Publication archive

Publication archive

The X-ray telescope forms the core of the high energy astrophysics observatory XEUS, currently under study at ESA as a well positioned candidate for its Cosmic Visions 1525 Science Programme, which is presently under formulation. The science requirements of XEUS are particularly demanding, combining a large effective area (10m2 at 1 keV), moderate angular resolution (5" requirement, with a goal of 2"), and a low mass for the optics system. The preferred operational orbit for XEUS is a halo orbit around the Lagrangian Point 2 (L2). Background and costing considerations led to the requirement of a single focal plane location, which in combination with the required broad energy response function, in turn requires a focal length of 50m. The mission design is based on formation flying, with the Mirror Spacecraft (MSC) flying inertially, and the Detector Spacecraft (DSC) actively following the focal point. The ambitious XEUS telescope relies on the novel X-ray technology currently under development in Europe. The X-ray optics technology development activities and status as well as the telescope design in general are addressed.
Published: 11 August 2005
The Xeus mission is designed to explore the X-ray emission from objects in the Universe at high redshifts, and the success of the mission depends critically on the deployment of a 10 square metre class telescope system in a suitable orbit for science observations. The minimisation of the telescope mass and volume becomes of critical importance for such a large facility. We describe developments of novel light weight optics that enable a reduction in mass per unit area of more than an order of magnitude, compared with traditional replication optics technology. With such a large collection area, image confusion limits become a scientific driver as well, demanding arcsecond class resolution. We describe measurements that demonstrate the improvement in resolution that gives very high confidence that these requirements can be met. Some implementation details of the mission are briefly mentioned.
Published: 01 January 2004
If sensitive enough, future missions for nuclear astrophysics will be a great help in the understanding of supernovae explosions. In comparison to coded-mask instruments, both crystal diffraction lenses and grazing angle mirrors offer a possibility to construct a more sensitive instrument to detect gamma-ray lines in supernovae. We report on possible implementations of grazing angle mirrors and simulations carried out to determine the performance. In this study we differentiate between single and multilayer mirrors. Moreover we discuss the possibilities of double reflection implementations.
Published: 22 June 2004
We study the gas mass fraction, f(gas), behavior in the XMM-Newton Omega project. The typical f(gas) shape of high redshift galaxy clusters follows the global shape inferred at low redshift quite well. This result is consistent with the gravitational instability picture leading to self similar structures for both the dark and baryonic matter. However, the mean f(gas) in distant clusters shows some differences to local ones, indicating a departure from strict scaling. This result is consistent with the observed evolution in the luminosity-temperature relation. We quantitatively investigate this departure from scaling laws. Within the local sample we used, a moderate but clear variation of the amplitude of the gas mass fraction with temperature is found, a trend that weakens in the outer regions. These variations do not explain departure from scaling laws of our distant clusters. An important implication of our results is that the gas fraction evolution, a test of the cosmological parameters, can lead to biased values when applied at radii smaller than the virial radius. From our XMM clusters, the apparent gas fraction at the virial radius is consistent with a non-evolving universal value in a high matter density model and not with a concordance.
Published: 19 March 2005
We use XMM-Newton blank-sky and closed-cover background data to explore the background subtraction methods for large extended sources filling the EPIC field of view, such as nearby galaxy clusters, for which local background estimation is difficult. We find that to keep the 0.8-7.0 keV band background modeling uncertainty tolerable, one has to use a much more restrictive filter than that commonly applied. In particular, because flares have highly variable spectra, not all of them are identified by filtering the E>10 keV light curve. We tried using the outer part of the EPIC FOV for monitoring the background in a softer band (1-5 keV). We find that one needs to discard the time periods when either the hard-band or the soft-band rate exceeds the nominal value by more than 20% in order to limit the 90% CL background uncertainty to between 5% at E=4-7 keV and 20% at E=0.8-1 keV, for both MOS and PN. This compares to a 10-30% respective PN uncertainty when only the hard-band light curve is used for filtering, and to a 15-45% PN uncertainty when applying the commonly used 2-3 sigma filtering method. We illustrate our method on a nearby cluster A1795. The above background uncertainties convert into the systematic temperature uncertainties between 1% at r=3-4 arcmin and 20--25% (~1 keV for A1795) at r=10-15 arcmin. For comparison, the commonly applied 2-3 sigma clipping of the hard-band light curve misses a significant amount of flares, rendering the temperatures beyond r=10 arcmin unconstrained. Thus, the background uncertainties do not prohibit the EPIC temperature profile analysis of low-brightness regions, like outer regions of galaxy clusters, provided a conservative flare filtering such as the double filtering method with 20% limits is used.
Published: 17 April 2005
By considering model comet nuclei with a wide range of sizes, prolate ellipsoidal shapes, spin axis orientations, and surface activity patterns, constraints have been placed on the nucleus properties of the primary Rosetta target, Comet 67P/Churyumov-Gerasimenko. This is done by requiring that the model bodies simultaneously reproduce the empirical nucleus rotational lightcurve, the water production rate as function of time, and non-gravitational changes (per apparition) of the orbital period (Delta P), longitude of perihelion (Delta omega tilde), and longitude of the ascending node (Delta Omega). Two different thermophysical models are used in order to calculate the water production rate and non-gravitational force vector due to nucleus outgassing of the model objects. By requiring that the nominal water production rate measurements are reproduced as well as possible, we find that the semi--major axis of the nucleus is close to 2.5 km, the nucleus axis ratio is approximately 1.4, while the spin axis argument is either 60+/-15 or 240+/-15 degrees. The spin axis obliquity can only be preliminary constrained, indicating retrograde rotation for the first argument value, and prograde rotation for the second suggested spin axis argument. A nucleus bulk density in the range 100-370 kg/m^3 is found for the nominal Delta P, while an upper limit of 500 kg/m^3 can be placed if the uncertainty in Delta P is considered. Both considered thermophysical models yield the same spin axis, size, shape, and density estimates. Alternatively, if calculated water production rates within an envelope around the measured data are considered, it is no longer possible to constrain the size, shape, and spin axis orientation of the nucleus, but an upper limit on the nucleus bulk density of 600 kg/m^3 is suggested.
Published: 02 July 2005
Starting with nearby galaxy clusters like Virgo and Coma, and continuing out to the furthest galaxy clusters for which ISO results have yet been published (z=0.56), we discuss the development of knowledge of the infrared and associated physical properties of galaxy clusters from early IRAS observations, through the "ISO-era" to the present, in order to explore the status of ISO's contribution to this field. Relevant IRAS and ISO programmes are reviewed, addressing both the cluster galaxies and the still-very-limited evidence for an infrared-emitting intra-cluster medium. ISO made important advances in knowledge of both nearby and distant galaxy clusters, such as the discovery of a major cold dust component in Virgo and Coma cluster galaxies, the elaboration of the correlation between dust emission and Hubble-type, and the detection of numerous Luminous Infrared Galaxies (LIRGs) in several distant clusters. These and consequent achievements are underlined and described. We recall that, due to observing time constraints, ISO's coverage of higher-redshift galaxy clusters to the depths required to detect and study statistically significant samples of cluster galaxies over a range of morphological types could not be comprehensive and systematic, and such systematic coverage of distant clusters will be an important achievement of the Spitzer Observatory.
Published: 02 July 2005
The improved performance of cryogenic detectors has drastically enhanced their utilisation range, allowing a number of space-based applications, with particular emphasis on astronomical observations. In this paper we provide an overview of the main applications of cryogenic detectors onboard spacecraft, together with a description of the key technologies and detection techniques used or being considered for space science missions. A summary of the cryogenic instrumentation technologies is also presented. Specific emphasis is given to space based astronomy in the soft X-ray regime, where superconducting tunnel junctions and cryogenic calorimeters offer well identified advantages. Possible instruments for future astrophysics space missions are also discussed, using XEUS (X-ray Evolving Universe Spectroscopy mission, presently proposed by ESA as a post XMM-Newton project) as a reference.
Published: 05 September 2003
The inner magnetosphere's current mapping is one of the key elements for current loop closure inside the entire magnetosphere. A method for directly computing the current is the multi-spacecraft curlometer technique, which is based on the application of Maxwell-Ampère's law. This requires the use of four-point magnetic field high resolution measurements. The FGM experiment on board the four Cluster spacecraft allows, for the first time, an instantaneous calculation of the magnetic field gradients and thus a measurement of the local current density.
Published: 29 July 2005

The closest Wolf-Rayet star, WR 11 in the binary system 2 Velorum, is the only star for which the spectral signature of the 26Al produced in its core is expected to be detectable with current gamma-ray instruments, through the 1.8 MeV decay of that radioactive nucleus.

We present here the current status of both model predictions, from calculations of massive star evolution including rotation of stellar interior, and from data on 2 Velorum obtained by the ESA's gamma-ray satellite INTEGRAL over the first year of its mission.

Published: 26 July 2005
We investigate the nature of the diffuse intracluster ultraviolet light seen in 12 local starburst galaxies, using longslit ultraviolet spectroscopy obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST ). We take this faint intracluster light to be the field in each galaxy and compare its spectroscopic signature with Starburst99 evolutionary synthesis models and with neighboring star clusters. Our main result is that the diffuse ultraviolet light in 11 of the 12 starbursts lacks the strong O star wind features that are clearly visible in spectra of luminous clusters in the same galaxies. The difference in stellar features dominating cluster and field spectra indicates that the field light comes primarily from a different stellar population and not from scattering of UV photons originating in the massive clusters.
Published: 21 July 2005
Contents: HST News and Status; ST-ECF Update; Hubble's 15th Anniversary; New Development in aXe; STIS CTE Correction Science Case; MultiDrizzle in the Archive Pipelines.
Published: 16 July 2005
We present ISOPHOT observations at 120 and 200 µm of a 31 × 57 arcmin² region, with optical extinction AV ranging between ~ 0.5 and 11 mag, that encloses the Taurus molecular cloud TMC-2. The far-infrared emission is separated into a warm and a cold component using the ISOPHOT data and IRAS measurements at 60 and 100 µm. This separation is based on the very different morphologies of the 60 and 200 µm emission maps.
Published: 02 June 2005
upsilon Cyg is a Be star which shows variations at all timescales. We monitored its spectrum during several years from 1998 to 2004 and in particular during a spectroscopic multisite campaign in 2000. In this paper we present and analyse the data. We observed several outbursts including an intense one in 2000. Moreover, we found several periods of short-term variations, including two frequencies of non radial pulsations at f~3.0 and 2.6 c/d. The stellar rotation is probably also identified at f~1.5 c/d, which is coherent with the rotation frequency deduced from our determination of stellar parameters. The peak-to-peak amplitude of variations also seems to vary in time, maybe due to a beating effect between close frequencies, but the resolution in time of our data does not allow to separate such close frequencies. Finally, a longer timescale variation is maybe present, with a period around 11 years, which could be associated with a binary companion.
Published: 02 June 2005
The Solar Orbiter mission was first discussed at the Tenerife "Crossroads" workshop in 1998, in the framework of the ESA Solar Physics Planning Group. Following a pre-assessment study in ESA's Concurrent Design Facility in 1999, the mission was submitted to ESA in 2000. Solar Orbiter was selected by ESA's Science Programme Committee in October 2000. The mission was subsequently re-confirmed by the SPC in May 2002. on the basis of implementation as a mission group together with BepiColombo. A reassessment of BepiColombo was conducted in 2003, leading to an SPC decision in November 2003 to maintain Solar Orbiter in the Cosmic Vision programme, and to begin an assessment study of Solar Orbiter. At its 107th meeting on 7-8 June 2004, the SPC endorsed the recommendations of the advisory bodies (SSWG and SSAC), and confirmed the place of Solar Orbiter in the Cosmic Vision programme, with the objective of a launch in October 2013 and no later than May 2015.
Published: 23 June 2005
The ESA Huygens Probe entered and descended for nearly 2.5 hours through the atmosphere of Titan on 14 January 2005. Huygens survived impact on the surface and continued its telemetry broadcast to the NASA Cassini spacecraft on two separate radio links, denoted Channels A and B, respectively, for an additional 1.2 hours. The instrumentation for the Huygens Doppler Wind Experiment (DWE) consisting of two Ultra-Stable Oscillators in the transmitter (TUSO) and receiver (RUSO), were implemented only in Channel A. Whereas Channel B functioned flawlessly during the entire mission, the receiver for Channel A was never able to lock onto the Huygens signal because the DWE-RUSO had not been properly programmed into the critical probe radio relay sequence. All data on Channel A, including the DWE measurements and probe telemetry, were thus lost. In spite of this setback, the Channel A signal was successfully received at many radio telescopes on Earth. The precision of these Doppler measurements, considered as an aggregate, is roughly equivalent to that which had been foreseen from the measurements on board Cassini. We present an overview of the DWE ground-based observations and the Titan wind profile derived from them.
Published: 28 May 2005
In light of Huygens measurements, we present our improved model of thermal and photochemical evolution of Titan's atmosphere. Atreya et. al (1978) demonstrated that photolysis of ammonia on primordial Titan is capable of producing a nitrogen atmosphere substantially thicker than that measured by Voyager. E. Wilson (2001) carried this calculation one step further by including methane and water vapor explicitly in the ammonia photochemistry model, and arrived at a preliminary estimate of time required to accumulate different amounts of nitrogen. However, both models assumed an isothermal atmosphere. Since chemistry leading up to nitrogen occurs in the stratosphere, both the thermal structure and saturation effects are important for determining the time constants and amounts of nitrogen production. In this presentation, we discuss preliminary results of a radiative equilibrium model for the primordial middle and lower atmosphere of Titan. It includes CH4, NH3 and H2O in solar proportions for its initial composition, and CH4-CH4 pressure induced absorption, which presently controls the thermal structure in the troposphere. The temperature in the stratosphere is controlled by the haze, and we explore the effects of a haze layer at various altitudes for accelerating conversion of ammonia to nitrogen. Furthermore, we include the effects of enhanced solar flux during the T-Tauri phase, which could speed up both the loss of nitrogen and conversion of ammonia to nitrogen. We are in the process of coupling the radiative transfer model to a comprehensive photochemical model (Wilson and Atreya, 2004) to access the roles of trace species other than those included in this calculation.
Published: 28 May 2005
Titan's nitrogen-rich atmosphere is directly bombarded by energetic ions, due to its lack of a significant intrinsic magnetic field. Singly-charged energetic ions from Saturn's magnetosphere undergo charge exchange collisions with neutral atoms in Titan's exosphere, being transformed into energetic neutral atoms (ENAs). The Ion and Neutral Camera (INCA), one of the three sensors that comprise the Magnetosphere Imaging Instrument (MIMI) on the Cassini/Huygens mission to Saturn and Titan, images the ENA emissions from various ion/gas interaction regions in the Saturnian magnetosphere. During Cassini's second orbit around Saturn the spacecraft performed the Ta Titan flyby (October 26, 2004), at an altitude of only 1174 km. INCA data acquired during this targeted close flyby confirm model predictions of dominant finite ion gyroradii effects, but also reveal a much more complex interaction: maximum ENA emissions are originating at higher altitudes than predicted by a simple Chamberlain-type model of the Titan exosphere. These observations will be analyzed and a simulation will be presented of some of the exospheric features they reveal.
Published: 28 May 2005
During the Huygens probe mission at Titan on 14th January 2005, the Huygens Atmospheric Structure Instrument (HASI) obtained measurements of atmospheric properties from up above 1400 km down to the ground, thus inferring the atmospheric structure. The atmospheric profile along the Huygens probe trajectory during entry phase have been retrieved from the accelerometers data, while below 160 km direct pressure and temperature measurements have been performed. The vertical temperature profile retrieved from HASI data is in very good agreement with the model derived from Voyager's observations, confirms the evidence for a stratopause and the inversion layers in the upper atmosphere as observed during stellar occultations and yielded new details on atmospheric structure.
Published: 28 May 2005
31-Oct-2020 07:59 UT

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