GLEAM: The GaLactic and Extragalactic All-Sky MWA Survey

GLEAM, the GaLactic and Extragalactic All-sky MWA survey, is a survey of the entire radio sky south of declination + 25° at frequencies between 72 and 231 MHz, made with the MWA using a drift scan method that makes efficient use of the MWA’s very large field-of-view. We present the observation details, imaging strategies, and theoretical sensitivity for GLEAM. The survey ran for two years, the first year using 40-kHz frequency resolution and 0.5-s time resolution; the second year using 10-kHz frequency resolution and 2 s time resolution. The resulting image resolution and sensitivity depends on observing frequency, sky pointing, and image weighting scheme. At 154 MHz, the image resolution is approximately 2.5 × 2.2/cos (δ + 26.7°) arcmin with sensitivity to structures up to ~ 10° in angular size. We provide tables to calculate the expected thermal noise for GLEAM mosaics depending on pointing and frequency and discuss limitations to achieving theoretical noise in Stokes I images. We discuss challenges, and their solutions, that arise for GLEAM including ionospheric effects on source positions and linearly polarised emission, and the instrumental polarisation effects inherent to the MWA’s primary beam.

Interferometry and Stellar Magnetism

The classical detection of magnetic fields in Be stars remains a challenge due to the sensitivity threshold and geometrical cancelation of the field effects. We propose to study the Zeeman effect using Spectro-Polarimetric INterferometry (SPIN) which consists of the simultaneous use of polarimetry and very high angular resolution provided by long baseline interferometers. As monitoring of the instrumental polarisation is mandatory in order to calibrate interferometric observations in any case, the polarised signal is a natural by-product of interferometers. This method will be tested on the GI2T interferometer thanks to its high spectral resolution and its polarimetric capabilities.

Polarization Profile Near the 4430 Band

It has been reported by several authors that the extinction profile near the wavelengths of unidentified diffuse features shows fine structure (Brück and Nandy, 1970; Walker, 1971; York, 1971; Bromage, 1972); the question arises whether these features also exhibit fine structure in wavelength dependence of polarisation. It was first suggested by Greenberg et al. (1970, 1971) that interstellar bands produced by grain impurities may be polarized differently from the neighbouring continuum. Similar conclusions were also derived by Wickramasinghe and Nandy (1971) and Kelly (1971). Our first attempt to derive a profile of polarization against wavelength across the 4430 Å band was reported earlier (Nandy and Seddon, 1970). In this method we have used a Wollaston prism at the Cassegrain focus of the 36″ Telescope of the Royal Observatory, Edinburgh to produce orthogonally polarised spectra on the photographic plate. The results for the star 55 Cyg (Figure 1) indicated a definite profile. It was felt than an extension of the programme to acquire further profiles and to establish instrumental polarisation to a greater accuracy was necessary.