New study improves data calibration for MWA polarisation surveys
27 September 2013
Low frequency radio telescopes have recently been developed to search for and ultimately detect faint signals from the Epoch of Reionisation (EoR) during the first billion years of the Universe’s history. A publication by members of the Murchison Widefield Array (MWA) consortium presents an analysis of the instrument’s early smaller prototype ("32T"). The large field of view of the MWA and its capability to observe at frequencies below 200 MHz fill a gap left by previous surveys that have never been able to generate wide-band data on polarised Galactic diffuse emission and extragalactic radio sources. Galactic and extragalactic polarised emission is important to characterise and account for when searching for signals from the EoR.
The present zenith drift-scan observations covered a 2400 deg2 field of view and were made at a central frequency of 189 MHz, with images being produced from 5-minute segments ("snapshots") that were then processed and combined into mosaics. The team detected 137 radio sources with flux densities above 4 Jy. From previous studies it was expected that a number of these sources would be polarised but the team’s data analysis only identified a single polarised source (PMN J0351-2744) while all other sources exhibited polarisation fractions below 2%, as compared to 7% in previous surveys at 1.4 GHz.
The team also analysed the Rotation Measure of the well-studied radio Galaxy Fornax A but did not detect point source polarisation. Diffuse polarised emission, however, was abundant in 20° strips of Galactic latitude and showed up in patchy and filamentary patterns. This survey, which was sensitive up to angular scales of ~4°, revealed a polarisation peak of ~13 K and Faraday depths between 0-10 rad m-2. Based on these measurements, the emission appears to arise from within a few hundred parsecs of Earth and may be the result of vorticity or shear in the local Interstellar Medium. Overall, this study confirmed that continuous improvements to the data processing and calibration procedures can make it possible to remove polarised foreground signals to detect the EoR.