ESA Science & Technology - Publication Archive

This A&A issue features 13 articles describing the pre-flight status of the European Space Agency Planck mission, launched together with the Herschel satellite on 14 May 2009. The Planck mission is designed to image the anisotropies of the cosmic background radiation field over the whole sky, with unprecedented sensitivity and angular resolution, as well as a wide frequency range. As a byproduct of that main goal, it will simultaneously address a wide range of galactic and extragalactic science. The mission involves more than four hundred scientists, who are currently working on data processing, calibration, and data analysis.

The satellite is scheduled to continuously acquire high-quality science data until the end of 2011. An early release of the compact source catalogue will be delivered in January 2011, together with a small set of science papers related to foreground astrophysical sources. The first major cosmology results will be delivered in December 2012.

In this special feature, the telescope's optical system and the design, ground calibration, and performance of the Planck lowand high-frequency instruments are described in detail.

The 11-year activity cycle of the Sun is a consequence of a dynamo process occurring beneath its surface. We analyzed photometric data obtained by the CoRoT space mission, showing solarlike oscillations in the star HD49933, for signatures of stellar magnetic activity. Asteroseismic measurements of global changes in the oscillation frequencies and mode amplitudes reveal a modulation of at least 120 days, with the minimum frequency shift corresponding to maximum amplitude as in the Sun. These observations are evidence of a stellar magnetic activity cycle taking place beneath the surface of HD49933 and provide constraints for stellar dynamo models under conditions different from those of the Sun.

A careful analysis of the wave emissions for this event has shown that Cluster 4 passed through the wave source region. Simultaneous electron particle data from the PEACE instrument in the generation region indicated the presence of a mid-energy electron population (<100 s of eV) that had a highly anisotropic temperature distribution with the perpendicular temperature 10 times the parallel temperature. To understand this somewhat rare event in which the satellite passed directly through the wave generation region and in which a free energy source (i.e., temperature anisotropy) was readily identified, a linear theory and particle in cell simulation study has been carried out to elucidate the physics of the wave generation, wave-particle interactions, and energy redistribution. The theoretical results show that for this event the anisotropic electron distribution can linearly excite obliquely propagating whistler mode waves in the upper frequency band, i.e., above 0.5f_ce. Simulation results show that in addition to the upper band emissions, nonlinear wave-wave coupling excites waves in the lower frequency band, i.e., below 0.5f_ce. The instability saturates primarily by a decrease in the temperature anisotropy of the mid-energy electrons, but also by heating of the cold electron population. The resulting wave-particle interactions lead to the formation of a high-energy plateau on the parallel component of the warm electron velocity distribution. The theoretical results for the saturation time scale indicate that the observed anisotropic electron distribution must be refreshed in less than 0.1 s allowing the anisotropy to be detected by the electron particle instrument, which takes several seconds to produce a distribution.

The relationship between the average structure of the inner magnetospheric large-scale electric field and geomagnetic activity levels has been investigated by Double Star TC-1 data for radial distances between 4.5 R_e and 12.5 R_e and MLT between 18:00 h and 06:00 h from July to October in 2004 and 2005. The sunward component of the electric field decreases monotonically as radial distance increases and approaches zero as the distance off the Earth is greater than 10 R_e. The dawn-dusk component is always duskward. It decreases at about 6 R_e where the ring current is typically observed to be the strongest and shows strong asymmetry with respect to the magnetic local time. Surprisingly, the average electric field obtained from TC-1 for low activity is almost comparable to that observed during moderate activity, which is always duskward at the magnetotail (8 R_e~12 R_e).

The detection of circumstellar water vapour around the ageing carbon star IRC +10216 challenged the current understanding of chemistry in old stars, because water was predicted to be almost absent in carbon-rich stars. Several explanations for the water were postulated, including the vaporization of icy bodies (comets or dwarf planets) in orbit around the star, grain surface reactions, and photochemistry in the outer circumstellar envelope. With a single water line detected so far from this one carbon-rich evolved star, it is difficult to discriminate between the different mechanisms proposed. Here we report the detection of dozens of water vapour lines in the far-infrared and sub-millimetre spectrum of IRC +10216 using the Herschel satellite. This includes some high-excitation lines with energies corresponding to ~1000 K, which can be explained only if water is present in the warm inner sooty region of the envelope. A plausible explanation for the warm water appears to be the penetration of ultraviolet photons deep into a clumpy circumstellar envelope. This mechanism also triggers the formation of other molecules, such as ammonia, whose observed abundances are much higher than hitherto predicted.

The International X-ray Observatory (IXO) is designed to conduct spectroscopic, imaging, and timing studies of astrophysical phenomena that take place as near as in the solar system and as far as in the early universe. It is a collaborative effort of ESA, JAXA, and NASA. It requires a large X-ray mirror assembly with an unprecedented X-ray collection area and a suite of focal plane detectors that measure every property of each photon. This paper reports on our effort to develop the necessary technology to enable the construction of the mirror assembly required by IXO.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

The optics for the International X-Ray Observatory (IXO) require alignment and integration of about fourteen thousand thin mirror segments to achieve the mission goal of 3.0 square meters of effective area at 1.25 keV with an angular resolution of five arc-seconds. These mirror segments are 0.4 mm thick, and 200 to 400 mm in size, which makes it hard not to impart distortion at the sub-arc-second level. This paper outlines the precise alignment, verification testing, and permanent bonding techniques developed at NASA's Goddard Space Flight Center (GSFC). These techniques are used to overcome the challenge of aligning thin mirror segments and bonding them with arc-second alignment and minimal figure distortion. Recent advances in technology development in the area of permanent bonding have produced significant results. This paper will highlight the recent advances in alignment, testing, and permanent bonding techniques as well as the results they have produced.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

Platinum is being explored as an alternative to the sprayed boron nitride mandrel release coating under study at GSFC for the International X-ray Observatory (IXO). Two and three inch diameter, polished (PFS) and superpolished (SPFS) fused silica flat mandrels, were used for these tests. Pt was applied to the mandrels by DC magnetron sputtering. The substrate material was 400 micron thick D263 glass, the material which has been proposed for the IXO segmented optics. These substrates were placed on the mandrels and thermally cycled with the same thermal profile being used at GSFC in the development of the BN slumping for IXO. After the thermal cycle was complete, the D263 substrates were removed; new D263 substrates were placed on the mandrels and the process was repeated. Four thermal cycles have been completed to date. After initially coating the mandrels with Pt, no further conditioning was applied to the mandrels before or during the thermal cycles. The microroughness of the mandrels and of the D263 substrates was measured before and after thermal cycling. Atomic force microscopy (AFM) and 8 keV X-ray reflectivity data are presented.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

The Soft X-Ray Telescope (SXT) modules are the fundamental focusing assemblies on NASA's next major X-ray telescope mission, the International X-Ray Observatory (IXO). The preliminary design and analysis of these assemblies has been completed, addressing the major engineering challenges and leading to an understanding of the factors effecting module performance. Each of the 60 modules in the Flight Mirror Assembly (FMA) supports 200-300 densely packed 0.4 mm thick glass mirror segments in order to meet the unprecedented effective area required to achieve the scientific objectives of the mission. Detailed Finite Element Analysis (FEA), materials testing, and environmental testing have been completed to ensure the modules can be successfully launched. Resulting stress margins are positive based on detailed FEA, a large factor of safety, and a design strength determined by robust characterization of the glass properties. FEA correlates well with the results of the successful modal, vibration, and acoustic environmental tests. Deformation of the module due to on-orbit thermal conditions is also a major design driver. A preliminary thermal control system has been designed and the sensitivity of module optical performance to various thermal loads has been determined using optomechanical analysis methods developed for this unique assembly. This design and analysis furthers the goal of building a module that demonstrates the ability to meet IXO requirements, which is the current focus of the IXO FMA technology development team.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

The mirror design for the International X-ray Observatory (IXO) is currently following two paths: a segmented slumped glass shell Wolter-I design, and a Silicon Pore Optics (SPO) conical approximation to the Wolter-I design. The conical approximation used for the SPO imposes a lower limit to the angular resolution which puts this option at a potential disadvantage. In this paper we describe ways in which this can be circumvented. We analyse the surface profile modifications that can be made to lift this limitation and show that a much closer approximation to the Wolter I ideal is possible. We describe several ways in which a much tighter angular resolution limit could be achieved in practice and discuss ways in which this can be implemented in the manufacture of the SPO.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

The background that will be observed by IXO's X-ray detectors naturally separates into two components: (1) a Cosmic X-ray Background (CXB), primarily due to unresolved point sources at high energies (E>2 keV), along with Galactic component(s) at lower energies that are generated in the disk and halo as well as the Local Bubble and charge exchange in the heliosphere, and (2) a Non-X-ray Background (NXB) created by unvetoed particle interactions in the detector itself. These may originate as relativistic particles from the Sun or Galactic Cosmic Rays (GCR), creating background events due to both primary and secondary interactions in the spacecraft itself. Stray light and optical transmission from bright sources may also impact the background, depending upon the design of the baffles and filters. These two components have distinct effects on observations. The CXB is a sum of power-law, thermal, and charge exchange components that will be focused and vignetted by the IXO mirrors. The NXB, in contrast, is due to particle, not photon, interactions (although there will be some fluorescence features induced by particle interactions), and so will not show the same effects of vignetting or trace the effective area response of the satellite. We present the overall background rates expected from each of these processes and show how they will impact observations. We also list the expected rates for each CXB process using both mirror technologies under consideration and the predicted NXB for each detector.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

The International X-ray Observatory (IXO) is a collaborative effort between NASA, ESA, and JAXA. The IXO science goals are heavily based on obtaining high quality X-ray spectra. In order to achieve this goal the science payload will incorporate an array of gratings for high resolution, high throughput spectroscopy at the lowest X-ray energies, 0.3 - 1.0 keV. The spectrometer will address a number of important astrophysical goals such as studying the dynamics of clusters of galaxies, determining how elements are created in the explosions of massive stars, and revealing most of the "normal" matter in the universe which is currently thought to be hidden in hot filaments of gas stretching between galaxies. We present here a mature design concept for an Off-Plane X-ray Grating Spectrometer (OP-XGS). This XGS concept has seen recent significant advancements in optical and mechanical design. We present here an analysis of how the baseline OP-XGS design fulfills the IXO science requirements for the XGS and the optical and mechanical details of this design.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

High-resolution spectroscopy at energies below 1 keV covers the lines of C, N, O, Ne and Fe ions, and is central to studies of the Interstellar Medium, the Warm Hot Intergalactic Medium, warm absorption and outflows in Active Galactic Nuclei, coronal emission from stars, etc. The large collecting area, long focal length, and 5 arcsecond half power diameter telescope point-spread function of the International X-ray Observatory will present unprecedented opportunity for a grating spectrometer to address these areas at the forefront of astronomy and astrophysics. We present the current status of a transmission grating spectrometer based on recently developed high-efficiency critical-angle transmission (CAT) gratings that combine the traditional advantages of blazed reflection and transmission gratings. The optical design places light-weight grating arrays close to the telescope mirrors, which maximizes dispersion distance and thus spectral resolution and minimizes demands on mirror performance. It merges features from the Chandra High Energy Transmission Grating Spectrometer and the XMM-Newton Reflection Grating Spectrometer, and provides resolving power R = E/DeltaE = 3000 - 5000 (full width half max) and effective area >1000 cm² in the soft x-ray band. We discuss recent results on ray-tracing and optimization of the optical design, instrument configuration studies, and grating fabrication.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

The High Time Resolution Spectrometer (HTRS) is one of six scientific payload instruments of the International X-ray Observatory (IXO). HTRS is dedicated to the physics of matter at extreme density and gravity and will observe the X-rays generated in the inner accretion flows around the most compact massive objects, i.e. black holes and neutron stars. The study of their timing signature and in addition the simultaneous spectroscopy of the gravitationally shifted and broadened iron line allows for probing general relativity in the strong field regime and understanding the inner structure of neutron stars. As the sources to be observed by HTRS are the brightest in the X-ray sky and the studies require good photon statistics the instrument design is driven by the capability to operate at extremely high count rates. The HTRS instrument is based on a monolithic array of Silicon Drift Detectors (SDDs) with 31 cells in a circular envelope and a sensitive volume of 4.5 cm² × 450 µm. The SDD principle uses fast signal charge collection on an integrated amplifier by a focusing internal electrical field. It combines a large sensitive area and a small capacitance, thus facilitating good energy resolution and high count rate capability. The HTRS is specified to provide energy spectra with a resolution of 150 eV (FWHM at 6 keV) at high time resolution of 10 µsec and with high count rate capability up to a goal of 2106 counts per second, corresponding to a 12 Crab equivalent source. As the HTRS is a non-imaging instrument and will target only point sources it is placed on axis but out of focus so that the spot is spread over the array of 31 SDD cells. The SDD array is logically organized in four independent 'quadrants', a dedicated 8-channel quadrant readout chip is in development.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

The Wide Field Imager (WFI) of the International X-ray Observatory (IXO) is an X-ray imaging spectrometer based on a large monolithic DePFET (Depleted P-channel Field Effect Transistor) Active Pixel Sensor. Filling an area of 10 × 10 cm with a format of 1024 × 1024 pixels it will cover a field of view of 18 arcmin. The pixel size of 100 × 100 µm corresponds to a fivefold oversampling of the telescope's expected 5 arcsec point spread function. The WFI's basic DePFET structure combines the functionalities of sensor and integrated amplifier with nearly Fano-limited energy resolution and high efficiency from 100 eV to 15 keV. The development of dedicated control and amplifier ASICs allows for high frame rates up to 1 kHz and flexible readout modes. Results obtained with representative prototypes with a format of 256 × 256 pixels are presented.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

The International X-ray Observatory (IXO) is a merger of the former ESA XEUS and NASA Constellation-X missions, with additional collaboration from JAXA, proposed for launch ~2020. IXO will address the leading astrophysical questions in the 'hot universe' through its breakthrough capabilities in X-ray spectroscopy. The mission covers the 0.1 to 40 keV energy range, complementing the capabilities of the next generation observatories, such as ALMA, LSST, JWST and 30 meter ground-based telescopes. An X-ray Grating Spectrometer is baselined to provide science in the energy range 0.3-1.0 keV at a spectral resolution of E/DeltaE > 3,000 with an effective area greater than 1,000 cm². This will require an array of soft X-ray enhanced CCDs operating at a modest frame rate to measure the diffracted light in both position and energy. Here we describe the baseline camera for the Off-plane XGS instrument using mature CCD technology.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

The International X-ray Observatory (IXO) has a top level requirement that the observing efficiency be 85%. This is a challenging requirement, given that the observing efficiencies for CXO and XMM-Newton are between 60% and 70%. However, the L2 orbit for IXO means that it will not be subject to the earth block/radiation zone effects that are seen for CXO and XMM-Newton. Outside of these effects the efficiencies for CXO and XMM-Newton do approach 85%, so this requirement appears achievable for IXO. In this paper we itemize the effects which impact the observing efficiency, in order to guide the design of the observatory. Meeting the 85% requirement should be possible but will require careful attention to detail.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

Silicon pore optics is a technology developed to enable future large area X-ray telescopes, such as the International Xray Observatory (IXO), a candidate mission in the ESA Space Science Programme 'Cosmic Visions 2015-2025'. IXO uses nested mirrors in Wolter-I configuration to focus grazing incidence X-ray photons on a detector plane. The IXO mirrors will have to meet stringent performance requirements including an effective area of ~3 m² at 1.25 keV and ~1 m² at 6 keV and angular resolution better than 5 arc seconds. To achieve the collecting area requires a total polished mirror surface area of ~1300 m² with a surface roughness better than 0.5 nm rms. By using commercial high-quality 12" silicon wafers which are diced, structured, wedged, coated, bent and stacked the stringent performance requirements of IXO can be attained without any costly polishing steps. Two of these stacks are then assembled into a co-aligned mirror module, which is a complete X-ray imaging system. Included in the mirror module are the isostatic mounting points, providing a reliable interface to the telescope. Hundreds of such mirror modules are finally integrated into petals, and mounted onto the spacecraft to form an X-ray optic of four meters in diameter. In this paper we will present the silicon pore optics assembly process and latest X-ray results. The required metrology is described in detail and experimental methods are shown, which allow to assess the quality of the HPOs during production and to predict the performance when measured in synchrotron radiation facilities.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

The International X-ray Observatory (IXO) is being studied as a joint mission by the NASA, ESA and JAXA space agencies. The main goals of the mission are large effective area (>3m² at 1 keV) and a good angular resolution (<5 arcsec HEW at 1 keV). This paper reports on an activity ongoing in Europe, supported by ESA and led by the Brera Astronomical Observatory (Italy), aiming at providing an alternative method for the realization of the mirror unit assembly. This is based on the use of thin glass segments and an innovative assembly concept making use of glass reinforcing ribs that connect the facets to each-other. A fundamental challenge is the achievement with a hot slumping technique of the required surface accuracy on the glass segments. A key point of the approach is represented by the alignment of the mirror segments and co-alignment of the mirror pairs assembled together. In this paper we present the mirror assembly conceptual design, starting from the design of the optical unit, the error budgets contributing to the image degradation and the performance analysis to assess error sensitivities. Furthermore the related integration concept and the preliminary results obtained are presented.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.

The next large x-ray astrophysics mission launched will likely include soft x-ray spectroscopy as a primary capability. A requirement to fulfill the science goals of such a mission is a large-area x-ray telescope focusing sufficient x-ray flux to perform high-resolution spectroscopy with reasonable observing times. The IXO soft x-ray telescope effort in the US is focused on a tightly nested, thin glass, segmented mirror design. Fabrication of the glass segments with the required surface accuracy is a fundamental challenge; equally challenging will be the alignment of the ~7000 secondary mirror segments with their corresponding primary mirrors, and co-alignment of the mirror pairs. We have developed a system to perform this alignment using a combination of a coordinate measuring machine (CMM) and a double-pass Hartmann test alignment system. We discuss the technique, its ability to correct low-order mirror errors, and results of a recent pair alignment including progress toward the required alignment accuracy of < 2 arcseconds, and discuss the influence of the alignment process on mirror figure. We then look forward toward its scalability to the task of building the IXO telescope.

This paper was presented at the SPIE conference on Astronomical Instrumentation 2010 conference.