SKA SA

Pinelands, South Africa
Pinelands, South Africa

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Iacobelli M.,Leiden University | Iacobelli M.,Netherlands Institute for Radio Astronomy | Burkhart B.,University of Wisconsin - Madison | Haverkorn M.,Leiden University | And 10 more authors.
Astronomy and Astrophysics | Year: 2014

Aims. Radio synchrotron polarization maps of the Galaxy can be used to infer the properties of interstellar turbulence in the diffuse magneto-ionic medium (MIM). In this paper, we investigate the normalized spatial gradient of linearly polarized synchrotron emission (|∇ P|/|P|) as a tracer of turbulence, the relationship of the gradient to the sonic Mach number of the MIM, and changes in morphology of the gradient as a function of Galactic position in the southern sky. Methods. We used data from the S-band Polarization All Sky Survey (S-PASS) to image the normalized spatial gradient of the linearly polarized synchrotron emission (|∇ P|/|P|) in the entire southern sky at 2.3 GHz. The spatial gradient of linear polarization reveals rapid changes in the density and magnetic fluctuations in the MIM due to magnetic turbulence as a function of Galactic position. We made comparisons of these data to ideal MHD numerical simulations. To constrain the sonic Mach number (M s), we applied a high-order moments analysis to the observations and to the simulated diffuse, isothermal ISM with ideal magneto-hydrodynamic turbulence. Results. We find that polarization gradient maps reveal elongated structures, which we associate with turbulence in the MIM. Our analysis indicates that turbulent MIM is in a generally transonic regime. This result for the turbulent regime is more general than the ones deduced by the analysis of electron density variation data, because it is based on the stochastic imprints of the Faraday rotation effect, which is also sensitive to the magnetic field fluctuations. Filamentary structures are seen with typical widths down to the angular resolution, and the observed morphologies closely match numerical simulations and, in some cases, Hα contours. The |∇ P|/|P| intensity is found to be approximately log-normal distributed. No systematic variations in the sonic Mach number are observed as a function of Galactic coordinates, which is consistent with turbulence in the WIM, as inferred by the analysis of Hα data. We conclude that the sonic Mach number of the diffuse MIM appears to be spatially uniform towards the Galactic plane and the Sagittarius-Carina arm, but local variations induced by nearby extended objects are also found. © ESO, 2014.


Goedhart S.,SKA SA | Goedhart S.,Hartebeesthoek Radio Astronomy Observatory | Goedhart S.,North West University South Africa | Maswanganye J.P.,Hartebeesthoek Radio Astronomy Observatory | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We report the results of 10 years of monitoring of six regularly varying 6.7 GHz methanol masers using the Hartebeesthoek 26-m telescope. Observations were done at intervals of 1-2 weeks, with faster sampling during flaring episodes. Four of the sources were also monitored at 12.2 GHz and show correlated variations.We find the Lomb-Scargle periodogram to be the most sensitive method to search for periodicity but possibly prone to false detections. Periods range from 132.8 d (with 26 cycles observed) to 509 d (with seven cycles observed). Five of the sources show arguably periodic variations, while G331.13-0.24 shows strong periodicity in one peak, with large and variable delays in other peaks.©2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


D'Ammando F.,University of Perugia | D'Ammando F.,National Institute of Nuclear Physics, Italy | D'Ammando F.,National institute for astrophysics | Rau A.,Max Planck Institute for Extraterrestrial Physics | And 15 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

The flat spectrum radio quasar (FSRQ) PKS 2123-463 was associated in the first Fermi-Large Area Telescope (LAT) source catalogue with the γ-ray source 1FGL J2126.1-4603, but when considering the full first two years of Fermi observations, no γ-ray source at a position consistent with this FSRQ was detected, and thus PKS 2123-463 was not reported in the second Fermi-LAT source catalogue. On 2011 December 14 a γ-ray source positionally consistent with PKS 2123-463 was detected in flaring activity by Fermi-LAT. This activity triggered radio-to-X-ray observations by the Swift, Gamma-ray Optical/Near-Infrared Detector (GROND), Australia Telescope Compact Array (ATCA), Ceduna and Seven Dishes Karoo Array Telescope (KAT-7) observatories. Results of the localization of the γ-ray source over 41 months of Fermi-LAT operation are reported here in conjunction with the results of the analysis of radio, optical, ultraviolet (UV) and X-ray data collected soon after the γ-ray flare. The strict spatial association with the lower energy counterpart together with a simultaneous increase of the activity in optical, UV, X-ray and γ-ray bands led to a firm identification of the γ-ray source with PKS 2123-463. A new photometric redshift has been estimated as z = 1.46 ± 0.05 using GROND and Swift Ultraviolet/Optical Telescope (UVOT) observations, in rough agreement with the disputed spectroscopic redshift of z = 1.67. We fit the broad-band spectral energy distribution with a synchrotron/external Compton model. We find that a thermal disc component is necessary to explain the optical/UV emission detected by Swift/UVOT. This disc has a luminosity of ~1.8 × 1046ergs-1, and a fit to the disc emission assuming a Schwarzschild (i.e. non-rotating) black hole gives a mass of ~2 × 109M⊙. This is the first black hole mass estimate for this source. © 2012 The Authors Monthly Notices of the Royal Astronomical Society. © 2012 RAS.


Maswanganye J.P.,Hartebeesthoek Radio Astronomy Observatory | Maswanganye J.P.,North West University South Africa | Gaylard M.J.,Hartebeesthoek Radio Astronomy Observatory | Goedhart S.,North West University South Africa | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

Eight new class II methanol masers selected from the 6.7 GHz Methanol Multibeam survey catalogues I and II were monitored at 6.7 GHz with the 26mHartebeesthoek Radio Astronomy Observatory radio telescope for three years and seven months, from 2011 February to 2014 September. The sources were also observed at 12.2 GHz and two were sufficiently bright to permit monitoring. One of the eight sources, namely G358.460-0.391, was found to show periodic variations at 6.7 GHz. The period was determined and tested for significance using the Lomb-Scargle, epoch-folding and Jurkevich methods, and by fitting a simple analytic function. The best estimate for the period of the 6.7 GHz class II methanol maser line associated with G358.460-0.391 is 220.0 ± 0.2 d. © 2014 The Authors.


Sun X.H.,University of Sydney | Gaensler B.M.,University of Sydney | Carretti E.,CSIRO | Purcell C.R.,University of Sydney | And 5 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

We present high-sensitivity and absolutely calibrated images of diffuse radio polarization at a resolution of about 10 arcmin covering the range 10° < l < 34° and |b| < 5° at 2.3 GHz from the S-band Polarization All Sky Survey and at 4.8 GHz from the Sino-German δ6 cm polarization survey of the Galactic plane. Strong depolarization near the Galactic plane is seen at 2.3 GHz, which correlates with strong Ha emission. We ascribe the depolarization to spatial Faraday rotation measure fluctuations of about 65 radm-2 on scales smaller than 6-9 pc. We argue that most (about 90 per cent) of the polarized emission seen at 4.8 GHz originates from a distance of 3-4 kpc in the Scutum arm and that the random magnetic field dominates the regular field there. A branch extending from the North Polar Spur towards lower latitudes can be identified from the polarization image at 4.8 GHz but only partly from the polarization image at 2.3 GHz, implying that the branch is at a distance larger than 2-3 kpc. We show that comparison of structure functions of complex polarized intensity with those of polarized intensity can indicate whether the observed polarized structures are intrinsic or caused by Faraday screens. The probability distribution function of gradients from the polarization images at 2.3 GHz indicates that the turbulence in the warm ionized medium is transonic. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Passmoor S.,SKA SA | Passmoor S.,University of the Western Cape | Passmoor S.,Center for High Performance Computing | Cress C.,University of the Western Cape | And 7 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

The relationship between the clustering of dark matter and that of luminous matter is often described using the bias parameter. Here, we provide a new method to probe the bias of intermediate-to-high-redshift radio continuum sources for which no redshift information is available. We matched radio sources from the Faint Images of the Radio Sky at Twenty centimetres survey data to their optical counterparts in the Sloan Digital Sky Survey to obtain photometric redshifts for the matched radio sources.We then use the publicly available semi-empirical simulation of extragalactic radio continuum sources (S3) to infer the redshift distribution for all FIRST sources and estimate the redshift distribution of unmatched sources by subtracting the matched distribution from the distribution of all sources. We infer that the majority of unmatched sources are at higher redshifts than the optically matched sources and demonstrate how the angular scales of the angular two-point correlation function can be used to probe different redshift ranges. We compare the angular clustering of radio sources with that expected for dark matter and estimate the bias of different samples. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Thondikulam V.L.,SKA SA | Loots A.,SKA SA | Gaylard M.,HartRAO
IOP Conference Series: Materials Science and Engineering | Year: 2012

The African VLBI Network (AVN) is an initiative by the SKA-SA and HartRAO, business units of the National Research Foundation (NRF), Department of Science and Technology (DST), South Africa. The aim is to fill the existing gap of Very Long Baseline Interferometry (VLBI)-capable radio telescopes in the African continent by a combination of new build as well as conversion of large redundant telecommunication antennas through an Inter-Governmental collaborative programme in Science and Technology. The issue of human capital development in the Continent in the techniques of radio astronomy engineering and science is a strong force to drive the project and is expected to contribute significantly to the success of Square Kilometer Array (SKA) in the Continent.


Manti S.,Normal School of Pisa | Gallerani S.,Normal School of Pisa | Ferrara A.,Normal School of Pisa | Feruglio C.,Normal School of Pisa | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2016

We explore the possibility of detecting hydrogen radio recombination lines from 0 < z < 10 quasars. We compute the expected Hnα flux densities as a function of absolute magnitude and redshift by considering (i) the range of observed active galactic nucleus spectral indices from UV to X-ray bands, (ii) secondary ionizations from X-ray photons, and (iii) stimulated emission due to non-thermal radiation. All these effects are important to determine the line fluxes. We find that the combination of slopes: αX,hard = -1.11, αX,soft = -0.7, αEUV = -1.3, αUV = -1.7, maximizes the expected flux, fHnα ~ 10 μJy for z ~ 7 quasars with MAB = -27 in the n ~ 50 lines; allowed spectral energy distribution variations produce variations by a factor of 3 around this value. Secondaries boost the line intensity by a factor of 2-4 , while stimulated emission in high-z quasars with MAB ≈ -26 provides an extra boost to radio recombination line flux observed at v ~ 1 GHz if recombinations arise in H II regions with Te ≈ 103 - 5 K, ne ≈ 103 - 5 cm-3. We compute the sensitivity required for a 5s detection of Hna lines using the Square Kilometre Array (SKA), finding that the SKA-MID could detect sources with MAB ≲ -27 (MAB ≲ -26) at z ≲ 8 (z ≲ 3) in less than 100 h of observing time. These observations could open new paths to searches for obscured SMBH progenitors, complementing X-ray, optical/IR and sub-mm surveys. © 2015 The Authors.


Bernardi G.,SKA SA | Bernardi G.,Rhodes University | Bernardi G.,Harvard - Smithsonian Center for Astrophysics | McQuinn M.,University of California at Berkeley | Greenhill L.J.,Harvard - Smithsonian Center for Astrophysics
Astrophysical Journal | Year: 2015

The most promising near-term observable of the cosmic dark age prior to widespread reionization (z 15-200) is the sky-averaged λ21 cm background arising from hydrogen in the intergalactic medium. Though an individual antenna could in principle detect the line signature, data analysis must separate foregrounds that are orders of magnitude brighter than the λ21 cm background (but that are anticipated to vary monotonically and gradually with frequency, e.g., they are considered "spectrally smooth"). Using more physically motivated models for foregrounds than in previous studies, we show that the intrinsic spectral smoothness of the foregrounds is likely not a concern, and that data analysis for an ideal antenna should be able to detect the λ21 cm signal after subtracting a fifth-order polynomial in log ν. However, we find that the foreground signal is corrupted by the angular and frequency-dependent response of a real antenna. The frequency dependence complicates modeling of foregrounds commonly based on the assumption of spectral smoothness. Our calculations focus on the Large-aperture Experiment to detect the Dark Age, which combines both radiometric and interferometric measurements. We show that statistical uncertainty remaining after fitting antenna gain patterns to interferometric measurements is not anticipated to compromise extraction of the λ21 cm signal for a range of cosmological models after fitting a seventh-order polynomial to radiometric data. Our results generalize to most efforts to measure the sky-averaged spectrum. © 2015. The American Astronomical Society. All rights reserved..


Purcell C.R.,University of Sydney | Gaensler B.M.,University of Sydney | Sun X.H.,University of Sydney | Carretti E.,CSIRO | And 7 more authors.
Astrophysical Journal | Year: 2015

The Gum Nebula is 36°-wide shell-like emission nebula at a distance of only ∼450 pc. It has been hypothesized to be an old supernova remnant, fossil H ii region, wind-blown bubble, or combination of multiple objects. Here we investigate the magneto-ionic properties of the nebula using data from recent surveys: radio-continuum data from the NRAO VLA and S-band Parkes All Sky Surveys, and data from the Southern H-Alpha Sky Survey Atlas. We model the upper part of the nebula as a spherical shell of ionized gas expanding into the ambient medium. We perform a maximum-likelihood Markov chain Monte Carlo fit to the NVSS rotation measure data, using the data to constrain average electron density in the shell ne. Assuming a latitudinal background gradient in rotation measure, we find , angular radius , shell thickness , ambient magnetic field strength , and warm gas filling factor . We constrain the local, small-scale (∼260 pc) pitch-angle of the ordered Galactic magnetic field to , which represents a significant deviation from the median field orientation on kiloparsec scales (∼-7.°2). The moderate compression factor at the edge of the shell implies that the "old supernova remnant" origin is unlikely. Our results support a model of the nebula as a H ii region around a wind-blown bubble. Analysis of depolarization in 2.3 GHz S-PASS data is consistent with this hypothesis and our best-fitting values agree well with previous studies of interstellar bubbles. © 2015. The American Astronomical Society. All rights reserved.

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