ESA Science & Technology - Publication Archive

S-Cam 3 is the 3rd generation of a cryogenic camera, based on superconducting tunnel junctions (STJs), for ground-based optical astronomy, deployed at the 4.2m William Herschel Telescope (WHT) at La Palma (Spain). It exploits a 10x12 pixel array of Ta/Al STJs, covering a field of view of ~9°x11° on the sky. The wavelength band extends from 330-750nm, with a wavelength resolving power of ~10 at 500nm. The detectors are operated at ~285mK, achieved with a double stage ⁴He-³He sorption cooler. Each pixel has its own electronic readout chain at room temperature, with a JFET-based charge sensitive preamplifier. The instrument has undergone extensive testing and calibration, followed by the first observation campaign at La Palma in July 2004. This campaign has focused on point sources with time variability, exploiting the instrument's unique combination of spectro-photometry with high time resolution.

To overcome the limited field of view which can be achieved with single STJ arrays, DROIDs (Distributed Read Out Imaging Devices) are being developed. DROIDs consist of a superconducting absorber strip with proximized STJs on either end. The ratio of the two signals from the STJs provides information on the absorption position and the sum signal is a measure for the energy of the absorbed photon. In our devices the absorber is an epitaxial Ta strip that extends underneath the Ta/Al read-out STJs. Thus, the bottom electrode of the STJs is an integral part of the absorber. Due to the proximity effect, the STJs have a lower energy gap than the absorber, causing trapping of quasiparticles in the STJs. The trapping will change with thicker Al layers because the energy gap of the devices will decrease. A series of 50x200µm and 20x200µm absorbers (including 50x50µm STJs) and different Al trapping layer thicknesses, ranging from 65 to 130nm, have been tested. The devices have been illuminated with 6 keV 55Fe photons. The position resolution is found to improve with increasing Al thickness. It is found that the current model needs to be adapted for DROIDs to account for different injection of quasiparticles into the STJ and extra losses to the absorber.

To overcome the limited field of view that can be achieved with single STJ arrays, DROIDS (Distributed Read Out Imaging Devices) are being developed. DROIDs consist of a superconducting absorber strip with proximized STJs on either end. The ratio of the two signals from the STJs provides information on the absorption position and the sum signal is a measure for the energy of the absorbed photon. To produce a large field of view with the least number of connection wires possible, the size of the DROID is an important parameter. A set of devices with different lengths, ranging from 200 to 700µm, has been tested at optical wavelengths. The widths of the DROIDs are 30µm with 30x30µm STJs Ta/Al STJs on either side. With 30nm layer thickness of Al the trapping of quasiparticles in the STJ is not optimal, but the devices can comfortably be operated at 300mK. All devices have been processed on a single wafer and are located on the same chip. Thus the STJs are all identical and any variation in response can be attributed to a difference in geometry. The position resolution is found to be degraded for shorter absorbers due to cross talk between the two STJs. The charge output of the different devices decreases with length due to reduced tunnel probability and losses in the absorber.

In a quest to further improve the performance of superconducting tunnel junctions as photon detectors over the broad spectral range from optical to X-ray wavelengths, a fundamental understanding of their limits is required. Recently fabricated Ta/Al STJs have shown an exceptionally high responsivity (number of collected charge carriers versus absorbed photon energy) and spectral resolution (R = E/Delta_E > 22 at 2.5eV). This high spectral resolution has now revealed some unique features when plotted against photon wavelength. The experimental data indicate the important role of the photon absorption profile. We have shown that vertical inhomogeneity is a fundamental consequence of the quasiparticle generation process in the thin film such that pair breaking phonons emitted in the process of energy down conversion have a chance to escape depending on the absorption depth. This results in an inhomogeneous broadening of the detected signal. We also found that another, previously unknown fundamental noise source exists which is related to statistical fluctuations of the angular distribution of phonons emitted in the down-conversion process. We present the new experimental data and compare them to the predictions of the down-conversion theory. We show that, while the responsivity is rather constant in the optical wavelength range, the intrinsic resolution exhibits a number of features which can be explained by changing statistical variations of the phonon losses as function of absorption depth.

We present here the results of a deep (130 ks) XMM-Newton observation of the cluster of galaxies 2A 0335+096. The deep exposure allows us to study in detail its temperature structure and its elemental abundances. We fit three different thermal models and find that the multi-temperature wdem model fits our data best. We find that the abundance structure of the cluster is consistent with a scenario where the relative number of type Ia supernovae contributing to the enrichment of the intra-cluster medium is ~25%, while the relative number of core collapse supernovae is ~75%. Comparison of the observed abundances to the supernova yields does not allow us to put any constrains on the contribution of Pop III stars to the enrichment of the ICM. Radial abundance profiles show a strong central peak of both type Ia and core collapse supernova products. Both the temperature and iron abundance maps show an asymmetry in the direction of the elongated morphology of the surface brightness. In particular the temperature map shows a sharp change over a brightness edge on the southern side of the core, which was identified as a cold front in the Chandra data. This suggests that the cluster is in the process of a merger with a subcluster. Moreover, we find that the blobs or filaments discovered in the core of the cluster by Chandra are, contrary to the previous results, colder than the ambient gas and they appear to be in pressure equilibrium with their environment.

As part of the first far-IR line survey toward Orion KL, we present the detection of seven new rotationally excited OH Lambda-doublets (at ~48, ~65, ~71, ~79, ~98, and ~115 mum). Observations were performed with the Long Wavelength Spectrometer Fabry-Pérot on board the Infrared Space Observatory. In total, more than 20 resolved OH rotational lines, with upper energy levels up to ~620 K, have been detected at angular and velocity resolutions of ~80" and ~33 km/s, respectively. The OH line profiles show a complex behavior evolving from pure absorption, P Cygni type, to pure emission. We also present a large-scale, 6' declination raster in the OH ²Pi_3/2 J=5/2⁺-3/2^- and ²Pi_3/2 J=7/2^--5/2⁺ lines (at 119.441 and 84.597 mum) revealing a decrease of excitation outside the core of the cloud. From the observed profiles, mean intrinsic line widths, and velocity offsets between emission and absorption line peaks, we conclude that most of the excited OH arises from Orion outflow(s), that is, the "plateau" spectral component. We determine an averaged OH abundance relative to H₂of chi(OH)=(0.5-1.0)×10^-6, a kinetic temperature of more than ~100 K, and a density of n(H₂)~=5×10⁵ cm^-3. Even with these conditions, the OH excitation is heavily coupled with the strong dust continuum emission from the inner "hot core" regions and from the expanding flow itself. Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands, and the UK) and with the participation of ISAS and NASA.

We present a follow-up imaging study (HST + ground based observations) of the complex gravitational lens system RXS J113155.4-123155. The latter consists of a quadruply imaged QSO and of an Einstein ring. Thanks to the MCS deconvolution technique (Magain, Courbin, Sohy 1998), we retrieve accurate relative positions for the lensed QSO images and photometry in the Bessel B, V, R, I filters and in the J-SOFI, F160W and Ks filters. The HST frames unveil structures in the Einstein ring as well as an unidentified object X in the vicinity of the lensing galaxy. We discuss the lightcurves and the chromatic flux ratio variations observed in this system and deduce that both intrinsic variability and microlensing took place during a span longer than one year. We also demonstrate that microlensing may easily account for the so called anomalous flux ratios presented in the discovery paper. However, the actual flux ratios are still poorly reproduced when modelling the lens potential with a Singular Isothermal Ellipsoid+shear. We argue that this disagreement can hardly be explained by milli-lensing due to substructures in the lensing galaxy. A solution proposed elsewhere (Claeskens et al. 2005) consists in a more complex lens model including an octupole term to the lens gravitational potential.

Using the observations of three satellites of Cluster (C1, C3, and C4) during the periods July to October 2001 and July to October 2002, we study 209 active time bursty bulk flows (BBFs), the difference between single satellite observations and multisatellite observations, and the difference among three selection criteria (two about BBFs and one about rapid convection event). Single satellite observations show that the average duration of BBFs selected by the criterion of Angelopoulos et al. is 604 s, while multisatellite observations show that the average duration of BBFs is 1105 s. Single satellite sometimes misses the BBFs. The missing ratio of single satellite is 22.4% for the criterion of Angelopoulos et al. and 44.9 % for the criterion of Raj et al. Therefore the single satellite observations cannot tell the true number of BBFs. The multisatellite observations are more important for the criterion of Raj et al. The single satellite observations also show that 22% of substorms are not accompanied by BBFs, while multisatellite observations show that only 4.5% of substorms are not accompanied by BBFs. Thus it seems possible that all substorms are accompanied by BBFs. The occurrence frequency of RCEs in the central plasma sheet obtained by multisatellites is 12.2%. The occurrence frequency of BBFs in the central plasma sheet is 9.5% for single satellite observations and 19.4% for multisatellite observations. So BBFs may contribute more to the transport of magnetic flux, mass, and energy than what was estimated by previous studies based on single satellite observations.

We have used the FORS-1 camera on the VLT to study the main sequence (MS) of the globular cluster NGC 6218 in the V and R bands. The observations cover an area of 3.4 x 3.4 around the cluster centre and probe the stellar population out to the clusters half-mass radius (r_h ~ 2.2). The colour-magnitude diagram (CMD) that we derive in this way reveals a narrow and well defined MS extending down to the 5 sigma detection limit at V~25, or about 6 magnitudes below the turn-off, corresponding to stars of ~ 0.25 Msolar. The luminosity function (LF) obtained with these data shows a marked radial gradient, in that the ratio of lower- and higher-mass stars increases monotonically with radius. The mass function (MF) measured at the half-mass radius, and as such representative of the clusters global properties, is surprisingly flat. Over the range 0.4 - 0.8 Msolar, the number of stars per unit mass follows a power-law distribution of the type dN/dm \propto m^{0}, where, for comparison, Salpeters IMF would be dN/dm \propto m^{-2.35}. We expect that such a flat MF does not represent the clusters IMF but is the result of severe tidal stripping of the stars from the cluster due to its interaction with the Galaxys gravitational field. Our results cannot be reconciled with the predictions of recent theoretical models that imply a relatively insignificant loss of stars from NGC 6218 as measured by its expected very long time to disruption. They are more consistent with the orbital parameters based on the Hipparcos reference system that imply a much higher degree of interaction of this cluster with the Galaxy than assumed by those models. Our results indicate that, if the orbit of a cluster is known, the slope of its MF could be useful in discriminating between the various models of the Galactic potential.

We report on the analysis of the FUSE spectrum of the most metal-poor blue compact dwarf (BCD) galaxy I Zw 18. From a simultaneous fitting of HI and metal absorption lines arising from the neutral interstellar medium (ISM), we infer abundances of heavy elements (Fe, O, Si, Ar, and N) which are several times lower than in the HII regions. The only exception is Fe, whose abundance is the same. The abundance pattern of the ISM suggests ancient star formation (SF) activity with an age of at least a Gyr that enriched the H i phase. A more recent episode that started 10 to several 100 Myr ago is responsible for the additional enrichment of alpha-elements and N in the HII regions.

We examine magnetic flux closure during an extended substorm interval on 29 August 2004 involving a two-stage onset and subsequent re-intensifications. Cluster and Double Star provide observations of magnetotail dynamics, while the corresponding auroral evolution, convection response, and substorm current wedge development are monitored by IMAGE FUV, SuperDARN, and the Greenland magnetometer chain, respectively. The first stage of onset is associated with the reconnection of closed flux in the plasma sheet; this is accompanied by a short-lived auroral intensification, a modest substorm current wedge magnetic bay, but no significant ionospheric convection enhancement. The second stage follows the progression of reconnection to the open field lines of the lobes; accompanied by prolonged auroral bulge and westward-travelling surge development, enhanced magnetic bays and convection. We find that the tail dynamics are highly influenced by ongoing dayside creation of open flux, leading to flux pile-up in the near-tail and a step-wise down-tail motion of the tail reconnection site. In all, 5 dipolarizations are observed, each associated with the closure of ~0.1 GWb of flux. Very simple calculations indicate that the X-line should progress down-tail at a speed of 20 km s^-1, or 6 R_E between each dipolarization.

We examine magnetic flux closure during an extended substorm interval on 29 August 2004 involving a two-stage onset and subsequent re-intensifications. Cluster and Double Star provide observations of magnetotail dynamics, while the corresponding auroral evolution, convection response, and substorm current wedge development are monitored by IMAGE FUV, SuperDARN, and the Greenland magnetometer chain, respectively. The first stage of onset is associated with the reconnection of closed flux in the plasma sheet; this is accompanied by a short-lived auroral intensification, a modest substorm current wedge magnetic bay, but no significant ionospheric convection enhancement. The second stage follows the progression of reconnection to the open field lines of the lobes; accompanied by prolonged auroral bulge and westward-travelling surge development, enhanced magnetic bays and convection. We find that the tail dynamics are highly influenced by ongoing dayside creation of open flux, leading to flux pile-up in the near-tail and a step-wise down-tail motion of the tail reconnection site. In all, 5 dipolarizations are observed, each associated with the closure of ~0.1 GWb of flux. Very simple calculations indicate that the X-line should progress down-tail at a speed of 20 km s^-1, or 6 R_E between each dipolarization.

We have studied ion distributions measured in the Plasma Sheet Boundary Layer (PSBL) of the Earth's magnetotail by the Cluster spacecraft at X ~ -15Re. Field-aligned ion beams (beamlets) accelerated in the magnetotail to energies of ~30keV are typically observed within the interface region between the Plasma Sheet (PS) and the magnetotail lobes. PSBL beamlets are produced by non-adiabatic ion acceleration in the vicinity of X-line which is located, during quiet periods, in the distant parts of the tail. Earlier kinetic models attributed the filamentary and/or bursty nature of these processes to Current Sheet (CS) resonances and predicted the scaling law for the velocity of subsequent structures as V_N ~ N^2/3. Cluster-2 provides experimental evidence from the PSBL from of the existence of such resonant structures in ion velocity space and provides the first statistically proven identification of such a scaling law for quiet and moderately disturbed periods.

We use Cluster spacecraft observations to study in detail the structure of a magnetic reconnection separatrix region on the magnetospheric side of the magnetopause about 50 ion inertial lengths away from the X-line. The separatrix region is the region between the magnetic separatrix and the reconnection jet. It is several ion inertial lengths wide and it contains a few subregions showing different features in particle and wave data. One subregion, a density cavity adjacent to the separatrix, has strong electric fields, electron beams and intense wave turbulence. The separatrix region shows structures even at smaller scales, for example, solitary waves at Debye length scale. We describe in detail electron distribution functions and electric field spectra in the separatrix region and we compare them to a numerical simulation. Our observations show that while reconnection is ongoing the separatrix region is highly structured and dynamic in the electric field even if the X-line is up to 50 ion inertial lengths away.

Presented at the Lunar and Planetary Science Conference, Houston, Texas, special session on 15 March 2006.

Nulling interferometry, a powerful technique for high-resolution imaging of the close neighbourhood of bright astrophysical objets, is currently considered for future space missions such as Darwin or the Terrestrial Planet Finder Interferometer (TPF-I), both aiming at Earth-like planet detection and characterization. Ground-based nulling interferometers are being studied for both technology demonstration and scientific preparation of the Darwin/TPF-I missions through a systematic survey of circumstellar dust disks around nearby stars. In this paper, we investigate the influence of atmospheric turbulence on the performance of ground-based nulling instruments, and deduce the major design guidelines for such instruments. End-to-end numerical simulations allow us to estimate the performance of the main subsystems and thereby the actual sensitivity of the nuller to faint exozodiacal disks. Particular attention is also given to the important question of stellar leakage calibration. This study is illustrated in the context of GENIE, the Ground-based European Nulling Interferometer Experiment, to be installed at the VLTI and working in the L' band. We estimate that this instrument will detect exozodiacal clouds as faint as about 50 times the Solar zodiacal cloud, thereby placing strong constraints on the acceptable targets for Darwin/TPF-I.

Context: The morphology of massive star formation in the central regions of galaxies is an important tracer of the dynamical processes that govern the evolution of disk, bulge, and nuclear activity.

Aims: We present optical imaging of the central regions of a sample of 73 spiral galaxies in the H-a line and in optical broad bands, and derive information on the morphology of massive star formation.

Methods:We obtained images with theWilliam Herschel Telescope, mostly at a spatial resolution of below one second of arc. For most galaxies, no H-a imaging is available in the literature.We outline the observing and data reduction procedures, list basic properties, and present the I-band and continuum-subtracted H-a images. We classify the morphology of the nuclear and circumnuclear H-a emission and explore trends with host galaxy parameters.

Results: We confirm that late-type galaxies have a patchy circumnuclear appearance in H-a, and that nuclear rings occur primarily in spiral types Sa-Sbc. We identify a number of previously unknown nuclear rings, and confirm that nuclear rings are predominantly hosted by barred galaxies.

Conclusions: Other than in stimulating nuclear rings, bars do not influence the relative strength of the nuclear H-a peak, nor the circumnuclear H-a morphology. Even considering that our selection criteria led to an over-abundance of galaxies with close massive companions, we do not find any significant influence of the presence or absence of a close companion on the relative strength of the nuclear H-a peak, nor on the H-a morphology around the nucleus.

Using radio observations by FIRST and NVSS, we build a sample of 151 radio-variable quasars selected from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3). Six (and probably another two) of these objects are classified as broad absorption line (BAL) quasars and show radio flux variations of a few tens of percent within 1.5-5 yr. Such large amplitudes of variations imply brightness temperatures much higher than the inverse Compton limits (10¹² K) in all these BAL quasars, suggesting the presence of relativistic jets beaming toward the observer. The angles between the outflow and the jet are constrained to be less than ~20°. Such BAL quasars with polar outflows are beyond the simple unification models of BAL and non-BAL quasars, which hypothesize BAL quasars as otherwise normal quasars seen nearly edge-on.

We present the first results from SMART-1's science and technology payload. SMART-1 is Europe's first lunar mission and will provide some significant advances to many issues currently active in lunar science, such as our understanding of lunar origin and evolution. The mission also contributes a step in developing an international program of lunar exploration. The spacecraft was launched on 27 September 2003 on an Ariane 5, as an auxiliary passenger to Geostationary Transfer Orbit (GTO), performed a 14-month long cruise using the tiny thrust of electric propulsion alone, reached lunar capture in November 2004, and lunar science orbit in March 2005. SMART-1 carries seven hardware experiments (performing 10 investigations, including three remote sensing instruments, used during the cruise, the mission's nominal six months and one year extension in lunar science orbit). The remote sensing instruments will contribute to key planetary scientific questions related to theories of lunar origin and evolution, the global and local crustal composition, the search for cold traps at the lunar poles and the mapping of potential lunar resources.

Enthusiasm and commitment for a strong European scientific programme are expressed in Giovanni Bignami's book "Explorer l'Espace pour Remonter le Temps' (Exploring space to go back in time), by Odile Jacob, Paris, published in January 2006. This book follows his other book "La Storia nello Spazio" (History in Space), published in Italian by Mursia, Milano, in 2001.

His latest book tells science stories of several past, present and future space missions (Giotto, Galileo, Cassini, BeppoSAX, RossiXTE, BepiColombo) of the ESA, NASA and ASI space agencies.

These missions address a vast panorama of science in space, from planets to comets and from black holes to explosive events in the Universe. Bignami also gives room to the lives, times and innovative spirit of each mission namesakes, from the early 1300s, with a taste of Giottos painting (including Halleys comet) in the Papal Europe, to the 1600s of Galileo and Gian Domenico Cassini, who left Bologna to go to Paris in the service of the Roi Soleil.

The overview moves onto the twentieth century, with Beppo Occhialini, Italian physicist at the Fermi school, and Bruno Rossi, who fled Fascist Italy to become the founder, in the United States, of astronomy in space. Finally, the book moves onto Bepi Colombo, the great scientist and celestial mechanics visionary of the University of Padova.

G.F. Bignami is chairman of ESA's Space Science Advisory Committee. He is the Director of the Centre d'Etude Spatiale des Rayonnements (CNRS-Universitè de Toulouse) and Professor of Astronomy at the University of Pavia. He is a member of the Accademia dei Lincei and of the Academia Europaea.