DRAO Seminar Series Spring 2015

The Polarized Galactic Synchrotron Foreground: An Unlikely Calibrator

Michiel Brentjens (ASTRON)

Low frequency polarimetry is an important part of cosmic magnetic field research. It is, however, severely hampered by time-variable ionospheric Faraday rotation, which may wipe out any polarized signal over the course of a multi-hour observation. Correcting for the ionosphere using global ionosphere and magnetic field models only works adequately down to about 120-150 MHz. Bright in-field polarization calibrators, such as some pulsars, could provide much better results straight from the actual observation, but only a handful exist below 150 MHz.

Fortunately, the polarized Galactic foreground seems to be everywhere we look with LOFAR. Unfortunately, it is not compact and has a very messy spatial and spectral structure. I have therefore devised a method to calibrate the ionospheric Faraday rotation using *only* the Galactic foreground polarization, using no knowledge of its spatial and spectral structure other than that it is not supposed to change during the course of the observation.

Although my procedure does not provide absolute calibration unless other information is included, it re-aligns all data in an observation to a common RM scale with high precision, removing the time-variable ionosphere. Relative ionospheric RM accuracies for global ionosphere maps combined with terrestrial magnetic field models are typically of order 0.1-0.2 rad/m/m at 2 hour intervals. My method obtains relative accuracies of order 0.02 rad/m/m at 10 minute intervals at 115-170 MHz, which is good enough to, for the first time ever, enable accurate polarimetry in the LOFAR low band, below 80 MHz.

Channelizer and Resampler Design Studies for SKA1-Mid Correlator Beamformer

Thushara Gunaratne (DRAO)

The Square Kilometer Array (SKA) is expected to be 50 times more sensitive and 10,000 times faster in conducting sky surveys than any existing instrument for centimetre-wavelengths 70 MHz to 13.5 GHz. In design, the SKA is a collection of three telescopes: 1) The SKA-Low (70-450 MHz); 2) the SKA-Mid (350 MHz-13.5 GHz); 3) the SKA-Survey (500-1500 MHz). The requirements of high sensitivity, wide operational bandwidth and high dynamic range impose significant design challenges at all stages of the SKA. The final design stage of the SKA Phase-1, which includes only 10% of the final collection area, has just begun and the construction stage is expected to start in early 2018.

For synthesis imaging in the SKA-Mid, a dual-stage Channelizer has been proposed. The first stage is consisted of an over-sampled Polyphase-DFT filter-bank (PDFB) and the second stage is consisted of a critically-sampled PDFB. The prototype filters for both stages have to be designed to meet the specifications in order to achieve the expected sensitivities. The hardware implementation of the critically-sampled PDFBs have been previously studied in detail but this is not the case for over-sampled PDFBs. Further, the target hardware platform for the SKA Phase-1 Mid-Band Central Signal Processor and Beamformer (SKA1-Mid-CSPB) is the next-generation Stratix-10 FPGA from Altera. Its novel “HyperFlex” architecture is significantly different from existing FPGAs and therefore imposes significant design challenges. A proposed design and implementation of an 8/7 over-sampled PDFB having 512 complex-channel outputs is discussed in detail in the first part of the presentation.

Digital resampling has been proposed to be used at several points along the signal processing chain in SKA1-Mid-CSPB. The proposed SKA1-Mid array will contain 190 SKA MID dishes and 64 MeerKAT dishes, both with Single Pixel Feeds. The L-band of MeerKAT covers the 770 MHz band from 900 MHz to 1670 MHz whereas Band 2 of SKA1-Mid covers the 810 MHz band from 950 MHz to 1760 MHz. Further, the L-band of MeerKAT is sampled at 1712 MHz whereas the Band 2 of SKA1-Mid is proposed to be sampled at 2000 MHz. In order to incorporate the L-band signals from MeerKAT dishes for the synthesis imaging process along with the Band 2 signals of SKA1-Mid, those L-band signals must be resampled to 2000 MHz. Also for VLBI applications, the SKA1-Mid signals have to be resampled at the VLBI standard rates prior/after the VLBI beamforming and before the VLBI correlation. Also, digital resampling has been proposed to be used in the frequency offset sampling/resampling scheme at the SKA1-Mid receiver and digitizer in order to mitigate self-interference due to ADC clocks. Therefore, it is important to study the effects of digital resampling schemes on the signal quality for SKA1-Mid. In the second part of the presentation, the designs for digital resampling schemes are described for MeerKAT L-band and SKA1-Mid Band-2 interfaces. Frequency-offset sampling/resampling for SKA1-Mid Bands is analyzed in terms of the sensitivity loss of cross-correlation coefficients.

The Potential to Form Planets in the Orion Nebula: The ALMA Perspective

Rita Mann (DAO)

The formation of planetary systems is intimately connected to the properties of the circumstellar disks in which they are born. Disk studies to date have focused on regions like Taurus and Ophiuchus for their proximity, however, stars rarely form in such isolated environments. Most stars form in massive star-forming regions and there is even clear evidence that our Sun formed near an OB association like that found in Orion. Using the Submillimeter Array (SMA) and the Atacama Large Millimeter/Submillimeter Array (ALMA), we surveyed 67 protoplanetary disks (“proplyds”) at 850 microns in the Orion Nebula to determine their masses. The SMA, as the world’s only sub-millimeter interferometer until ALMA, was uniquely capable of detecting dust emission from the Orion proplyds, making these results the first successful measurements of disk masses in an OB association. These observations have revealed the range of influence of nearby massive stars on disk evolution and allowed us to answer the long-standing question about whether enough material remains in the Orion disks to potentially form Solar System analogs.

The Cryogenic Phased Array Feed: Fact or Fiction? Boojum or Snark? Here Be Dragons.

Lisa Locke (Millimetre Technology Group, NRC Herzberg)

Astronomers: “We want more!”
Engineers: “Hmm… Let’s see.”

Optimizing astronomy receiver throughput usually involves an increase in antenna reflector size or number of antennas, both of which can be prohibitively expensive. Another option is to reduce the system temperature by using new and improved LNA devices and circuitry but device noise is fast approaching the quantum noise limit.

A viable alternative is to exploit the telescope’s imaging capability by placing multiple independent feed-receiver systems on the focal plane of the reflector system. This results in an enlarged field of view of the reflector system which in turn decreases the time required to observe a certain solid angle on sky. Multi-beam horn feeds did exactly that and decreased observation time by a factor equal to the number of horn feeds. Unfortunately feedhorns are wide (~2 wavelengths) so the focal plane is under-sampled and multiple interleaved pointings are required reducing the net improvement factor compared with a single pixel feed (SPF). However, by using narrow antenna elements (~0.5 wavelength) phased array feeds (PAFs) can fully Nyquist sample the focal plane. The increased field of view is proportionate to the number of synthesized beams, reducing the imaging time by an order of magnitude compared with an SPF.

PAF development for astronomy instrumentation research started in the late 1990s and has been underway worldwide mostly at L-band (0.7–1.5 GHz) with room temperature dual-polarization antenna arrays and coherent receivers. Building on these lessons, the DRAO/DAO team is currently working on an S-band (1.5–4.0 GHz) design incorporating a 96-element (48-element dual linear) Vivaldi array and low-noise (2.5 K) amplifiers, all cryogenically cooled in a scary-large 0.5 m diameter dewar. Estimated system noise temperature is 25 K including mutual coupling and sky contributions. All 96 coaxial RF inputs are summed with complex weights in the beamformer, producing 36 dual-polarization wideband beams, each 600 MHz on fiber, direct to your computer.

Please join me for an exciting (relatively) review of the cryoPAF design and who’s doing what in radio camera land.

Highlights from the JCMT Gould Belt Survey

Helen Kirk (DAO)

The JCMT Gould Belt Survey, an ambitious survey of nearby star-forming regions visible from the northern hemisphere, finished its final observations in January of this year. The dataset includes SCUBA2 observations of the continuum emission from the cold dust present in molecular clouds, as well as a smaller subset of HARP observations of CO line emission. The SCUBA2 observations trace dust properties including column density and temperature, while the HARP observations trace the gas motions and CO abundance. I will highlight some of the first exciting results arising from the SCUBA2 data, including the properties of filamentary structures, the stability and clustering properties of stellar progenitors, disk sources, and changes in dust grain properties. These are just the first of many results which will be coming out in the next few years from the GBS which will help us to better understand what shapes the formation of stars, locally, and cloud-wide.

Gas Clouds in the Local Group Where They Should Not Be, and Other Recent Discoveries of the Green Bank Telescope

Felix J. (Jay) Lockman (NRAO)

In this talk I’ll review some recent discoveries made with the Green Bank Radio Telescope in areas ranging from the structure of the Lunar surface, to astrochemistry, pulsar physics, rocks in Orion, and cosmology.

My special focus will be on current work on the evolution of galaxies that has led to the discovery of gas clouds in unexpected and puzzling locations. Some of these clouds may be the remnants of past galaxy interactions, while others may mark the infall of new gas onto the Milky Way. One large cloud may be a “failed” galaxy containing gas and dark matter without stars. I’ll discuss the properties and possible origins of this material and its implication for the evolution of the Milky Way.

From Filaments, to Cores, to… Filaments?! The Role of Magnetic Fields in Multi-scale, Filamentary Star Formation

Chat Hull (Jansky Fellow, Harvard-Smithsonian Center for Astrophysics/NRAO)

In just the past few years, it has become clear that filamentary structure is present in the star-formation process across many orders of magnitude in spatial scale, from the galactic scales probed by Planck and Herschel all the way down to the AU-scale structures that ALMA has revealed within protoplanetary disks. A similar story can be told of magnetic fields, which play a role in star formation across the same vast range of size scales. Here I will first review my work on 1000 AU-scale dust polarization and magnetic fields in Class 0 protostellar envelopes, which were observed as part of the TADPOL survey using the 1.3 millimeter dual-polarization receiver system at CARMA. I will then highlight two 1000 AU-scale filamentary structures seen with CARMA before I reveal new, high resolution (150 AU!) ALMA 1.3 mm continuum observations of three protostars in Serpens. Even at such high resolution, these sources have a number of nearby, filamentary blobs/condensations/companions, most of which coincide in a tantalizing way with the magnetic fields we mapped with CARMA. I will muse on what this all means, and on what questions may soon be answered by ALMA polarization observations of the same three sources.

The Beauty in the Extreme: Supernova Remnants and Associated Compact Objects

Samar Safi-Harb (Manitoba)

Supernova Remnants (SNRs) are among the most energetic and fascinating astrophysical objects in the universe. Observational studies of SNRs across the electromagnetic spectrum have allowed us to address a host of science topics ranging from: the formation of the heavy elements essential to life, to the efficient acceleration of cosmic rays to extremely high energies, to the formation of the most exotic compact objects (neutron stars including `magnetars’) — thus providing nearby laboratories for probing extreme physics. To date, we know of 310 (380 including candidates) SNRs in our Galaxy, with a growing fraction discovered or observed with modern high-energy X-ray and gamma-ray missions. As a result, our understanding of these energetic objects has witnessed a tremendous jump during the past decade, yet many fundamental questions remain to be answered. I provide a brief overview of this fast-growing field with focus on the growing diversity of neutron stars and nebulae in SNRs, then highlight some fundamental science questions related to core-collapse supernova explosions. These include (1) addressing the apparent age discrepancy between SNRs and their associated compact objects, and (2) probing their debated supernova progenitors and energetics via X-ray spectroscopy. I will conclude with prospects for future studies in this field, particularly with the upcoming ASTRO-H X-ray mission.