Interplanetary scintillation found to contaminate the EoR signal
20 October 2015
The Epoch of Reionisation (EoR) is a period in the early Universe when the first stars and galaxies began radiating. Before this time, the Universe existed in the Dark Ages, a time marked by a lack of radiating sources and a neutral hydrogen intergalactic medium. During the EoR, the ionising radiation from these first sources stripped the electrons from these hydrogen atoms, transitioning the Universe from dark and neutral to bright and ionised. The details of this transition promise to provide a wealth of information about structure formation in the Universe but the radio signal we are trying to detect to trace it is extremely weak. Much more prominent in the radio sky are the numerous foreground sources, such as Active Galactic Nuclei and radio galaxies, that ‘contaminate’ our signal. In the EoR experiments with the Murchison Widefield Array (MWA), and others, we use our knowledge of these bright foregrounds, along with some signal processing tricks, to discriminate the EoR signal from the contaminants. However, if the contaminants differ from our expectations, then residual signal may affect our ability to observe the early Universe. Interplanetary scintillation (IPS) is a potential candidate to create this kind of issue.
IPS is typically observed as the twinkling of radio sources due to their light interacting with solar plasma before reaching our telescopes. Electrons and other ionised particles (plasma) flowing in the solar wind interact with the light from distant objects, distorting and refracting the wavefronts. The constructive and destructive interference of these wavefront distortions, as seen by the telescope, apparently increases or decreases the strength of the signal, compared with expectations. Such behaviour has the potential to add unexpected and time-dependent power to the EoR data and to further contaminate the signal. Following the measurement of IPS in two bright radio sources in the MWA EoR field by Kaplan et al. (2015), Curtin University based CAASTRO members Dr Cathryn Trott and Prof Steven Tingay explored the importance of considering IPS contamination for EoR experiments in their recent publication.
Taking a statistical approach, they took the measured spatial and temporal properties of IPS from results in the literature spanning 50 years and imprinted this signature on the expected, static properties of foreground radio sources in EoR data. The researchers found that IPS has different spatial and spectral properties to the static radio sources themselves, producing a unique, but low-level, signature in the EoR data. Having considered normal IPS conditions, they concluded that IPS would not be a major contributor to EoR contamination but that it should be considered in the modelling due to its distinct behaviour to avoid bias in the final results. This conclusion has potential implications for future, large-scale EoR experiments, such as with the Square Kilometre Array (SKA).