A brief overview of radio pulsars and the ISM propagation effects on pulsar radio emission have been introduced in this Chapter. Chapter 2 presents the related instrumentation and observation methodology, including a detailed description of the MWA, its data processing procedures, the RM synthesis technique and its applications relevant in this work. Chapter 3 presents the initial census of Southern pulsars with the MWA, as published in Xue et al. (2017). Chapter 4 presents the polarimetric verification of the MWA tied-array beamforming process using the analysis of two bright Southern pulsars, as published in Xue et al.
(2019). Chapter 5 presents polarimetric studies for six pulsars in the general direction of the Gum Nebula using the MWA to probe the ISM properties in the Gum Nebula. A summary and future prospects are in Chapter 6.
Instrumentation and
Methodologies
2.1
Overview of the Murchison Widefield Array
The Murchison Widefield Array (MWA), operating in the frequency range 80–300 MHz, is located at the Murchison Radio-astronomy Observatory (MRO) in West- ern Australia. It is the low-frequency precursor telescope of the Low Frequency Aperture Array component of the Square Kilometre Array (SKA-Low), planned to be built at the same site. The MRO is located in a radio quiet zone protected by the Australian Government, and therefore has a relatively low level of radio frequency interference (RFI), mostly caused by aircrafts and satellites.
The Phase I MWA (Tingay et al., 2013) consisted of 128 ‘tiles’, where each tile is a dipole array composed of 16 dual-polarisation dipole antennas that are regularly arranged in a 4×4 grid with ∼1.1 m separation and mounted on a 5×5 m steel wire mesh ground screen, as shown in Figure2.1. These tiles are distributed across a ∼ 3 km diameter region; 25% of them are closely placed within a dense ∼ 100-m diameter core.
As an aperture array radio telescope, the MWA has no moving parts. All telescope functions, including pointing, are performed by electronic manipulation
of dipole signals (Lonsdale et al., 2009). Signals from the 16 dipoles of a tile are combined by an analog beamformer, where a set of switchable delays (between 0 and 13.5 ns) are employed to steer the ‘tile beam’ (or ‘primary beam’) toward one of the 197 predetermined (and coarsely spread) sky positions (known as ‘sweet spots’). The analog signal from each tile is then amplified, digitised, and channelised in the receiver box (Prabu et al.,2015). There are, in total, 16 receiver boxes deployed at the site. In each receiver box, 16 streams of dual-polarisation analog signals from a group of eight tiles are filtered to a bandpass of 80−300 MHz and then channelised to 24 × 1.28 MHz coarse frequency channels by an Polyphase Filter Bank (PFB) based on Field Programmable Gate Array (FPGA). This first stage channelisation produces a signal stream with a total bandwidth of 30.72 MHz, which can be either contiguous (the most common observing mode) or non-contiguous (the so-called ‘picket-fence’ observing mode) (Prabu et al.,2015). The signal then undergoes a second stage channelisation, producing 128 × 10 kHz fine channels, resulting in a time resolution of 100 µs. This task is performed by dedicated hardware that is designed to perform the second (fine) PFB operation. (Ord et al., 2015).
Between late 2016 and mid-2017, the MWA underwent a major upgrade. This involved the addition of 128 new tiles to the Phase I configuration to create a larger array (i.e. the Phase II MWA) consisting of 256 deployed tiles extending out to a maximum baseline of ∼ 6 km (Wayth et al., 2018). In this upgrade, the existing signal path and electronics (receivers and correlator) has remained the same. As a result, 128 tiles are available at any given time because of the limitation set by the maximum signal throughput of the MWA hardware. In order to satisfy the requirements of the MWA’s various science goals, the array switches between two types of configuration, namely, compact and extended, and the switching is done approximately once a year. The compact configuration makes use of the 56 tiles in the MWA’s core, along with 72 new tiles that are arranged in two regular hexagonal configurations (Wayth et al., 2018) located
Figure 2.1: A photograph of three MWA tiles with the corresponding analog beamformer units (the ‘white box’) at the MRO site. Within each tile, the centres of 16 dual-polarisation dipoles are equally spaced by ∼1.1 m over a 5 × 5 m steel wire mesh ground screen. Thus, the observing element has effectively a physical size of approximately 4.4 × 4.4 m.
nearby. This regular antenna arrangement is especially useful for the study of the Epoch of Reionisation (EoR) power spectrum (one of the MWA’s key science goals), because the large number of identical (and therefore, redundant) baselines help both boost the sensitivity (e.g. Parsons et al.,2012) and enable the recovery of antenna gains during calibration without requiring a sky model (e.g. Li et al.,
2018). The compact configuration is also advantageous for doing pulsar surveys as it provides a larger tied-array beam (because of the shorter baselines, see section2.4.2), thereby increasing the survey efficiency significantly. The extended configuration replaces the 56 closely-spaced core tiles of the Phase I MWA with 56 sparsely-spaced tiles placed within a ∼ 5.3 km diameter region. This allows for higher spatial resolution imaging (∼1.300 at 154 MHz), and reducing the expected classical confusion by a factor of ∼ 8, comparing with the Phase I MWA (Wayth et al., 2018). For the work presented in this thesis, we used Phase I MWA observations (Chapters 3 and 4), and a combination of Phase I and Phase II
compact- and extended-configuration observations (Chapter 5).