Brent R. Carlson

The Expanded Very Large Array

In almost 30 years of operation, the Very Large Array (VLA) has proved to be a remarkably flexible and productive radio telescope. However, the basic capabilities of the VLA have changed little since it was designed. A major expansion utilizing modern technology is currently under way to improve the capabilities of the VLA by at least an order of magnitude in both sensitivity and in frequency coverage. The primary elements of the Expanded Very Large Array (EVLA) project include new or upgraded receivers for continuous frequency coverage from 1 to 50 GHz, new local oscillator, intermediate frequency, and wide bandwidth data transmission systems to carry signals with 16 GHz total bandwidth from each antenna, and a new digital correlator with the capability to process this bandwidth with an unprecedented number of frequency channels for an imaging array. Also included are a new monitor and control system and new software that will provide telescope ease of use. Scheduled for completion in 2012, the EVLA will provide the world research community with a flexible, powerful, general-purpose telescope to address current and future astronomical issues.

Tracing Electron Beams in the Sun's Corona with Radio Dynamic Imaging Spectroscopy

We report observations of type III radio bursts at decimeter wavelengths (type IIIdm bursts)—signatures of suprathermal electron beams propagating in the low corona—using the new technique of radio dynamic imaging spectroscopy provided by the recently upgraded Karl G. Jansky Very Large Array. For the first time, type IIIdm bursts were imaged with high time and frequency resolution over a broad frequency band, allowing electron beam trajectories in the corona to be deduced. Together with simultaneous hard X-ray and extreme ultraviolet observations, we show that these beams emanate from an energy release site located in the low corona at a height below ~15 Mm, and propagate along a bundle of discrete magnetic loops upward into the corona. Our observations enable direct measurements of the plasma density along the magnetic loops, and allow us to constrain the diameter of these loops to be less than 100 km. These overdense and ultra-thin loops reveal the fundamentally fibrous structure of the Suns corona. The impulsive nature of the electron beams, their accessibility to different magnetic field lines, and the detailed structure of the magnetic release site revealed by the radio observations indicate that the localized energy release is highly fragmentary in time and space, supporting a bursty reconnection model that involves secondary magnetic structures for magnetic energy release and particle acceleration.

The S2 VLBI system

Abstract The S2 VLBI system is described in some detail, including the S2 data acquisition system, data recorder and playback terminals, and the correlator.

NOISE IN THE CROSS-POWER SPECTRUM OF THE VELA PULSAR

We compare the noise in interferometric measurements of the Vela pulsar from ground- and space-based antennas with theoretical predictions. The noise depends on both the flux density and the interferometric phase of the source. Because the Vela pulsar is bright and scintillating, these comparisons extend into both the low and high signal-to-noise regimes. Furthermore, our diversity of baselines explores the full range of variation in interferometric phase. We find excellent agreement between theoretical expectations and our estimates of noise among samples within the characteristic scintillation scales. Namely, the noise is drawn from an elliptical Gaussian distribution in the complex plane, centered on the signal. The major axis, aligned with the signal phase, varies quadratically with the signal, while the minor axis, at quadrature, varies with the same linear coefficients. For weak signal, the noise approaches a circular Gaussian distribution. Both the variance and covariance of the noise are also affected by artifacts of digitization and correlation. In particular, we show that gating introduces correlations between nearby spectral channels.

Size of the Vela pulsar's emission region at 18 cm wavelength

We present measurements of the linear diameter of the emission region of the Vela pulsar at observing wavelength λ = 18 cm. We infer the diameter as a function of pulse phase from the distribution of visibility on the Mopra-Tidbinbilla baseline. As we demonstrate, in the presence of strong scintillation, finite size of the emission region produces a characteristic W-shaped signature in the projection of the visibility distribution onto the real axis. This modification involves heightened probability density near the mean amplitude, decreased probability to either side, and a return to the zero-size distribution beyond. We observe this signature with high statistical significance, as compared with the best-fitting zero-size model, in many regions of pulse phase. We find that the equivalent FWHM of the pulsars emission region decreases from more than 400 km early in the pulse to near zero at the peak of the pulse and then increases again to approximately 800 km near the trailing edge. We discuss possible systematic effects and compare our work with previous results.

A broadband fpga digital beamformer for the advanced focal array demonstrator (AFAD)

Broadband Focal Array Beamformers offer the potential to increase the field of view of reflector antennas. Such systems require a processing bandwidth of order 1 GHz and need to beamform tens of polarized beams from over 100 array elements. A number of such systems have been built but none with the bandwidth and number of elements required by the Square Kilometre Array. In this paper we outline the digital beamformer for the Advanced Focal Array Demonstrator being developed at the National Research Council¿s Dominion Radio Astrophysical Observatory ¿ a system that meets the bandwidth and beam requirements proposed for the Square Kilometre Array.

Measuring the Size of the Vela Pulsar's Radio Emission Region

We describe measurements of the size of the Vela pulsar via scintillation, using both fits to the distribution of intensity and measurements of the modulation index. We briefly discuss systematic effects other than source size that can affect the distribution, including gain variations, self-noise, scintillation shot noise, and correlator saturation. Modulation index, a single number, can be biased by all of these, whereas the distribution of intensity is affected in different ways by different effects, providing means of distinguishing among them. Self-noise and gain variations are likely more important at long observing wavelengths, and correlator saturation and scintillation shot noise at short wavelengths. We find a size of about 500 km at decimeter wavelengths. Interestingly, this agrees with measurements of modulation index by Roberts & Ables at the same wavelength. Their results (and more recently that reported by Macquart et al.) suggest that size decreases with increasing wavelength. Although consistent with the observations, this conclusion is perhaps surprising from the standpoint of the traditional radius-to-frequency mapping. However, these measurements are of size rather than height; and of course systematic effects may play a role.

High-performance hardware platform for the square kilomtre array mid correlator & beamformer

The ‘TALON’ architecture has been proposed to meet the unprecedented processing requirements and flexibility required for the Square Kilometre Array-Phase-1 (SKA1) Mid telescope, Correlator & Beamformer (Mid. CBF). The high-performance hardware platform of the TALON architecture incorporates two variants of TALON line-replaceable-units (LRUs); TALON-SX and TALON-MX. Each LRU features a single Intel Stratix-10 FPGA, 2 DDR4 DIMM modules, 4 100GE QSFP28 ports and 48 26 Gbps bi-directional optical channels that connect to a custom optical-backplane. Each LRU facilitates up to 7 TMAC/s of processing capability, 512 GB/s of memory bandwidth and 1.648 Tb/s of I/O capacity.

Signal processing aspects of the sample clock frequency offset scheme for the SKA1 mid telescope array

In the proposed sample clock frequency offset (SCFO) scheme, the received signals at each antenna of the Square Kilometre Array Mid Telescope Array are sampled at slightly different sample rates. Subsequently, these sampled sequences are digitally re-sampled to a common sample rate prior to channelization and correlation/ beamforming. In this paper, the signal processing aspects of the SCFO scheme as it relates to the correlator and beamformer (CBF) of the SKA1 Mid Telescope are discussed. Here, it is shown that sample clock-related self-interference and out-of-band interference de-correlates improving the quality of correlated/ beamformed results.

The contribution of Canada to the VSOP mission

Abstract The Canadian contribution to the VSOP Space VLBI mission is based on the S2 recording system, a correlation centre, and a data analysis and archive centre. The S2 record/playback terminals are a low cost and reliable means for recording VLBI data and are presently distributed worldwide at 17 observatories involved in the VSOP mission. The Canadian S2 correlator is a lag-based correlator, designed specifically to handle Space VLBI signal processing. Fringe de-rotation and delay compensation are station-based, and the correlators 24576 lag channels can be distributed in a variety of ways to accommodate both continuum and spectral line observations. Unique design features include a station-based zoom filter for high spectral resolution, multiple pulsar gates, and a fast dump capability (1 kHz), principally used for pulsar observations. Subsequent data analysis and archiving are carried out to provide feed-back to the mission on data quality.