Telescope arrays measure how ionosphere affects pulsar signals
29 August 2013
Precise measurements of the effect of the Interstellar Medium and the Earth’s ionosphere on incoming radio waves, so-called Faraday Rotation, promise to be a powerful tool to map the structure of the Galactic magnetic field. In a recent publication in the journal Astronomy & Astrophysics, an international research team provides evidence that their approach to measuring Faraday Rotation will be applicable to Phase I of the Square Kilometre Array at low radio frequencies, using pulsars, without the need for Global Positioning System receivers at each observation station.
CAASTRO members Dr Martin Bell and Prof Michael Kramer contributed to LOFAR commissioning and are co-authors on this publication. The team presents their publicly available, Python-based code ionFR and demonstrate its performance in calculating rotation measures from the position of the source in sky, the geographic location of the observing site, the date of observation, and a ionospheric map (e.g. from the Center for Orbit Determination in Europe or the Royal Observatory of Belgium, as used here). Results from the model are compared to observational data for pulsars from two observing sessions (in 2009 and 2011) with LOFAR, the Low-Frequency Array for Radio Astronomy, and the Westerbork Synthesis Radio Telescope (in 2005 and archival data), both located in The Netherlands.
The researchers found their model agreed well with the observations from both instruments, generating an error of less than
0.1 rad m-2. All measurements are consistent with previously published data for these particular pulsars but are more precise. The study therefore convincingly verifies ionF as a tool to derive Faraday Rotations from interferometric imaging data. This is an important contribution to pre-SKA science and highlights the need for high-precision calibration procedures for the analysis of data from next generation radio telescopes.