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

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

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

Analysis of the strut and feed blockage effects in radio telescopes with compact UWB feeds

The international radio astronomy community is currently pursuing the development of a giant radio telescope known as the Square Kilometre Array (SKA). The SKA reference design consists of several wideband antenna technologies, including reflector antennas fed with novel multi-beam Phased Array Feeds (PAF) and/or wide band Single Pixel Feeds (SPFs) that can operate at frequencies from 1 to 10 GHz [1], [2]. The baseline of this design represents an array of several hundred to a few thousand reflector antennas of 15-m diameter and that will realize sensitivity of 10,000 m 2/K. During the past years, several different reflector and feed concepts have been proposed and examined, but only a small number of these design options (that have a sufficient level of maturity) will be built and tested in a set-up that is closely resembling the final SKA system [3]. These tests are aimed to evaluate the overall system performance as well as construction and operational costs. The final choices for the dish and feed evaluation tests might include: (i) off-set Gregorian and axi-symmetric reflector antennas and; (ii) an optimized octave corrugated horn and the single-pixel wideband feeds such as quad-ridged horn and Eleven antenna [2], [4]

An Axi-Symmetric Segmented Composite SKA Dish Design: Performance and Production Analysis

A concept of an axi-symmetric dish as antenna reflector for the next generation radio telescope - the Square Kilometre Array (SKA) - is presented. The reflector is based on the use of novel thermoplastic composite material (reinforced with carbon fibre) in the context of the telescope design with wide band single pixel feeds. The baseline of this design represents an array of 100's to 1000's reflector antennas of 15-m diameter and covers frequencies from <1 to 10 GHz. The purpose of our study is the analysis of the production cost of the dish and its performance in combination with a realistic wideband feed (such as the 'Eleven Antenna' feed) over a wide frequency band and a range of elevation angles. The presented initial simulation results inidicate the potential of the proposed dish concept for low-cost and mass production and demonstrate sensitivity comparable to that of the presently considered off-set Gregorian reflector antenna with the same projected aperture area. We expect this observation to be independent of the choice of the feed, as several other single-pixel wideband feeds (that have been reported in the literature) have similar beamwidth and phase center location, both being rather constant with frequency.Comment: Invited paper for the Asia-Pacific Microwave Conference 2011 (APMC 2011), Melbourne, 5-8 Dec., Australia, 201

Design, Environmental and Sustainability Constraints of new African Observatories: The example of the Mozambique Radio Astronomy Observatory

The Mozambique Radio Astronomy Observatory (MRAO) will be a first milestone towards development of radioastronomy in Mozambique. Development of MRAO will constitute a preparation step towards participation in the upcoming Africa VLBI Network and the Square Kilometer Array project. The MRAO first antenna is planned to serve as a capacitation and training facility and will be installed after the conversion of a 7-meter telecom dish in South Africa. Therefore, this first radiotelescope design has to comply with local spectral and environmental constraints. Furthermore, power availability and long term sustainability with potential inclusion of solar power and control of Radio Frequency Interference are analyzed. Here we outline some of the design, environmental and power sustainability constraints.Comment: 5 pages, 3 Figures; Proceedings of the URSI BEJ Session 'Large Scale Science Projects: Europa-Africa Connects', IEEE Africon 2013 Conference Mauritius (9-12 Sep) 2013, Accepted for Publication at IEEE Xplorer, Nov 201

Optimized Trigger for Ultra-High-Energy Cosmic-Ray and Neutrino Observations with the Low Frequency Radio Array

When an ultra-high energy neutrino or cosmic ray strikes the Lunar surface a radio-frequency pulse is emitted. We plan to use the LOFAR radio telescope to detect these pulses. In this work we propose an efficient trigger implementation for LOFAR optimized for the observation of short radio pulses.Comment: Submitted to Nuclear Instruments and Methods in Physics Research Section

A LOFAR observation of ionospheric scintillation from two simultaneous travelling ionospheric disturbances

This paper presents the results from one of the first observations of ionospheric scintillation taken using the Low-Frequency Array (LOFAR). The observation was of the strong natural radio source Cassiopeia A, taken overnight on 18–19 August 2013, and exhibited moderately strong scattering effects in dynamic spectra of intensity received across an observing bandwidth of 10–80 MHz. Delay-Doppler spectra (the 2-D FFT of the dynamic spectrum) from the first hour of observation showed two discrete parabolic arcs, one with a steep curvature and the other shallow, which can be used to provide estimates of the distance to, and velocity of, the scattering plasma. A cross-correlation analysis of data received by the dense array of stations in the LOFAR “core” reveals two different velocities in the scintillation pattern: a primary velocity of ~20–40 ms−1 with a north-west to south-east direction, associated with the steep parabolic arc and a scattering altitude in the F-region or higher, and a secondary velocity of ~110 ms−1 with a north-east to south-west direction, associated with the shallow arc and a scattering altitude in the D-region. Geomagnetic activity was low in the mid-latitudes at the time, but a weak sub-storm at high latitudes reached its peak at the start of the observation. An analysis of Global Navigation Satellite Systems (GNSS) and ionosonde data from the time reveals a larger-scale travelling ionospheric disturbance (TID), possibly the result of the high-latitude activity, travelling in the north-west to south-east direction, and, simultaneously, a smaller-scale TID travelling in a north-east to south-west direction, which could be associated with atmospheric gravity wave activity. The LOFAR observation shows scattering from both TIDs, at different altitudes and propagating in different directions. To the best of our knowledge this is the first time that such a phenomenon has been reported

Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies

Context. The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (&lt; 100 MHz) can reach several thousands of janskys, and they often contaminate observations of the low-frequency sky by interfering with image processing. Furthermore, these sources are foreground objects for all-sky observations hampering the study of faint signals, such as the cosmological 21 cm line from the epoch of reionisation. Aims. We aim to produce robust models for the surface brightness emission as a function of frequency for the A-team sources at ultra-low frequencies. These models are needed for the calibration and imaging of wide-area surveys of the sky with low-frequency interferometers. This requires obtaining images at an angular resolution better than 15\u2033 with a high dynamic range and good image fidelity. Methods. We observed the A-team with the Low Frequency Array (LOFAR) at frequencies between 30 MHz and 77 MHz using the Low Band Antenna system. We reduced the datasets and obtained an image for each A-team source. Results. The paper presents the best models to date for the sources Cassiopeia A, Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to obtain the aimed resolution and dynamic range in all cases. Owing to its compactness and complexity, observations with the long baselines of the International LOFAR Telescope will be required to improve the source model for Cygnus A further