Anomalous a.c. resistivity effect in Nb and Sn at high static fields in the liquid helium temperature range

The a.c. susceptibilities of Nb and Sn cylinders in the normal state at temperatures below T>c have been measured. The results cannot be described by the formalism of the classical skin-effect. Therefore a formal description is given

CCH <i>N</i> = 4-3 emission from dense interstellar clouds

The authors have searched for N = 4 - 3 rotational line emission from the ethynyl radical CCH, at 349 GHz toward a number of galactic molecular clouds. They have detected emission from ten giant molecular clouds and have derived CCH column densities on the order of 1014 - 1015cm-2. They find that CCH emission arises from dense gas, n(H2) ~ 104 - 105cm-3, but not from very dense material, n(H2) > 106cm-3, nor from hot gas such as the "hot core" region in Orion

Redundancy Calibration of Phased Array Stations

Our aim is to assess the benefits and limitations of using the redundant visibility information in regular phased array systems for improving the calibration. Regular arrays offer the possibility to use redundant visibility information to constrain the calibration of the array independent of a sky model and a beam models of the station elements. It requires a regular arrangement in the configuration of array elements and identical beam patterns. We revised a calibration method for phased array stations using the redundant visibility information in the system and applied it successfully to a LOFAR station. The performance and limitations of the method were demonstrated by comparing its use on real and simulated data. The main limitation is the mutual coupling between the station elements, which leads to non-identical beams and stronger baseline dependent noise. Comparing the variance of the estimated complex gains with the Cramer-Rao Bound (CRB) indicates that redundancy is a stable and optimum method for calibrating the complex gains of the system. Our study shows that the use of the redundant visibility does improve the quality of the calibration in phased array systems. In addition it provides a powerful tool for system diagnostics. Our results demonstrate that designing redundancy in both the station layout and the array configuration of future aperture arrays is strongly recommended. In particular in the case of the Square Kilometre Array with its dynamic range requirement which surpasses any existing array by an order of magnitude.Comment: 16 pages, 15 figures, accepted for publication in the A&A in Section 13, acceptance date: 1st May 2012. NOTE: Please contact the first author for high resolution figure

Single particle imaging with FEL using photon correlations

Scattering experiments with femtosecond high-intensity free-electron laser pulses provide a new route to macromolecular structure determination without the need for crystallization at low material usage. In these experiments, the X-ray pulses are scattered with high repetition on a stream of identical single biomolecules and the scattered photons are recorded on a pixelized detector. The main challenges are the unknown random orientation of the molecule in each shot and the extremely low signal to noise ratio due to the very low expected photon count per scattering image, typically well below the number of over 100 photons required by available analysis methods. The latter currently limits the scattering experiments to nano-crystals or larger virus particles, but the ultimate goal remains to retrieve the atomic structure of single biomolecules. Here, we use photon correlations to overcome the issue with low photon counts and present an approach that can determine the molecular structure de novo from as few as three coherently scattered photons per image. We further validate the method with a small protein (46 residues), show that near-atomic resolution of 3.3 Å is within experimental reach and demonstrate structure determination in the presence of isotropic noise from various sources, indicating that the number of disordered solvent molecules attached to the macromolecular surface should be kept at a minimum. Our correlation method allows to infer structure from images containing multiple particles, potentially opening the method to other types of experiments such as fluctuation X-ray scattering (FXS)

Near term measurements with 21 cm intensity mapping: neutral hydrogen fraction and BAO at z<2

It is shown that 21 cm intensity mapping could be used in the near term to make cosmologically useful measurements. Large scale structure could be detected using existing radio telescopes, or using prototypes for dedicated redshift survey telescopes. This would provide a measure of the mean neutral hydrogen density, using redshift space distortions to break the degeneracy with the linear bias. We find that with only 200 hours of observing time on the Green Bank Telescope, the neutral hydrogen density could be measured to 25% precision at redshift 0.54<z<1.09. This compares favourably to current measurements, uses independent techniques, and would settle the controversy over an important parameter which impacts galaxy formation studies. In addition, a 4000 hour survey would allow for the detection of baryon acoustic oscillations, giving a cosmological distance measure at 3.5% precision. These observation time requirements could be greatly reduced with the construction of multiple pixel receivers. Similar results are possible using prototypes for dedicated cylindrical telescopes on month time scales, or SKA pathfinder aperture arrays on day time scales. Such measurements promise to improve our understanding of these quantities while beating a path for future generations of hydrogen surveys.Comment: 6 pages, 5 figures. Submitted to Phys. Rev. D. Addressed reviewer comments. Changed figure format, added more detailed technical discussion, and added forecasts for aperture arrays. Added references

Mid-frequency aperture arrays: the future of radio astronomy

Aperture array (AA) technology is at the forefront of new developments and discoveries in radio astronomy. Currently LOFAR is successfully demonstrating the capabilities of dense and sparse AA's at low frequencies. For the mid-frequencies, from 450 to 1450MHz, AA's still have to prove their scientific value with respect to the existing dish technology. Their large field-of-view and high flexibility puts them in an excellent position to do so. The Aperture Array Verification Program is dedicated to demonstrate the feasibility of AA's for science in general and SKA in particular. For the mid-frequency range this has lead to the development of EMBRACE, which has already demonstrated the enormous flexibility of AA systems by observing HI and a pulsar simultaneously. It also serves as a testbed to demonstrate the technological reliability and stability of AA's. The next step will put AA technology at a level where it can be used for cutting-edge science. In this paper we discuss the developments to move AA technology from an engineering activity to a fully science capable instrument. We present current results from EMBRACE, ongoing tests of the system, and plans for EMMA, the next step in mid-frequency AA technology.Comment: 8 pages, 7 figures, proceedings of Resolving The Sky - Radio Astronomy: Past, Present and Future (RTS2012), April 17-20, 2012, Manchester, U

SKA antenna systems; outlook for non-astronomy applications

The globally endorsed Square Kilometre Array project primarily aims to advance high sensitivity radio astronomy using a distributed collection of radio telescope stations spiraling outward from the core along three to five arms out to 3000km. This planned highly sensitive instrument covering a frequency range from 70MHz up to 10GHz will be used as wideband, high resolution, wide observing field interferometer of which the first phase will be realized this decade. With these SKA telescope capabilities and with the underlying technologies, there are many space related applications outside the immediate radio astronomy domain especially in the ground segment area. Examples are tracking space debris, precision orbit determination, simultaneous deep space tracking of multiple spacecrafts, GNSS and for other ground segment applications such as search and rescue tracking. After a brief introduction to the SKA, this paper will explore these potential application areas using the SKA based on its underlying approaches in the antenna and receiving subsystems

Nadeije Laneyrie-Dagen. L’Invention de la nature

Sous titrée « Les quatre éléments à la Renaissance ou le peintre premier savant », cette étude apparaît d’emblée comme importante, et est appelée à faire date. Consacrée au nouveau regard que les artistes, avec la Renaissance, posent sur la nature, dont ils dressent bientôt le paysage précis, L’Invention de la nature s’inscrit en cohérence dans la foulée de L’Invention du corps, ouvrage publié par Nadeije Laneyrie-Dagen voici une dizaine d’années. La manière appliquée dont les artistes de la ..