High Dynamic Range Observations Research Articles

Calibration requirements for epoch of reionization 21-cm signal observations – I. Effect of time-correlated gains

ABSTRACT The residual gain errors add to the systematics of the radio interferometric observations. In case of the high dynamic range observations, these systematic effects dominates over the thermal noise of the observation. In this work, we investigate the effect of time-correlated residual gain errors in the estimation of the power spectrum of the sky brightness distribution in high dynamic range observations. Particularly, we discuss a methodology to estimate the bias in the power spectrum estimator of the redshifted 21-cm signal from neutral hydrogen in the presence of bright extragalactic compact sources. We find, that for the visibility-based power spectrum estimators, particularly those use nearby baseline correlations to avoid noise bias, the bias in the power spectrum arises mainly from the time correlation in the residual gain error. The bias also depends on the baseline distribution for a particular observation. Analytical calculations show that the bias is dominant for certain types of baseline pairs used for the visibility correlation. We perform simulated observation of extragalactic compact sources in the presence of residual gain errors with the Giant Metrewave Radio Telescope like array and estimate the bias in the power spectrum. Our results indicate that in order to estimate the redshifted 21-cm power spectrum, better calibration techniques, and estimator development are required.

Galactic neutral hydrogen and the magnetic ISM foreground

AbstractThe interstellar medium is suffused with magnetic fields, which inform the shape of structures in the diffuse gas. Recent high-dynamic range observations of Galactic neutral hydrogen, combined with novel data analysis techniques, have revealed a deep link between the morphology of neutral gas and the ambient magnetic field. At the same time, an observational revolution is underway in low-frequency radio polarimetry, driven in part by the need to characterize foregrounds to the cosmological 21-cm signal. A new generation of experiments, capable of high angular and Faraday depth resolution, are revealing complex filamentary structures in diffuse polarization. The relationship between filamentary structures observed in radio-polarimetric data and those observed in atomic hydrogen is not yet well understood. Multiwavelength observations will enable new insights into the magnetic interstellar medium across phases.

Science enabled by high precision inertial formation flying

The capability of maintaining two satellites in precise relative position, stable in a celestial coordinate system, would enable major advances in a number of scientific disciplines and with a variety of types of instrumentation. The common requirement is for formation flying of two spacecraft with the direction of their vector separation in inertial coordinates precisely controlled and accurately determined as a function of time. We consider here the scientific goals that could be achieved with such technology and review some of the proposals that have been made for specific missions. Types of instrumentation that will benefit from the development of this type of formation flying include 1) imaging systems, in which an optical element on one spacecraft forms a distant image recorded by a detector array on the other spacecraft, including telescopes capable of very high angular resolution; 2) systems in which the front spacecraft of a pair carries an occulting disk, allowing very high dynamic range observations of the solar corona and exoplanets; 3) interferometers, another class of instrument that aims at very high angular resolution and which, though usually requiring more than two spacecraft, demands very much the same developments.

IONOSPHERIC POWER-SPECTRUM TOMOGRAPHY IN RADIO INTERFEROMETRY

A tomographic method is described to quantify the three-dimensional power-spectrum of the ionospheric electron-density fluctuations based on radio-interferometric observations by a two-dimensional planar array. The method is valid to first-order Born approximation and might be applicable to correct observed visibilities for phase variations due to the imprint of the full three-dimensional ionosphere. It is shown that not the ionospheric electron density distribution is the primary structure to model in interferometry, but its autocorrelation function or equivalent its power-spectrum. An exact mathematical expression is derived that provides the three dimensional power-spectrum of the ionospheric electron-density fluctuations directly from a rescaled scattered intensity field and an incident intensity field convolved with a complex unit phasor that depends on the w-term and is defined on the full sky pupil plane. In the limit of a small field of view, the method reduces to the single phase screen approximation. Tomographic self-calibration can become important in high-dynamic range observations at low radio frequencies with wide-field antenna interferometers, because a three-dimensional ionosphere causes a spatially varying convolution of the sky, whereas a single phase screen results in a spatially invariant convolution. A thick ionosphere can therefore not be approximated by a single phase screen without introducing errors in the calibration process. By applying a Radon projection and the Fourier projection-slice theorem, it is shown that the phase-screen approach in three dimensions is identical to the tomographic method. Finally we suggest that residual speckle can cause a diffuse intensity halo around sources, due to uncorrectable ionospheric phase fluctuations in the short integrations, which could pose a fundamental limit on the dynamic range in long-integration images.

MeerKAT—The South African Array With Composite Dishes and Wide-Band Single Pixel Feeds

The MeerKAT is a precursor for the ldquosmall dish plus single-pixel wide-band feedrdquo scenario for the Square Kilometre Array (SKA). The current goal specification is for an array of 80 12-m dishes operating over a continuous frequency range of 0.7-10 GHz, thus the individual receptors match the SKA draft specification for midband dishes. The proposed array configuration is centrally concentrated with 70% of the collecting area within a 700-m-diameter circle and the remaining 30% extending out to baselines of 10 km. The site for MeerKAT lies within a radio quiet reserve established in the remote and Northern Cape province of South Africa. The technologies and techniques that MeerKAT will develop and evaluate include wide-bandwidth feeds, low-cost dish antennas, instrumentation for high dynamic range observations, packet-switched array processing architectures, shielding of component radio-frequency interference (RFI), mitigation of external RFI, and low-cost and reliable remote telescope operation.

A differentially rotating disc in a high-mass protostellar system

A strong signature of a circumstellar disc around a high-mass protostar has been inferred from high resolution methanol maser observations in NGC7538-IRS1 N. This interpretation has however been challenged with a bipolar outflow proposed as an alternative explanation. We compare the two proposed scenarios for best consistency with the observations. Using a newly developed formalism we model the optical depth of the maser emission at each observed point in the map and LOS velocity for the two scenarios. We find that if the emission is symmetric around a central peak in both space and LOS velocity then it has to arise from an edge-on disc in sufficiently fast differential rotation. Disc models successfully fit ~100 independent measurement points in position-velocity space with 4 free parameters to an overall accuracy of 3-4%. Solutions for Keplerian rotation require a central mass of at least 4 solar masses. Close to best-fitting models are obtained if Keplerian motion is assumed around a central mass equaling ~30 solar masses as inferred from other observations. In contrast we find that classical bipolar outflow models cannot fit the data, although could be applicable in other sources. Our results strongly favour the differentially rotating disc hypothesis to describe the main feature of the 12.2 (and 6.7) GHz methanol maser emission in NGC7538 IRS1 N. Furthermore, for Keplerian rotation around a ~30 solar masses protostar we predict the position and velocity at which tangentially amplified masers should be detected in high dynamic range observations. [abridged]

Astrophysical Results from Geodetic VLBI Campaigns

Geodetic VLBI observations of extragalactic radio sources –designed to measure earth orientation parameters– are performed regularly since more than 10 years now. In different campaigns about 20 Quasars and BL Lacs are being monitored at 2.3 and 8.4 GHz. The high duty-cycle of these observations (every month for IRIS-S) and the resulting enormous data base allow to study the milliarcsecond structures and their variations simultaneously at two frequencies on short timescales. Since the accuracy of baseline determination and measurement of earth rotation parameters has reached a limit where source structure contributions become significant, there is substantial interest to monitor and correct for the source structure “effects”. Therefore we perform a regular analysis of sources selected from IRIS-S (high north-south resolution and almost circular beam for low declination sources) and EUROPE (high sensitivity due to large telescopes) campaigns covering a maximum time base of 10 years (1983–1993). The (undesired) structure of the sources leads to an occasional updating of the source catalogue in order to have “pointlike” sources which are observed 10–15 times for three to seven minutes during 24 hours. Meanwhile about 50 sources have been observed in geodetic campaigns. The comparatively poor data quality requires careful calibration with a source whose structure is well–known from high dynamic range observations (e.g. the blazar 1803+784). The dynamic range of these maps is limited to 1:50. The merit of this database lies obviously in the monthly persecution of structural changes in extragalactic radio sources over at best 10 years time and the filling of the gaps between high dynamic range observations. Sources which are presently of particular interest to us, include 1803+784, 4C39.25, 0528+134, 3C454.3 and OJ287.

High dynamic range VLA observations of eight core-dominated quasars

view Abstract Citations (44) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High Dynamic Range VLA Observations of Eight Core-Dominated Quasars Kollgaard, R. I. ; Wardle, J. F. C. ; Roberts, D. H. Abstract The Very Large Array has been used at 5 GHz to make high dynamic range observations of eight quasars with prominent cores. All exhibit one-sided jets, and all but one show evidence of diffuse halos. The luminosity of the extended emission by itself is sufficient for these to be Fanaroff-Riley Class II radio sources. This interpretation is confirmed by the polarization of the extended structure: the inferred magnetic field is parallel to the jet axis in all cases, and in all but one instance turns to be nearly perpendicular to the jet at its outer end. We identify these latter structures as classical terminal hotspots. Both the total intensity data and especially the polarization data strongly support the notion that these quasars are similar to the classical lobe-dominated quasars, but are oriented with jet axes close to the line of sight. If this is so, then the relatively high degrees of polarization observed in the terminal hotspots appear to require that the downstream fluid velocities in the hotspots are mildly relativistic, in the range 0.2 <= v/c <= 0.8. This, in turn, implies that the jets are at least moderately relativistic over their entire length. Publication: The Astronomical Journal Pub Date: October 1990 DOI: 10.1086/115579 Bibcode: 1990AJ....100.1057K Keywords: Luminous Intensity; Polarization (Waves); Quasars; Stellar Cores; Very Large Array (Vla); Emission Spectra; Halos; Stellar Luminosity; Stellar Spectra; Astrophysics; QUASARS full text sources ADS | data products NED (12) SIMBAD (8)

High Resolution VLA Observations of Quasars with Distorted Radio Stucture

Most quasars with extended radio structures have the classical linear double structure (Class II) which is also characteristic of the most luminous radio galaxies. A very small fraction of quasars do, however, show deviations from the classical double structure which are similar to the distortions observed in some low luminosity radio galaxies, e.g., bent double sources. Among radio galaxies, structural distortions seem to be associated with membership in a cluster of galaxies. Indeed, Scott and Hintzen, 1978, have suggested that such distortions might be indicators of cluster membership for quasars as well. To test this hypothesis and examine the physics of distorted quasars, we have mapped the quasars 3C 270.1 and 3C 275.1 in both I and P using the VLA A-Array at 20 cm and 6 cm. Our high resolution (0.1 arc sec at 6 cm) and high dynamic range (200:1) observations demonstrate that the displaced components reported previously for these sources are indeed connected with the quasars themselves and, therefore, the distortions are not simply projection effects. We also report the discovery of a very narrow northwesterly pointing jet in the quasar 3C 275.1.