Murchison Widefield Array Report

MWA status

The MWA’s Phase II expansion phase reached a major milestone in September with the successful commissioning of the compact configuration Phase II array. Regular observations began at the start of October for the 2016-B observing semester, with a significant focus on observations for the Epoch of Reionisation (EoR) key science programme. The new compact configuration of the Phase II MWA has 72 new “hex configuration” (see previous eNews) tiles optimised for EoR science. The new hex configuration tiles improve the theoretical sensitivity of the MWA by almost an order of magnitude for EoR power spectrum science.

The MWA is preparing to host the SKA Low Aperture Array Verification System 1 (AAVS1) prototype, to be completed early 2017. Although AAVS1 can work in a standalone mode, the MWA operations team has been working behind the scenes to enable integration of the AAVS1 digital signals into the MWA correlator, which will be mutually beneficial for both AAVS1 and MWA.

MWA Science

The sky GLEAMs with MWA data: The first large data product from the MWA’s GaLactic and Extragalactic All-sky MWA (“GLEAM”) survey was published in MNRAS in October. The GLEAM extragalactic catalogue contains over 300,000 sources covering most of the extragalactic sky south of +30O declination, with 20 flux density measurements between 72 and 230 MHz for all sources. The catalogue improves by more than an order of magnitude the number of sources with measured flux densities at low frequency in the southern hemisphere, and the fractional bandwidth from the GLEAM survey is unprecedented.

The GLEAM extragalactic catalogue is described in:

GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey – I. A low-frequency extragalactic catalogue” Hurley-Walker et al., 2016. MNRAS. ADS link.

Images and movies available here: http://www.icrar.org/gleam/

GLEAM-Data survey

The GLEAM view of the centre of the Milky Way, in radio colour. Red indicates the lowest frequencies, green indicates the middle frequencies and blue the highest frequencies. Each dot is a galaxy, with around 300,000 radio galaxies observed as part of the GLEAM survey. Credit: Natasha Hurley-Walker (Curtin / ICRAR) and the GLEAM Team.

Other recent publication highlights include:

Galactic synchrotron emissivity measurements between 250° < l < 355° from the GLEAM survey with the MWA“, Su et al. 2016. MNRAS in press. arXiv link.

The MWA’s GLEAM all-sky survey is sensitive to temperature variations on degree scales due to the MWA’s many short baselines. This paper uses GLEAM data to create a coarse map the overall synchrotron emissivity throughout our galaxy by looking at how the apparent temperature of the sky changes due to absorption of radio waves by interstellar HII regions.

Hii_Su

An HII absorption region see by the MWA (left) coincides with a known HII region seen in optical emission (right). The HII region blocks background radio waves, creating a “hole” in the sky at MWA frequencies.

Delay Spectrum with Phase-Tracking Arrays: Extracting the HI power spectrum from the Epoch of Reionization” Paul et al., 2016. ApJ in press. ADS link.

This paper is the next instalment in the toolkit of the MWA’s Epoch of Reionisation (EoR) science team. The paper describes a method to extract the faint signal from the EoR using the delay spectrum technique.

First Season MWA EoR Power Spectrum Results at Redshift 7” Beardsley et al. 2016. ApJ in press. ADS link.

The MWAs’ EoR science team is continuing the pursuit to understand the stars and galaxies that drove the Epoch of Reionization (EoR). In a paper recently accepted to the Astrophysical Journal, the EoR team from the U.S. and Australia presents their analysis of several months’ worth of MWA observations. While digging deeper and deeper into the data the team has learned new things about the telescope and its incredible sensitivity. This result is the deepest limit on the cosmological signal to date from an imaging array, and pushes the frontier towards the cosmic dawn.

EoR_Beardsley

(Left) The N-S, z = 6.8 two dimensional power spectrum. The figures on the right show cuts through the power spectrum and the expected noise level given the integration time.

PUMA: The Positional Update and Matching Algorithm” Line et al., 2016. PASA in press. ADS link.

Members of the MWA’s EoR science team have developed new software that combines radio source catalogues that have significantly different resolution and frequency, automatically matching and classifying sources using a Bayesian probabilistic approach. The team show that by using more accurate positional information of sources from higher frequency radio catalogues, the residual noise from subtracting sources will be reduced for EoR power spectrum science.

Line_img

This example figure from the paper shows the PUMA match of a single, unresolved source in the MWACS catalogue with a multiple component source from the VLSSR, SUMSS and NVSS catalogues.

A search for long-timescale, low-frequency radio transients” Murphy et al., 2016. MNRAS in press.

Members of the MWA’s Transient science team collaborated with the GLEAM survey team and some members of the Alternative Data Release of the 150 MHz TIFR GMRT Sky Survey (TGSS ADR1) to search for radio sources that appeared or disappeared between the GLEAM observations in 2014 and observations from 2011 with the Giant Metrewave Radio Telescope (GMRT). Ultimately, after considering over 500,000 sources, only a single radio source appeared to have appeared or disappeared between those two catalogs. This sets one of the lowest limits to date on the rate of slow radio transients at these frequencies, and moreover gives the collaboration an intriguing new source to study. Further multi-wavelength follow-up observations are underway.

MWA1

TGSS ADR1 (left) and GLEAM (right) images of the transient candidate TGSSADR J183304.4−384046 (shown with a red circle). On the TGSS ADR1 image, the blue circles show the location of NVSS sources with a 1.4 GHz flux density greater than 5 mJy.

The GLEAM image is overlaid with a TGSS contour at 80 mJy/beam to show the position of TGSS sources.


Report provided by Randall Wayth