The MWA completed the 2016-B semester after a successful reconfiguration of the array into the compact configuration that utilises the two new 36-tile redundant “hexes”. The completion of the two new hexes were the first major milestone of the MWA’s Phase II upgrade. The remaining 56 tiles of the Phase II upgrade will be deployed on long baselines to double the diameter of the MWA to roughly 5 km. Infrastructure work to install the new long baseline tiles is ongoing, with commissioning expected mid-year. The MWA will be reconfigured into the extended configuration for the beginning of 2017-B.
The list of external instruments that the MWA hosts gained a significant new member with the initial deployment of “Aperture Array Verification System 1” (AAVS1) in March (see eNews by the AADC). AAVS1 is a prototype system for the future SKA-low. It is planned that the digital output of AAVS1 be incorporated into the MWA digital systems and correlator, so that AAVS1 data can be correlated with the full sensitivity (and antenna distribution) of the MWA to help characterise and verify the performance of AAVS1.
Caption: An aerial view of the MWA’s infrastructure hub region with the two new hex configuration sets of tiles (top right) and the AAVS1 station area (bottom left). The existing “core” region of the MWA is visible top centre. The radial lines show the path of buried power and comms infrastructure for the MWA.
The MWA continues to be scientifically productive and there are now around 90 collaboration-led publications since the commencement of operations in mid-2013. There are also now around a dozen papers using MWA data led by external teams from around the world, showing the increasing interest in using publicly available MWA data.
The following are some science highlights from the recent quarter.
“154 MHz Detection of Faint, Polarized Flares from UV Ceti” Lynch et al, 2017. ApJ in press.
Flaring activity is a common characteristic of magnetically active stars. Flare events produce emission throughout the electromagnetic spectrum, implying a range of physical processes. The number of stars where radio emission has been detected is few, with numbers limited to a few tens of objects. Observations of a wider sample of active stellar systems are necessary in order to establish the fraction that exhibit radio bursts and to relate occurrence of these bursts to basic physical parameters of these stellar objects.
To better constrain stellar flare rates at 154 MHz, Dr Christene Lynch (University of Sydney, CAASTRO) and collaborators used the MWA to observe UV Ceti, an M dwarf star known to exhibit magnetically driven flares. They detected four circularly polarised, dim flares from UV Ceti. Using the detected flares they measured the first flare rates for stellar flares with intensities < 100 mJy at 154 MHz.
“Low Frequency Radio Constraints on the Synchrotron Cosmic Web” Vernstrom et al., 2017. MNRAS in press.
On large scales galaxies clump together to form clusters connected by filaments, so there’s dense heavily populated areas and fairly empty void areas, connected by string like areas. This makes a web like structure known as the “cosmic web”. Theories suggest that ~50% of all the visible (not dark matter) matter/mass exists in the space between galaxies, but because this gas is such low density it is very difficult to detect. But this space between galaxies also has intrinsic magnetic fields, as well as magnetic fields generated from the galaxies merging to form this structure, which means that the cosmic web should glow with faint radio emission. This emission also has yet to be detected, but theories all say it should exist.
Using data from the MWA, Dr. Tessa Vernstrom (University of Toronto) and collaborators have put new limits on the brightness of the radio emission coming from the cosmic web, as well as limits on the magnetic field strength by using a technique known as cross correlation, which uses the position and numbers of known galaxies to increase the faint the radio signal.
“Spectral energy distribution and radio halo of NGC 253 at low radio frequencies” Kapinska et al, 2017. ApJ in press.
Using the MWA’s GLEAM all-sky survey, Dr Anna Kapinska (University of Western Australia, CAASTRO) and collaborators have studied in detail one of the archetypal starburst galaxies, NGC 253. The galaxy is one of our closest neighbours outside the Local Group, only 3.5 Mpc (11 million light-years) away from the Milky Way. NGC 253 is undergoing very powerful star formation, especially in its nucleus, and has a spectacular synchrotron radio halo.
This study has modelled and uncovered the physical conditions in the starburst nucleus responsible for its radio emission – something that has been unsolved since the suggestions were put forward 20 years ago- and discovered absorption processes occurring in the southern parts of the galaxy halo. The MWA observations used in this study are some of the most sensitive ones of NGC 253 at these low radio frequencies (<300MHz).