arXiv: Instrumentation and Methods for Astrophysics, 2012
An important design decision for the first phase of the Square Kilometre Array is whether the low... more An important design decision for the first phase of the Square Kilometre Array is whether the low frequency component (SKA1-low) should be implemented as a single or dual-band aperture array; that is, using one or two antenna element designs to observe the 70-450 MHz frequency band. This memo uses an elementary parametric analysis to make a quantitative, first-order cost comparison of representative implementations of a single and dual-band system, chosen for comparable performance characteristics. A direct comparison of the SKA1-low station costs reveals that those costs are similar, although the uncertainties are high. The cost impact on the broader telescope system varies: the deployment and site preparation costs are higher for the dual-band array, but the digital signal processing costs are higher for the single-band array. This parametric analysis also shows that a first stage of analogue tile beamforming, as opposed to only station-level, all-digital beamforming, has the pote...
This paper presents the results from one of the first observations of ionospheric scintillation t... more 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...
The Sun’s activity leads to bursts of radio emission, among other phenomena. An example is type-I... more The Sun’s activity leads to bursts of radio emission, among other phenomena. An example is type-III radio bursts. They occur frequently and appear as short-lived structures rapidly drifting from high to low frequencies in dynamic radio spectra. They are usually interpreted as signatures of beams of energetic electrons propagating along coronal magnetic field lines. Here we present novel interferometric LOFAR (LOw Frequency ARray) observations of three solar type-III radio bursts and their reverse bursts with high spectral, spatial, and temporal resolution. They are consistent with a propagation of the radio sources along the coronal magnetic field lines with nonuniform speed. Hence, the type-III radio bursts cannot be generated by a monoenergetic electron beam, but by an ensemble of energetic electrons with a spread distribution in velocity and energy. Additionally, the density profile along the propagation path is derived in the corona. It agrees well with three-fold coronal densit...
Higher frequency Aperture Array technology, up to 1.5GHz, is being prepared for large scale deplo... more Higher frequency Aperture Array technology, up to 1.5GHz, is being prepared for large scale deployment in radio astronomy. Novel approaches in order to increase performance and lower the production costs are a key element of the success as well as scientific demonstration. This paper describes the current status, technology innovations and planned demonstrators.
Page 1. ESPRIT – A study concept for a far-infrared interferometer in space W. Wild*a,f, Th. de G... more Page 1. ESPRIT – A study concept for a far-infrared interferometer in space W. Wild*a,f, Th. de Graauwa ... given. 7. CORRELATOR DESIGN 7.1 Distributed correlator For ESPRIT a distributed correlator implementation is proposed. In ...
Pondering Pulsars Pulsars are rapidly rotating, magnetized neutron stars that are powered by the ... more Pondering Pulsars Pulsars are rapidly rotating, magnetized neutron stars that are powered by the loss of rotational energy. Because their emission is beamed, their light appears to pulse on and off at regular intervals. Changes in radio emission behavior have been observed for a number of pulsars, manifesting themselves as switches between ordered and disordered variations in intensity and pulse shapes, but these changes have not been seen at other wavelengths. Based on simultaneous radio and x-ray observations of pulsar PSR B0943+10, Hermsen et al. (p. 436 ) show that changes in emission state identified in radio measurements show counterpart fluctuations in the strength and temporal behavior of x-rays. Some of these changes were unexpected in their character and physical properties, challenging pulsar emission theories.
arXiv: Instrumentation and Methods for Astrophysics, 2012
An important design decision for the first phase of the Square Kilometre Array is whether the low... more An important design decision for the first phase of the Square Kilometre Array is whether the low frequency component (SKA1-low) should be implemented as a single or dual-band aperture array; that is, using one or two antenna element designs to observe the 70-450 MHz frequency band. This memo uses an elementary parametric analysis to make a quantitative, first-order cost comparison of representative implementations of a single and dual-band system, chosen for comparable performance characteristics. A direct comparison of the SKA1-low station costs reveals that those costs are similar, although the uncertainties are high. The cost impact on the broader telescope system varies: the deployment and site preparation costs are higher for the dual-band array, but the digital signal processing costs are higher for the single-band array. This parametric analysis also shows that a first stage of analogue tile beamforming, as opposed to only station-level, all-digital beamforming, has the pote...
This paper presents the results from one of the first observations of ionospheric scintillation t... more 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...
The Sun’s activity leads to bursts of radio emission, among other phenomena. An example is type-I... more The Sun’s activity leads to bursts of radio emission, among other phenomena. An example is type-III radio bursts. They occur frequently and appear as short-lived structures rapidly drifting from high to low frequencies in dynamic radio spectra. They are usually interpreted as signatures of beams of energetic electrons propagating along coronal magnetic field lines. Here we present novel interferometric LOFAR (LOw Frequency ARray) observations of three solar type-III radio bursts and their reverse bursts with high spectral, spatial, and temporal resolution. They are consistent with a propagation of the radio sources along the coronal magnetic field lines with nonuniform speed. Hence, the type-III radio bursts cannot be generated by a monoenergetic electron beam, but by an ensemble of energetic electrons with a spread distribution in velocity and energy. Additionally, the density profile along the propagation path is derived in the corona. It agrees well with three-fold coronal densit...
Higher frequency Aperture Array technology, up to 1.5GHz, is being prepared for large scale deplo... more Higher frequency Aperture Array technology, up to 1.5GHz, is being prepared for large scale deployment in radio astronomy. Novel approaches in order to increase performance and lower the production costs are a key element of the success as well as scientific demonstration. This paper describes the current status, technology innovations and planned demonstrators.
Page 1. ESPRIT – A study concept for a far-infrared interferometer in space W. Wild*a,f, Th. de G... more Page 1. ESPRIT – A study concept for a far-infrared interferometer in space W. Wild*a,f, Th. de Graauwa ... given. 7. CORRELATOR DESIGN 7.1 Distributed correlator For ESPRIT a distributed correlator implementation is proposed. In ...
Pondering Pulsars Pulsars are rapidly rotating, magnetized neutron stars that are powered by the ... more Pondering Pulsars Pulsars are rapidly rotating, magnetized neutron stars that are powered by the loss of rotational energy. Because their emission is beamed, their light appears to pulse on and off at regular intervals. Changes in radio emission behavior have been observed for a number of pulsars, manifesting themselves as switches between ordered and disordered variations in intensity and pulse shapes, but these changes have not been seen at other wavelengths. Based on simultaneous radio and x-ray observations of pulsar PSR B0943+10, Hermsen et al. (p. 436 ) show that changes in emission state identified in radio measurements show counterpart fluctuations in the strength and temporal behavior of x-rays. Some of these changes were unexpected in their character and physical properties, challenging pulsar emission theories.
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Papers by A. Gunst