Wideband spectropolarimetry of extragalactic radio sources with KAT 7 and commissioning phase MeerKAT

The study of cosmic magnetism is vital in fully understanding the role that magnetic fields play in the evolution of cosmological objects such as galaxies. The exact role played by magnetism in galaxy evolution is still not fully understood. This work aims to use wide band radio polarimetry to probe the frequency dependent polarization properties in a small sample of unresolved known disc and AGN powered radio galaxies and also in a larger faint radio source sample from a deep blind survey. The second sample is composed of lesser known faint radio sources spanning ∼ 4 square degrees. We also aim to develop broad band radio polarimetry techniques that can be utilised in the future. We observed six radio galaxies at frequencies spanning 1200−1900 MHz – divided into three 256 MHz bands centred on 1350, 1600, and 1850 MHz. We also analysed deeper wide band and wide field observations from the commissioning phase of the South African SKA precursor, MeerKAT. The MeerKAT observations are of the DEEP field, a radio quiet region in the southern sky. These observations were made at frequencies 890 MHz to 1702 MHz. Following data calibrations, we made sensitive (down to ∼ 7 µJy/beam) high resolution (6”) total intensity image maps for DEEP field source finding and characterization, as well as lower resolution (14”) IQU image cubes. We analysed sources with total flux density greater than one milliJy. The technique of Faraday rotation measure synthesis (RM synthesis) was employed through a python based algorithm to probe the broad band structure of the polarisation spectra. RM synthesis has the power to resolve individual contributions to the overall observed spectropolarimetric features along a line of sight. We found agreement with literature with regard to total radio intensities and the expected polarisation levels of order 1%. Our polarisation detection rate in the wide field case was estimated to be 24 ± 9% per square degree. This translates to 30 ± 13 polarised sources per square degree, in agreement with projections from the literature. The results confirm two main categories of polarised radio sources. These are: (1) sources with simple polarisation profiles, theoretically resulting from a scenario where incident polarised emission is Faraday rotated by a foreground region of coherent magnetic field such as in the case of a radio bright galactic nucleus emitting synchrotron radiation that then traverses disk and halo regions filled with a uniform magnetic field; (2) sources with complex Faraday spectra, indicating depolarisation and/or repolarisation suggesting other complex behaviour originating from several combinations of Faraday rotating and synchrotron emitting regions along the line of sight. We found that the most complex polarisation profiles emanate from the most polarised radio sources. From the sample of known sources observed with KAT 7, we found that unresolved disk dominated galaxies tended to be less polarised than compact sources suggesting a morphological dependence of the polarisation spectrum where disk galaxies may tend to have more regions within them, such as pockets of turbulent magnetic fields in the material disk, that may cause depolarisation to a larger degree as compared to earlier type non-disk dominated galaxies. Steep spectrum sources of different types (radio lobes and most polarised compact sources) show the most complexity with multiple Faraday emission components, depolarisation, and repolarisation. The more sensitive MeerKAT data allowed us to probe the broad band polarimetric properties of faint never observed radio sources. Our study is an early step to future campaigns with MeerKAT to explore relations between polarimetry and source properties, probing the correlations of magnetism with galaxy evolution and the evolution of large-scale fields in galaxies, galaxy groups and clusters

Pebble bed: reflector treatment and pressure\ud velocity coupling

In this report, we describe some models and numerical methods used to simulate the flow and temperature in a pebble bed modular nuclear reactor. The reactor core is filled with around 450000 spheres containing low enriched uranium and helium is forced through these hot pebbles to cool the system down. The group first investigated the flow model in the pebbles. Numerical aspects were then considered to tackle difficulties encountered with the flow simulation and the temperature inside the pebbles. Numerical schemes are presented that can significantly improve the accuracy of the computed results

Electrical Characterization of 1.8 MeV Proton-Bombarded ZnO

We report on the electrical characterization of single-crystal ZnO and Au Schottky contacts formed thereon before and after bombarding them with 1.8 MeV protons. From capacitance–voltage measurements, we found that ZnO is remarkably resistant to high-energy proton bombardment and that each incident proton removes about two orders of magnitude less carriers than in GaN. Deep level transient spectroscopy indicates a similar effect: the two electron traps detected are introduced in extremely low rates. One possible interpretation of these results is that the primary radiation-induced defects in ZnO may be unstable at room temperature and anneal out without leaving harmful defects that are responsible for carrier compensation

Structural, morphological, optical and electrical properties of Schottky diodes based on CBD deposited ZnO : Cu nanorods

Undoped and copper doped zinc oxide (ZnO) nanorods have been synthesized by a simple chemical bath deposition (CBD) method at a temperature of 90 C. Structural, morphological, optical and electrical properties of the synthesized ZnO nanorods were found to be dependent on the Cu doping percentage. X-ray diffraction (XRD) patterns revealed strong diffraction peaks of hexagonal wurtzite of ZnO, and no impurity phases from metallic zinc or copper. Scanning electron microscopy (SEM) images showed changes in diameter and shape of nanorods, where by those doped with 2 at.% and 3 at.% aggregated and became compact. Selected area electron diffraction (SAED) patterns indicates high quality, single crystalline wurtzite structure ZnO and intensities of bright spots varied with copper doping concentration. UVevisible absorption peaks of ZnO red shifted with increasing copper doping concentration. Raman studies demonstrated among others, strong and sharp E2 (low) and E2 (high) optical phonon peaks confirming crystal structure of ZnO. Current-voltage measurements based on the gold/ZnO nanorods/ITO showed good rectifying behavior of the Schottky diode. The predicted Schottky barrier height of 0.60 eV was obtained which is not far from the theoretical Schottky-Mott value of 0.80 eVThe University of Pretoria and National Research Foundation (NRF), South Africa, Grant No: 91550 and 94166.http:// www.elsevier.com/locate/superlattices2018-07-31hb2017Physic

The Curious Case of the “Heartworm” Nebula

© 2022. The Author(s). Published by the American Astronomical Society. This work is licensed under the terms of the Creative Commons Attribution 4.0 licence. https://creativecommons.org/licenses/by/4.0/The curious Galactic features near G357.2−0.2 were observed with the MeerKAT radio interferometer array in the UHF and L bands (0.56–1.68 GHz). There are two possibly related features: a newly identified faint heart-shaped partial shell (the “heart”), and a series of previously known but now much better imaged narrow, curved features (the “worm”) interior to the heart. Polarized emission suggests that much of the emission is nonthermal and is embedded in a dense plasma. The filaments of the worm appear to be magnetic structures powered by embedded knots that are sites of particle acceleration. The morphology of the worm broadly resembles some known pulsar wind nebulae (PWNe) but there is no known pulsar or PWN which could be powering this structure. We also present eROSITA observations of the field; no part of the nebula is detected in X-rays, but the current limits do not preclude the existence of a pulsar/PWN of intermediate spin-down luminosity.Peer reviewe

Electrical characterization of 1.8 MeV proton-bombarded ZnO

We report on the electrical characterization of single-crystal ZnO and Au Schottky contacts formed thereon before and after bombarding them with 1.8 MeV protons. From capacitance–voltage measurements, we found that ZnO is remarkably resistant to high-energy proton bombardment and that each incident proton removes about two orders of magnitude less carriers than in GaN. Deep level transient spectroscopy indicates a similar effect: the two electron traps detected are introduced in extremely low rates. One possible interpretation of these results is that the primary radiation-induced defects in ZnO may be unstable at room temperature and anneal out without leaving harmful defects that are responsible for carrier compensation

The MPIfR-MeerKAT Galactic Plane Survey - I. System set-up and early results

Galactic plane radio surveys play a key role in improving our understanding of a wide range of astrophysical phenomena. Performing such a survey using the latest interferometric telescopes produces large data rates necessitating a shift towards fully or quasi-real-time data analysis with data being stored for only the time required to process them. We present here the overview and set-up for the 3000-h Max-Planck-Institut für Radioastronomie (MPIfR)-MeerKAT Galactic Plane Survey (MMGPS). The survey is unique by operating in a commensal mode, addressing key science objectives of the survey including the discovery of new pulsars and transients and studies of Galactic magnetism, the interstellar medium and star formation rates. We explain the strategy coupled with the necessary hardware and software infrastructure needed for data reduction in the imaging, spectral, and time domains. We have so far discovered 78 new pulsars including 17 confirmed binary systems of which two are potential double neutron star systems. We have also developed an imaging pipeline sensitive to the order of a few tens of micro-Jansky () with a spatial resolution of a few arcseconds. Further science operations with an in-house built S-band receiver operating between 1.7 and 3.5 GHz are about to commence. Early spectral line commissioning observations conducted at S-band, targeting transitions of the key molecular gas tracer CH at 3.3 GHz already illustrate the spectroscopic capabilities of this instrument. These results lay a strong foundation for future surveys with telescopes like the Square Kilometre Array (SKA)

Comparative study of the electrical properties of Pd/ZnO Schottky contacts fabricated using electron beam deposition and resistive/thermal evaporation techniques

A systematic investigation to check the quality of Pd Schottky contacts deposited on ZnO has been performed on electron beam (e-beam) deposited and resistively/thermally evaporated samples using current-voltage, IV, and conventional deep level transient spectroscopy (DLTS) measurements. Room temperature IV measurements reveal the dominance of pure thermionic emission on the resistively evaporated contacts, while the e-beam deposited contacts show the dominance of generation recombination at low voltages,<0.30 V, and the dominance of pure thermionic emission at high voltages, greater than 0.30 V. The resistively evaporated contacts have very low reverse currents of the order of 10 10 A at a reverse voltage of 1.0 V whereas the e-beam deposited contacts have reverse currents of the order of 10 6 A at 1.0 V. Average ideality factors have been determined as (1.4360.01) and (1.6660.02) for the resistively evaporated contacts and e-beam deposited contacts, respectively. The IV barrier heights have been calculated as (0.72160.002) eV and (0.62460.005) eV for the resistively evaporated and e-beam deposited contacts, respectively. Conventional DLTS measurements reveal the presence of three prominent defects in both the resistive and e-beam contacts. Two extra peaks with energy levels of 0.60 and 0.81 eV below the conduction band minimum have been observed in the e-beam deposited contacts. These have been explained as contributing to the generation recombination current that dominates at low voltages and high leakage currents. Based on the reverse current at 1.0 V, the degree of rectification, the dominant current transport mechanism and the observed defects, we conclude that the resistive evaporation technique yields better quality Schottky contacts for use in solar cells and ultraviolet detectors compared to the e-beam deposition technique. The 0.60 eV has been identified as possibly related to the unoccupied level for the doubly charged oxygen vacancy, Vо2+.The National Research Foundation of South Africahttp://dx.doi.org/10.1063/1.3658027nf201

Identifying anomalous radio sources in the Evolutionary Map of the Universe Pilot Survey using a complexity-based approach

The Evolutionary Map of the Universe (EMU) large-area radio continuum survey will detect tens of millions of radio galaxies, giving an opportunity for the detection of previously unknown classes of objects. To maximize the scientific value and make new discoveries, the analysis of these data will need to go beyond simple visual inspection. We propose the coarse-grained complexity, a simple scalar quantity relating to the minimum description length of an image that can be used to identify unusual structures. The complexity can be computed without reference to the broader sample or existing catalogue data, making the computation efficient on new surveys at very large scales (such as the full EMU survey). We apply our coarse-grained complexity measure to data from the EMU Pilot Survey to detect and confirm anomalous objects in this data set and produce an anomaly catalogue. Rather than work with existing catalogue data using a specific source detection algorithm, we perform a blind scan of the area, computing the complexity using a sliding square aperture. The effectiveness of the complexity measure for identifying anomalous objects is evaluated using crowd-sourced labels generated via the Zooniverse.org platform. We find that the complexity scan identifies unusual sources, such as odd radio circles, by partitioning on complexity. We achieve partitions where 5 per cent of the data is estimated to be 86 per cent complete, and 0.5 per cent is estimated to be 94 per cent pure, with respect to anomalies and use this to produce an anomaly catalogue

Investigating atmospheric corrosion behavior of carbon steel in coastal regions of Mauritius using Raman Spectroscopy

Low carbon steel was exposed at two sites in Mauritius, namely Port Louis and Belle Mare. The site at Port Louis is basically an industrial marine one whereas the one at Belle Mare is a purely marine site. Though the corrosion loss trend at both sites follow the power law, the corrosion loss at Port Louis was found to be higher than that at Belle Mare. This study has been performed to investigate the surface characteristics of the rust layers of the samples exposed at the two sites, through Raman spectroscopy and SEM, so as to get a better insight into the mechanism of the atmospheric corrosion process. For Port Louis, it was observed that there was not much change in the corrosion products in the rust layer over the 3 years period. The structure was less compact than that at Belle Mare with the presence of lepidocrocite and akaganeite as commonly observed corrosion products. The corrosion rate at Port Louis is, therefore, expected to follow the same trend over the long term. For Belle Mare, the corrosion products changed significantly after 3 years of exposure. Though lepidocrocite and akaganeite were observed on the surface after 0.2 years of exposure, magnetite was the most probable corrosion product in the more compact rust layer after 3 years of exposure. This compactness of the rust layer is expected to have reduced the corrosion rate as compared to that of Port Louis. Significant changes in the corrosion rate at Belle Mare are, therefore, expected over the medium and the long term