T. J. Cornwell

Propagation of coherence in scattering: an experiment using interplanetary scintillation

It is well known that high-time-resolution imaging of the optical seeing disk reveals the existence of speckles inside the envelope. Since analogs of seeing disks occur in many different fields, one might expect that analogs of speckles also exist. Using radio-astronomical techniques, we have studied one such example in depth: interplanetary scintillation. By using a radio-interferometric array, we measured directly the coherence of the radiation after scattering by the solar wind and confirmed the existence of fine-scale structure similar to speckles. In order to understand further the behavior of the coherence function after scattering, we adapted and extended the theory previously developed by Cornwell and Napier [in Proceedings of the National Radio Astronomy Observatory Workshop on Radio Astronomy in Space ( National Radio Astronomy Observatory, Greenbank, W. Va., 1986); Radio Sci.23, 739 ( 1988)] for the case of coherence measurements at the focal plane of a single-reflector radio telescope. We develop and demonstrate a test for coherence of the scattered radiation, and we discuss the prospects for high-angular-resolution imaging of objects. This treatment also leads to new insights into the effect of spatial coherence of emitters on interferometric measurements.

Imaging with ultra-resolution in the presence of strong scattering

When there is random scattering between a source and an imaging telescope or interferometer, it is known that the angular resolution that can be attained with a long exposure observation is below the ideal limit. Techniques such as speckle and self-calibration permit the use of short exposure data to restore the full resolution of the instrument. We show in this Letter that, under certain circumstances involving very strong scattering, short exposure data can actually be used to enhance the angular resolution to well beyond the ideal limit. Schematic designs for an interferometer to make use of this effect are presented

Extragalactic sources with very asymmetric radio structure: VLA and MERLIN observations

As part of our study to understand the nature of extragalactic radio sources which are very asymmetric in the surface brightness of the two lobes, often with radio emission on only one side of the nucleus, we have observed a large number of them with high angular resolution and good surface brightness sensitivity at radio frequencies. In this paper we present VLA and MERLIN observations of 15 such sources. We discuss their observed structures and spectra, and possible explanations for their morphologies. We report evidence of a possible correlation between the hot-spot brightness ratio and the degree of core prominence, used as a Statistical measure of source orientation, suggesting that relativistic beaming of the hot-spot emission does play a significant role in the observed brightness asymmetry. To explain the apparently one-sided sources within the relativistic beaming framework, the velocities required are in the range of 0.2 to 0.8c. We discuss the possibility that the lobe which is seen to the south of the jet in 3C273 is the counter-lobe seen in projection. We also draw-attention to a number of one-sided sources with very weak cores, and discuss their possible nature.

Extragalactic Sources with Asymmetric Radio Structure II. Further Observations of the Quasar B2 1320 + 299

We present VLA A-array observations at λ20, 6 and 2 cm and B-array observations at λ20 and 6 cm of the quasar B2 1320 + 299, which has a very unusual radio structure. In addition to a component, A, coincident with the quasar, there are two lobes of radio emission, B and C, on the same side of A. These are located at distances of -25 and 50 arcsec respectively from A. The present observations show that A has a flat-spectrum component coincident with the quasar and a weak outer component at a distance of-4 arcsec along PA - 100°. The morphology of B resembles a head-tail type of structure with its tail towards the north-east. The magnetic field lines in component B appear to follow the bend in the tail. Component C exhibits some extension towards the north-west. We discuss the possible nature of B2 1320 + 299 and suggest that while A appears to be an independent source, the relation between B and C, if they are associated at all, is unclear. Deep optical observations are essential to help clarify the situation.

Spatial coherence in a scatter‐broadened image‐observations of 3C279 close to the sun

Using the NRAO Very Large Array radio‐telescope, we have imaged the IPS scattered disk of 3C279 close to the Sun with high spatial and temporal resolution. The resulting images have speckles, similar to those seen in short‐exposure optical photographs of the seeing disk. We describe a test for coherence in the scattered radiation, and confirm that, under suitable observing conditions, the observed radiation is indeed spatially coherent.

Source-noise in radio synthesis images

Open Access. Radio interferometry: Theory, techniques, and applications; Proceedings of the 131st IAU Colloquium, Socorro, NM, Oct. 8-12, 1990.