Precision Polarimetry with the Jansky Very Large Array

Rick Perley (NRAO)

Accurate polarimetry using circularly-polarized feeds requires observation of at least one polarized calibrator with known electric vector position angle (EVPA). The only sources whose EVPAs are known with absolute certainty are thermal bodies — the Moon, Mars, and Venus (at frequencies below ~8 GHz). Using VLA observations of these objects, we have determined that the EVPAs of the standard calibrator 3C286 (J1331+3030) at frequencies above 1 GHz to be significantly lower than the 33 degrees which has been in use for decades.

Below ~2 GHz, an additional complicating factor becomes important — the Faraday rotation of the Earth’s ionosphere. Using six separate observations of the Moon, taken with the JVLA with its P-band receivers (295 — 450 MHz), we have determined that the global IONEX models significantly overestimate the ionospheric rotation measure, while the local-based ALBUS software model correctly estimates the ionospheric rotation with an accuracy better than 0.1 rad/m^2 — equivalent to <5 degrees at a frequency of 300 MHz. As a byproduct of this work, we have determined the intrinsic polarization properties of three polarized calibrators -- DA240, 3C345, and 3C303. [su_divider]

Properties of Supernova Remnants from 1-D Models

Denis Leahy (University of Calgary)

Supernova remnants are the interaction of a supernova explosion with the interstellar medium, and result in spectacular radio and X-ray images. Here is described how 1-dimensional models (from simple to complex) can be used to learn about the physical properties of supernova remnants, such as their ages and explosion energies. Separately is described recent work on the significant effects of a simple (incorrect) assumption of the most common X-ray spectrum fitting software (XSPEC) and how to correct for this assumption to obtain physical values inferred from the X-ray spectrum.

Latest results of LOFAR’s Epoch of Reionization Key Science Project

Carolin Höfer (University of Groningen)

The Low Frequency Observing Array (LOFAR) is European’s largest radio interferometer operating between 30 – 240 MHz and a pathfinder for the SKA. Utilising a phased-array design, the antennae are grouped together into stations, with a dense core located near Exoloo, the northeast of The Netherlands. The Epoch of Reionization Key Science project uses the high-band antenna system, which covers LOFAR’s spectral response between 110-240 MHz corresponding to a redshift range z ~ 6 – 10. Its science goal is to answer how the first stars, galaxies and black holes formed by measuring the spatial fluctuations of the redshifted 21-cm emission of neutral hydrogen.

One of greatest challenges is to calibrate the instrument precisely, considering that astrophysical foregrounds are several orders brighter than the cosmological signal. I will discuss LOFAR’s latest data processing of the NCP and present our current upper limits on the 21-cm power spectrum. I will then discuss in more detail our current calibration strategy and some of the challenges we are still facing. I implemented a simulation pipeline and will show the effects of incomplete sky models on calibration solutions, images and power spectra.