

Attitude and orbit control of the XMM spacecraft relies on two subsystems on-board: the Attitude and Orbit Control Subsystem (AOCS) and the Reaction Control Subsystem (RCS). Together they must provide:

• stable Sun-pointing after the spacecraft's separation from the launcher and during solar-array deployment
• increase the spacecraft's perigee altitude by means of a chemical propulsion delta-V of about 210 ms^-1
• provide undisturbed, high-accuracy, three-axis pointing during scientific observations lasting up to 40 h
• slew the spacecraft between observations, and before and after perigee ensure that the Sun remains more than 15 deg away from the telescope at all times.

The XMM spacecraft has a conventional structure and thermal design. Due to the long focal length of the telescopes (7.5 m), the mirrors are far removed from the instruments. On the ground and during the launch, the structure has to maintain the integrity of the whole spacecraft. The thermal control does not make use of onboard software. In orbit, the functions of the structure and the thermal control are mixed. Their global common requirement is to relate and align the set of mirrors at one end of the spacecraft with the set of instruments at the other. Some parts of the instruments will be kept at cryogenic temperatures, but most of the spacecraft will be kept at about room temperature. It is the task of the thermal control to maintain these diverse requirements by passive and active means. Under inevitably varying thermal conditions, the structure has to stay very 'straight'.

The XMM observatory has, at its heart, three large X-ray telescopes, which will provide a large collecting area (1430 cm² each at 1.5 keV, and 610 cm² each at 8.0 keV) with a spatial resolution of around 14-15 arcsec. At the end of 1998, three months ahead of schedule, the three flight and the two spare models of the X-ray telescope were handed over to the XMM Prime Contractor Daimler Chrysler Aerospace (D). The three flight models were integrated onto the spacecraft's optical platform at ESTEC at the end of March 1999.

The payload carried by the X-Ray Multi-Mirror Mission (XMM), the second Cornerstone of the ESA Horizon 2000 Science Programme, consists of three scientific instruments: the Reflection Grating Spectrometer (RGS), the European Photon Imaging Camera (EPIC), and the Optical Monitor (OM). This article provides a general overview of the main characteristics of all three instruments.

The X-Ray Multi-Mirror Mission (XMM) is an X-ray astrophysics observatory scheduled for launch in December 1999. With a projected lifetime of 10 years, it will enable astronomers to conduct sensitive spectroscopic observations of a wide variety of cosmic sources.

Two years after the launch of the XMM-Newton Observatory saw a gathering of about 350 scientists at ESTEC for the Conference 'New Vision of the X-ray Universe'. This huge interest in the mission and the rapidly increasing number of scientific papers published as a result of XMM-Newton observations show the importance of ESA's latest observatory for astrophysics in the 21st century.

The term 'Target of Opportunity' (ToO) is used in astronomy to identify unpredictable events whose study is of the highest scientific interest. For Xmm-Newton a ToO is an astronomical event observable by its instruments, which cannot be predicted and scheduled on the time scale of one year, yet is scientifically significant enough to justify the interruption of the ongoing observing programme. Here we discuss the kinds of objects that are suitable for observation as ToOs with XMM-Newton, their scientific interest, and how they are incorporated into the overall observing schedule in terms of target selection and mission planning. What has been done so far and the preliminary results for a few ToO examples are then presented

The XMM-Newton space observatory - a Cornerstone mission in ESA's Horizon 2000 Programme and originally referred to as the High- Throughput X-ray Spectroscopy Mission - was placed into a 48-hour orbit by the first commercial Ariane-5 launch (V504) on 10 December 1999. This brief survey of the scientific results obtained during the first year of XMM-Newton operations clearly illustrates that the observatory is more than living up to expectations and already providing unique and promising results, even before full scientific data analysis gets officially underway.

The launch of the X-ray Multi-Mirror (XMM) telescope on flight V119, the first commercial launch of Ariane-5, will mark another important milestone for European Space and the Agency's Scientific Programme. The mission, conceived in the late 1970s, entered the assessment-study phase in 1982 and was adopted as a 'Cornerstone Mission' in the then new Horizon 2000 Programme approved by the Science Programme Committee in 1984. The industrial studies were completed in 1988 and the scientific payload selected in 1989.

Cataclysmic Variables' (CVs) are a distinct class of interacting binaries, transferring mass from a donor star to a degenerate accretor, a white dwarf (WD). In all observational determinations, and as is required by theory for stable mass transfer, the donor star is of lower mass than the accretor.

The XMM-OM instrument extends the spectral coverage of the XMM-Newton observatory into the ultraviolet and optical range. It provides imaging and time-resolved data on targets simultaneously with observations in the EPIC and RGS. It also has the ability to track stars in its field of view, thus providing an improved post-facto aspect solution for the spacecraft. An overview of the XMM-OM and its operation is given, together with current information on the performance of the instrument.

The EPIC focal plane imaging spectrometers on XMM-Newton use CCDs to record the images and spectra of celestial X-ray sources focused by the three X-ray mirrors. There is one camera at the focus of each mirror; two of the cameras contain seven MOS CCDs, while the third uses twelve PN CCDs, defining a circular field of view of 30' diameter in each case. The CCDs were specially developed for EPIC, and combine high quality imaging with spectral resolution close to the Fano limit. A filter wheel carrying three kinds of X-ray transparent light blocking filter, a fully closed, and a fully open position, is fitted to each EPIC instrument. The CCDs are cooled passively and are under full closed loop thermal control. A radio-active source is fitted for internal calibration. Data are processed on-board to save telemetry by removing cosmic ray tracks, and generating X-ray event files; a variety of different instrument modes are available to increase the dynamic range of the instrument and to enable fast timing. The instruments were calibrated using laboratory X-ray beams, and synchrotron generated monochromatic X-ray beams before launch; in-orbit calibration makes use of a variety of celestial X-ray targets. The current calibration is better than 10% over the entire energy range of 0.2 to 10 keV. All three instruments survived launch and are performing nominally in orbit. In particular full field-of-view coverage is available, all electronic modes work, and the energy resolution is close to pre-launch values. Radiation damage is well within pre-launch predictions and does not yet impact on the energy resolution. The scientific results from EPIC amply fulfil pre-launch expectations.

he European Photon Imaging Camera (EPIC) consortium has provided the focal plane instruments for the three X-ray mirror systems on XMM-Newton. Two cameras with a reflecting grating spectrometer in the optical path are equipped with MOS type CCDs as focal plane detectors (Turner 2001), the telescope with the full photon flux operates the novel pn-CCD as an imaging X-ray spectrometer. The pn-CCD camera system was developed under the leadership of the Max-Planck-Institut für extraterrestrische Physik (MPE), Garching. The concept of the pn-CCD is described as well as the different operational modes of the camera system. The electrical, mechanical and thermal design of the focal plane and camera is briefly treated. The in-orbit performance is described in terms of energy resolution, quantum efficiency, time resolution, long term stability and charged particle background. Special emphasis is given to the radiation hardening of the devices and the measured and expected degradation due to radiation damage of ionizing particles in the first 9 months of in orbit operation.

The ESA X-ray Multi Mirror mission, XMM-Newton, carries two identical Reflection Grating Spectrometers (RGS) behind two of its three nested sets of Wolter I type mirrors. The instrument allows high-resolution (E/DE 100 to 500) measurements in the soft X-ray range (6 to 38 Å or 2.1 to 0.3 keV) with a maximum effective area of about 140 cm² at 15 Å. Its design is optimized for the detection of the K-shell transitions of carbon, nitrogen, oxygen, neon, magnesium, and silicon, as well as the L shell transitions of iron. The present paper gives a full description of the design of the RGS and its operational modes. We also review details of the calibrations and in-orbit performance including the line spread function, the wavelength calibration, the effective area, and the instrumental background.

This Special Letters Issue of Astronomy & Astrophysics is dedicated to the first results from XMM-Newton. One year after the launch of the X-Ray space observatory, this series of 56 publications describes the spacecraft and the instruments onboard, and provides a first look at their exciting scientific capabilities.

XMM is the largest scientific observatory developed by ESA and dedicated to exploring the Universe in the soft-X-ray portion of the electromagnetic spectrum. This article presents XMM as a complete system, composed by the space segment (the satellite and its launch vehicle) and the ground segment (all of ground-based infrastructure needed to control the satellite and gather the scientific data). The definition of the XMM orbit, which is also described, is a key element in meeting the primary mission objectives and satisfying the project programmatic requirements and constraints.

The in-orbit imaging performance of the three X-ray telescopes on board of the X-ray astronomy observatory XMM-Newton is presented and compared with the performance measured on ground at the MPE PANTER test facility. The comparison shows an excellent agreement between the on ground and in-orbit performance.