Murchison Widefield Array Report

Phase II expansion complete!

The Murchison Widefield Array achieved a major milestone in October with the completion of the “Phase II” expansion – the first major upgrade of the MWA since operations commenced in mid-2013. The upgrade consisted of installing 128 new phased-array antenna “tiles”, which doubles the resolution and sensitivity of the MWA for continuum imaging, and brings almost an order of magnitude improvement in sensitivity for the EoR power spectrum key science case.

The first stage of the upgrade installed 72 tiles in two compact regular hexagonal configurations, and was completed in September 2016. The second stage of the upgrade installed 56 remote tiles to double the diameter of the array. The new remote tiles contain novel hardware, signal transport and solar power systems suited for autonomous self-contained operation in the Murchison.

Photos of new long baseline tiles with solar power and beamformer controller units:

https://photos.app.goo.gl/jqWzy9ztFTq0Jjo63

https://photos.app.goo.gl/SToyfroKrAyZhtYj1

For more information, see: http://news.curtin.edu.au/media-releases/major-milestone-key-radio-astronomy-project/

MWA Science

The MWA continues to be scientifically productive and there are now over 90 collaboration-led publications since the commencement of operations in mid-2013. The publications cover a diverse range of science areas, from solar science through to the Epoch of Reionisation.

The following are some science highlights from the recent quarter.

4D Data Cubes from Radio-Interferometric Spectroscopic Snapshot Imaging
Atul Mohan, Divya Oberoi. https://arxiv.org/abs/1710.10525

Low radio frequency solar emission spans a very large range in intensity, as well as temporal, spectral and spatial scales. Often multiple processes are going on simultaneously at different locations on the Sun, giving rise to different emissions. These emissions can differ greatly in their strengths and till recently one could usually only study the most intense of these sources. The significantly improved imaging dynamic range of the MWA is now making it possible to study comparatively weaker emissions in presence of more intense ones. In order to facilitate such studies, Dr. Divya Oberoi and his student Atul Mohan have recently developed a new data product which will enable scientists to study the frequency and time variations of the emission coming from any specific patch on the Sun. Called SPREDS, an acronym for SPatially REsolved Dynamic Spectrum; it is named in analogy with the usual definition of a dynamic spectrum, which shows the variations of the emission in the time-frequency plane. They also presented the first flux calibrated solar images from the MWA, made with a resolution of 0.5 s and 160 kHz.

The top left panel shows a radio image of the Sun with some regions of interest marked on it; the top right panel shows the dynamic spectrum for the entire Sun, the most commonly used radio data product at these frequencies; the remaining panels show the SPREDS from the corresponding regions marked on the solar disc. Note that the colour scale for these panels is in log scale, the differences between emissions from different regions on the Sun are self-evident.

The Spectral Energy Distribution of Powerful Starburst Galaxies I: Modelling the Radio Continuum
Galvin et al. https://arxiv.org/abs/1710.01967

Star forming galaxies, like their name suggests, are galaxies that are actively producing young stars. Because they are normally faint at radio wavelengths, astronomers have often been restricted to looking at fairly local, and rare, representative galaxies. The Square Kilometre Array will be probing a volume of the Universe for the first time, where these types of objects are far more common.

In preparation, using data from the Murchison Widefield Array and the Australian Telescope Compact Array, Western Sydney University PhD student Tim Galvin and collaborators studied a set of starburst galaxies in detail from low to high radio frequencies. The study shows that the radio emission as a function of frequency in these galaxies is far more complex than typically assumed, and overly simple models may overestimate the expected radio emission at low frequencies (most relevant to SKA-low) by a factor of up to 12. The combination of data over a broad frequency range from multiple radio telescopes, including the MWA, provides the full picture of the astrophysics underlying the radio emission for these star forming galaxies.

Example SED of IRAS F23389-6139 showing complex frequency structure and a turnover at low frequencies.

Spectral Flattening at Low Frequencies in Crab Giant Pulses
Meyers et al. https://arxiv.org/abs/1709.03651

Pulsars emit beams of radiation along their magnetic axes, which we see as a series of periodic bursts (or pulses) each time those beams sweep over our line-of-sight. The young and energetic pulsar that resides in the Crab Nebula (PSR J0534+2200) has been of special interest to both observers and theorists alike. Unlike the vast majority of known pulsars, the Crab pulsar was discovered through its “giant pulses” – extremely bright, very short-duration bursts, whose energetics are many orders of magnitude higher than those of regular pulses. These giant pulses therefore provide excellent avenues to explore how pulsars emit in the first place. The strength of the pulsar emission (brightness or flux density) rapidly declines with increasing frequency, and can be characterised in terms of spectral energy distributions (SEDs), which describe how much energy is produced as a function of frequency. For regular pulsar emission, this generally follows a power-law, and in some cases tends to exhibit either a break or turn-over at lower frequencies. Similar characterisation has been inherently difficult in the case of giant pulses because of their sporadic nature.

Curtin University PhD student Bradley Meyers and co-authors leveraged the wide frequency coverage that is achievable via simultaneous use of the MWA and the CSIRO Parkes radio telescope to study the Crab pulsar’s giant pulse emission, spanning frequencies from ~100 MHz (MWA) to ~4 GHz (Parkes). This study shows that the SED of Crab giant pulses is not well described by a single power-law over such a large range in frequency, and instead, flattens at low frequencies. An important implication of this is that giant pulses are not as bright as expected at low frequencies. Moreover, the physical processes that give rise to such a spectral flattening in giant-pulse emission also remain unknown. This result makes a compelling case for undertaking similar studies for other giant-pulse emitters, and if it holds up, may also have implications for the detectability of pulsars in external galaxies (via giant pulses), or potentially for certain models of Fast Radio Bursts, where they are thought to be ultra-luminous giant pulses.

example of a single Crab giant pulse over an order of magnitude in radio frequency.

A First Look for Molecules between 103 and 133 MHz using the Murchison Widefield Array
Tremblay et al. http://adsabs.harvard.edu/abs/2017MNRAS.471.4144T

Led by ICRAR/Curtin University PhD student Chenoa Tremblay, the largest ever low-frequency spectral line survey of the Galactic Plane has recently been completed and has found evidence for Nitric Oxide (NO). The NO molecule is a key ingredient for forming amino acids, which in turn are building blocks for the DNA that powers all life on Earth. The NO molecule, along with a detection of the mercapto radical (SH), was found in cold gas surrounding evolved stars. This evidence was found thanks to the development of a new data-processing pipeline which allows the Murchison Widefield Array to study the chemistry of our Galaxy. This new capability is now being put to use in order to study the complex and interesting gas, dust and stars in the Orion constellation.

The field around the galactic centred observed by the MWA for low frequency molecular line transitions.


Report provided by Randall Wayth