The Square Kilometre Array (SKA)

Abstract:

Faraday rotation measurements using the current and next generation of low-frequency radio telescopes will provide a powerful probe of astronomical magnetic fields. However, achieving the full potential of these measurements requires accurate removal of the time-variable ionospheric Faraday rotation contribution. We present ionFR, a code that calculates the amount of ionospheric Faraday rotation for a specific epoch, geographic location, and line-of-sight. ionFR uses a number of publicly available, GPS-derived total electron content maps and the most recent release of the International Geomagnetic Reference Field. We describe applications of this code for the calibration of radio polarimetric observations, and demonstrate the high accuracy of its modeled ionospheric Faraday rotations using LOFAR pulsar observations. These show that we can accurately determine some of the highest-precision pulsar rotation measures ever achieved. Precision rotation measures can be used to monitor rotation measure variations - either intrinsic or due to the changing line-of-sight through the interstellar medium. This calibration is particularly important for nearby sources, where the ionosphere can contribute a significant fraction of the observed rotation measure. We also discuss planned improvements to ionFR, as well as the importance of ionospheric Faraday rotation calibration for the emerging generation of low-frequency radio telescopes, such as the SKA and its pathfinders.

Abstract:

This is the first paper in a series that presents a multiwavelength analysis of the archetype ultraluminous infrared galaxy IRAS F10214+4724, a gravitationally lensed, starburst/active galactic nucleus at z = 2.3. Here we present a new lens model and spatially resolved radio data, as well as a deep Hubble Space Telescope (HST) F160W map. The lens modelling employs a Bayesian Markov chain Monte Carlo algorithm with extended source, forward ray tracing. Using these high-resolution HST, Multi-Element Radio Linked Interferometer Network (MERLIN) and Very Large Array (VLA) maps, the algorithm allows us to constrain the level of distortion to the continuum spectral energy distribution resulting from emission components with differing magnification factors, due to their size and proximity to the caustic. Our lens model finds that the narrow-line region, and by proxy the active nucleus, is preferentially magnified. This supports previous claims that preferential magnification could mask the expected polycyclic aromatic hydrocarbon spectral features in the Spitzer mid-infrared spectrum which roughly trace the star-forming regions. Furthermore, we show that the arc-to-counter-image flux ratio is not a good estimate of the magnification in this system, despite its common use in the IRAS F10214+4724 literature. Our lens modelling suggests magnifications of μ ∼ 15-20 ± 2 for the HST F814W, MERLIN 1.7 GHz and VLA 8 GHz maps, significantly lower than the canonical values of μ = 50-100 often used for this system. Systematic errors such as the dark matter density slope and colocation of stellar and dark matter centroids dominate the uncertainties in the lens model at the 40 per cent level. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.