Instrumentation Development for Radio Astronomy

The development of receivers for radio astronomy involves a number of technical domains in order to have a system which brings the electromagnetic signal from space through to the telescope, antenna, and various electronics before arriving at the system of signal processing and ultimately, analysis by the astronomer. I have worked in several of these domains for a number of instruments. This includes antenna analysis for the Submillimetre Common User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (Torchinsky 1990, Torchinsky PhD, U. Edinburgh 1991). I worked on the design and scale modeling of a terahertz heterodyne mixer (Torchinsky et al. 1993, 1994). I worked on optics design and system integration and test for the Swedish/Canadian/French/Finnish satellite called Odin (Torchinsky 2000, Torchinsky et al. 2000, Torchinsky et al. 2002, Frisk et al. 2003, Olberg et al. 2003, Kwok et al. 2004). For the Heterodyne Instrument for the Far Infrared (HIFI, de Graauw et al 1998, Whyborn et al 1998) on the ESA space observatory called Herschel, I worked on optics design (Belitsky & Torchinsky 1996, Torchinsky & Belitsky 1997), and I was responsible as the Canadian co-Investigator for HIFI for overseeing the development of the Local Oscillator Source Unit which was done by an industrial partner.

For the past several years, I have been responsible for the characterization and operation of the Square Kilometre Array (SKA) pathfinder called EMBRACE (Torchinsky et al. 2016a, 2015, 2013, Renaud et al. 2011, Bosse et al 2010, Torchinsky et al 2010, Olofsson et al. 2009, Wijnholds et al. 2009, Torchinsky et al. 2008b). EMBRACE, the Electronic MultiBeam Radio Astronomy ConcEpt, is a completely novel instrument for radio astronomy. There is no paraboloid dish to focus radiation onto a receiving element, and instead, a large area is “carpeted” with small antennas, all of which are combined together in a large and operationally flexible phased array system. EMBRACE is the first astronomically capable instrument using this densely packed phased-array technique. The Mid Frequency Aperture Array (MFAA) of the SKA will be nearly 10 000 times bigger than EMBRACE (Bij de Vaate et al. 2014a, 2014b, Faulkner et al. 2010, Bolton et al. 2009, Faulkner et al. 2008, Alexander et al. 2007).

As an astronomer with a technical background, I have often been involved in the definition of technical specifications which require an interpretation of the scientific requirements. The scientific goals have requirements in terms of sensitivity, velocity resolution, imaging fidelity and extent, or cosmological redshift range, to name a few examples. These have to be translated into technical requirements such as system noise temperature, spectral resolution, field of view, antenna profile, etc. This work requires a good understanding of the astrophysical goals of the observatory, as well as a good technical knowledge of the entire receiver signal chain (for example, for SKA, see Torchinsky et al. 2016b, Faulkner et al. 2010, Bolton et al. 2009). The responsibilities as Project Scientist also include preparing documents and making presentations for various audiences which could be scientific, technical, administrative, or the general public (for example, Torchinsky 2011a, Torchinsky 2009, Torchinsky et al. 2007a, Torchinsky & van Driel 2007, and Public Outreach activities).

Aside from SKA, I am also interested in the projects focusing on radio surveys of the sky in order to detect the signature of Baryon Acoustic Oscillations. There are a number of instruments proposed and under construction which are optimized for neutral hydrogen intensity mapping. At Nançay, a 4-dish interferometer called PAON4 serves as a prototype for testing electronics and data processing algorithms. This project feeds into the Chinese TIANLAI project which has an array of 3 cylinder antennas, and another array of 16 parabolic dishes. We are planning to run comparison tests between PAON4 and EMBRACE. I am also associated with the BINGO project led by the University of Manchester who have invited me to join their external advisory committee. EMBRACE has provided some Radio Frequency Interference measurements for BINGO. The SKA Aperture Array Mid Consortium is proposing a pathfinder instrument called MANTIS (The Mid-Frequency Aperture Array Transient and Intensity-Mapping System, van Cappellen et al 2016).

Gravitation and Cosmology

Perhaps the greatest mystery in physics today is related to the observed accelerated expansion of the Universe. This unexplained phenomenon is called “Dark Energy” and behaves as a negative pressure acting against gravity. It might be due to an exotic force not yet properly explained in modern theories of particle physics. It may also be simply explained by the “Cosmological Constant” in the General Theory of Relativity, or by alternative theories of gravity. Observational cosmology is a very powerful technique for learning more about the nature of Dark Energy, and in particular, observations of the distribution of matter in the Universe throughout its history will give us important information on the evolution of Dark Energy.

The early Universe, in a state of a hot dense plasma, was subject to perturbations resulting in pressure waves. These “baryon acoustic oscillations” (BAO) created a pattern of dense and less-dense volumes, which eventually stabilized when the Universe cooled enough. The signature of the BAO can be seen in the all-sky maps at microwave frequencies, such as has been done with the satellites COBE, WMAP, and finally Planck. The current standard model predicts that those early perturbations are the source of overdensities which eventually developed into the structure we observe today. It is expected that structure should follow the pattern of the BAO throughout the history of the Universe. Radio maps of the sky are sensitive to the ubiquitous neutral hydrogen (H_I) which has a spin-flip transition at 1420 MHz. By observing at frequencies at and below 1420 MHz, we effectively probe the Universe at different times because of the redshifted signal due to the expansion of the Universe. In this way we can make a map of the distribution of H_I gas at different epochs in the history of the Universe. This is one of the key science projects of the proposed Square Kilometre Array radio telescope.

The study of molecules at low frequency is motivated not only by a desire to further our understanding of the chemistry of star forming regions, but also to investigate the possibility that low frequency spectral lines can potentially “pollute” the H_I extragalactic survey at low to mid redshifts. For example the 834 MHz transition of methanol could be misidentified as H_I at z=0.7. Another possibility is the detection of OH megamasers which could be mistakenly identified as objects in nearby galaxies rather than objects at z>0.17. Currently, the map-cleaning strategies for the BAO surveys assume that the galactic contribution is purely continuum synchrotron, and that any spectral feature below 1420 MHz must be redshifted H_I. This may be erroneous as we know there are molecular transitions below 1420 MHz, as well as atomic Radio Recombination Lines, and to date, nine molecules have been detected at frequencies between 700 MHz and 1500 MHz. The SKA will bring us into a whole new regime of sensitivity, and many more molecules will be detected. We are looking into estimates of line strength and distribution of sources on the sky, both galactic and extragalactic, and eventually, an estimate of the effect of erroneously identified sources on the inferred BAO signal.