Low-Frequency Observations of the Moon With the Murchison Widefield Array

Abstract

A new generation of low frequency radio telescopes is seeking to observe the redshifted 21 cm signal from the Epoch of Reionization (EoR), requiring innovative methods of calibration and imaging to overcome the difficulties of widefield -2low frequency radio interferometry. Precise calibration will be required to separate the small expected EoR signal from the strong foreground emission at the frequencies of interest between 80 and 300 MHz. The Moon may be useful as a calibration source for detection of the EoR signature, as it should have a smooth and predictable thermal spectrum across the frequency band of interest. Initial observations of the Moon with the Murchison Widefield Array 32 tile prototype show that the Moon does exhibit a similar trend to that expected for a cool thermally emitting body in the observed frequency range, but that the spectrum is corrupted by reflected radio emission from Earth. In particular, there is an abrupt increase in the observed flux density of the Moon within the internationally recognised Frequency Modulated (FM) radio band. The observations have implications for future low frequency surveys and EoR detection experiments that will need to take this reflected emission from the Moon into account. The results also allow us to estimate the equivalent isotropic power emitted by the Earth in the FM band and to determine how bright the Earth might appear at metre wavelengths to an observer beyond our own solar system.

Bayesian constraints on the global 21-cm signal from the Cosmic Dawn

“…While current experiments targeting 21-cm fluctuations are well placed to constrain the reionization process statistically, only the upcoming interferometric arrays like the Hydrogen Epoch of Reionization Array (Pober et al 2014;DeBoer et al prep) and the Square Kilometre Array (Koopmans et al 2015) will have sufficient sensitivity and frequency coverage to probe the Lyα and Xray heating epochs (Mesinger et al 2015;Ewall-Wice et al 2016a). Therefore increased attention has recently been devoted to observations targeting the global (sky-averaged) 21-cm emission (e.g., Pritchard & Loeb 2010;Morandi & Barkana 2012;Liu & Parsons 2016), including novel ways to use interferometric arrays to probe the global 21-cm signal (McKinley et al 2013;Presley et al 2015;Singh et al 2015;Vedantham et al 2015). Albeit challenged by the same requirements of accurate subtraction of bright foreground emission and control over systematic effects that affect its sibling 21-cm fluctuation observations, the global 21-cm signal may represent an alternative, relatively inexpensive way to achieve the mil-liKelvin sensitivity needed to access the pre-reionization epoch.…”

A Serendipitous MWA Search for Narrowband Signals from ‘Oumuamua

“…Moreover, it could be argued that the FM band is a good place to search for transmissions from a spacecraft visiting our solar system. It would be quickly apparent to the visitors that the FM band is a highly used frequency range, given that the Earth appears to have an approximate 77 MW EIRP in the FM band (McKinley et al 2013), and mainly used for information broadcast rather than two-way communications. Thus, the FM band would certainly be an interesting frequency range for monitoring and a logical choice for communications-there would be many people listening on Earth.…”

A new MWA limit on the 21 cm Power Spectrum at Redshifts $\sim$ 13 $-$ 17

“…Although the MRO is an extremely radio-quiet site, RFI is unavoidable in the ultralow data, particularly in the FM band (87-108 MHz) where bright terrestrial signals can come over the horizon, be ducted through the troposphere, or be reflected off aircraft or even the Moon (McKinley et al 2013). We investigated the RFI environment in the ultralow band by considering the flagging statistics of AOFlagger as well as SSINS (Wilensky et al 2019), a different RFI detection algorithm developed for EoR data.…”