J. M. Anderson

Max Planck Institute for Radio Astronomy, Bonn, North Rhine-Westphalia, Germany

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Publications (4)15.24 Total impact

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    ABSTRACT: DiFX is a correlator for VLBI data based on the FX architecture (first Fourier transform and then cross-multiply). DiFX is a free licensed software written in C++, developed and maintened by an international group of programmers. A new DiFX version (dra) has been developed at Max-Planck-Institut f\"ur Radioastronomie (MPIfR), in order to manage the correlation of a space-based antenna with ground stations. The dra version is running on the High Performance Computer cluster (HPC) in Bonn, and it is used for the data processing of the three AGN imaging RadioAstron Key Science Projects ongoing, based at the MPIfR.
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    ABSTRACT: RadioAstron space-ground VLBI observations of the pulsar B0950+08, conducted with the 10-m space radio telescope in conjunction with the Arecibo 300-m telescope and Westerbork Synthesis Radio Telescope at a frequency of 324 MHz, were analyzed in order to investigate plasma inhomogeneities in the direction of this nearby pulsar. The observations were conducted at a spacecraft distance of 330,000 km, resulting in a projected baseline of 220,000 km, providing the greatest angular resolution ever achieved at meter wavelengths. Our analysis is based on fundamental behavior of structure and coherence functions. We find that the pulsar shows scintillation on two frequency scales, both much less than the observing frequency; but modulation is less than 100%. We infer that the scattering is weak, but a refracting wedge disperses the scintillation pattern. The refraction angle of this "cosmic prism" is measured as theta_0=1.1 - 4.4 mas, with the refraction direction being approximately perpendicular to the observer velocity. We show that the observed parameters of scintillation effects indicate that two plasma layers lie along the line of sight to the pulsar, at distances of 4.4 - 16.4 pc and 26 - 170 pc, and traveling in different directions relative to the line of sight. Spectra of turbulence for the two layers are found to follow a power law with the indices gamma_1 = gamma_2 = 3.00 +/- 0.08, significantly different from the index expected for a Kolmogorov spectrum of turbulence, gamma=11/3.
    The Astrophysical Journal 02/2014; 786(2). DOI:10.1088/0004-637X/786/2/115 · 6.28 Impact Factor
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    ABSTRACT: We report the development of a semi-automatic pipeline for the calibration of 86 GHz full-polarization observations performed with the Global Millimeter-VLBI array (GMVA) and describe the calibration strategy followed in the data reduction. Our calibration pipeline involves non-standard procedures, since VLBI polarimetry at frequencies above 43 GHz is not yet well established. We also present, for the first time, a full-polarization global-VLBI image at 86 GHz (source 3C 345), as an example of the final product of our calibration pipeline, and discuss the effect of instrumental limitations on the fidelity of the polarization images. Our calibration strategy is not exclusive for the GMVA, and could be applied on other VLBI arrays at millimeter wavelengths. The use of this pipeline will allow GMVA observers to get fully-calibrated datasets shortly after the data correlation.
    Astronomy and Astrophysics 03/2012; 542(0004-6361). DOI:10.1051/0004-6361/201218958 · 4.48 Impact Factor
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    ABSTRACT: Starting from December 2004, a program for the monitoring of intraday variable sources at a frequency of 5 GHz was performed at the Urumqi Observatory. The analysis of the variability characteristics of the flat-spectrum radio source AO 0235+164 revealed the existence of an annual cycle in the variability amplitude. This appears to correlate with the solar elongation of the source. A thorough analysis of the results of the MASIV IDV survey --- which provides the variability characteristics of a large sample of compact radio sources --- confirms that there is a small but detectable component of the observed fractional modulation which increases with decreasing solar elongation. We discuss the hypothesis that the phenomenon is related to interplanetary scintillation.
    Astronomy and Astrophysics 03/2011; 530. DOI:10.1051/0004-6361/201016033 · 4.48 Impact Factor