DRAO Seminar Series Fall 2015

Spatial Interference Mitigation for Radio Astronomy

Gregory Hellbourg (CSIRO Astronomy & Space Science)

Radio Frequency Interference (RFI) constitutes a serious threat for radio astronomy due to an increasing spectrum occupancy of human origin. The protection of some dedicated frequency bandwidths is not sufficient for high red-shifted or continuum cosmic sources studies. Similarly, radio quiet zones do not provide enough protection against moving targets such as cars, airplanes or satellites.

Although research on RFI mitigation techniques has been very active over the last decades, astronomers usually prefer the option of flagging and blanking corrupted time-frequency data. Despite its obvious efficiency, this approach can lead to a significant loss of data and is time consuming when applied manually. Moreover, it is incompetent in processing continuous interferers.

RFI mitigation applied in the spatial domain theoretically allows the recovery of uncorrupted time-frequency data – a feature of high interest for astronomers. Deep nulling techniques consist of designing the main beam of the instrument while rejecting all the power coming from the direction of the source of RFI. In this talk, I will present several approaches for performing spatial RFI mitigation, and discuss the main limitations of these techniques. A specific focus will be placed on real data processing.

Using Radio Polarimetry To Study the Interstellar Medium

Xiaohui Sun (Sydney)

Polarized radio emission provides information on both the magnetic field at locations where the emission is generated and the magnetic field that pervades the ionized interstellar medium through which the emission then propagates. Therefore, radio polarization observations provide a unique probe of the Galactic magnetic field coupled with both thermal and relativistic electrons. In this talk, I will demonstrate how we can reveal 3D structures of interstellar medium with polarization images and how we can study magnetism with Faraday rotation measures. I will then show that the GMIMS survey and other forthcoming surveys such as POSSUM will advance our understanding of the Galaxy.

New Frontiers in Solar Radio Astronomy

Stephen White (US Air Force Research Lab)

Solar radio astronomy is undergoing a period of rapid development, with new telescopes enabling a range of studies that were not previously possible. This talk will discuss these developments and focus on science topics including space weather, the magnetic field in the solar atmosphere, solar science with ALMA, and the relationship between the 10.7 cm solar radio flux and solar EUV emission.

Extragalactic Circuits, Black Holes, and the Ultimate Energy Transfers

Phil Kronberg (Toronto/LANL)

A non-negligible fraction of a Supermassive Black Hole’s (SMBH) rest mass energy gets transported into extragalactic space—by remarkable processes in jets which are not completely understood. The bulk of the energy flow from the SMBH (e.g. in excess of 10 million solar masses) appears to be electromagnetic, rather than via a particle beam flux. Also, remarkably, these jets contain current flows that remain largely intact over multi-kiloparsec distances. Accretion disk models have independently calculated that a 100 million solar mass SMBH should generate of order 1-10 quintillion (1E18-1E19) ampères in the vicinity of the SMBH.

I describe the first and best yet observational estimate of the current flow along the axis of a jet that extends from the nucleus of the active elliptical galaxy in 3C303. This is a current of roughly I = 1E18 ampères at a projected 40 kpc from the AGN. This points to the existence of cosmic scale electric circuits. The power flow is P = (I^2)Z watts, where the impedance Z is roughly 30 ohms, which is of order the impedance of free space Z(epsilon_0 ,mu_0), where (epsilon_0, mu_0) are the permittivity and magnetic permeability. These, in turn, uniquely determine the speed of light c. The electric potential drop (~ 1E20 volts) across the jet diameter (which is roughly a few times the gravitational radius of the SMBH) is, interestingly roughly equal to that required to accelerate the Ultra High Energy Cosmic Rays (UHECR).

The Social Network Outburst: A Multiwavelength Perspective on the 2015 V404 Cygni Outburst

Greg Sivakoff (Alberta)

Accretion disks and jets are ubiquitous astrophysical phenomena. Given the potential feedback between supermassive black holes and galaxy evolution, understanding the physics of accretion discs and relativistic radio jets around Active Galactic Nuclei (AGN) has gained increased motivation; however, the outbursts of AGN likely last millions of years. Since the outbursts of black hole X-ray binaries, stellar mass cousins of AGN, typically lasts weeks to months, they are ideal targets for probing the physics of accretion disks and jets. On 2015 June 15, the nearby black hole X-ray binary V404 Cyg went into outburst. Over the next ~50 days, professional astronomers and citizen scientists undertook the most extensive and coordinated set of multiwavelength observations of a black hole X-ray binary outburst to date. This was enabled not only by “classical tools” like GCNs, ATels, and emails between small sets of astronomers, but also by social networks and their ethos, leading to an extensive mailing list and interactive web site for communicating about observing V404 Cyg. In this talk, I present some early fruit from this labor, including a multiwavelength perspective (which is still evolving) of the entire outburst and specific epochs of strong (nearly-)simultaneous multiwavelength coverage.

The Thirty Meter Telescope: Science, Technology and People

Luc Simard (DAO)

The Thirty-Meter Telescope International Observatory (TIO) will enable transformational observations over the full cosmic timeline all the way from the first luminous objects in the Universe to the planets and moons of our own Solar System. To realize its full scientific potential, TMT will rely on cutting-edge technologies in structures, optics and control systems. It will also be equipped with a powerful suite of adaptive optics systems and science instruments to maximize its observing power. Members of the TMT partnership include the California Institute of Technology, the University of California, Canada, China, Japan and India. Such a unique partnership fosters new and exciting collaborations that will last for decades to come.

Modelling Molecular Cloud Cores and Turbulent Magnetic Fields Using Submillimetre Polarization

Jason Fiege (Manitoba)

Magnetic fields are believed to play an important role in star formation, and they can be mapped using submillimetre polarimetry. In this talk, I focus on recent progress in modelling these fields. I introduce a software package, called “PolCat”, for modelling the continuum and submillimetre polarization due to magnetized molecular cloud cores. PolCat builds a three-dimensional core model by using a sequence of carefully constructed, parametrized coordinate transformations to transform the density and the field. We generate polarization maps from the model, allowing us to fit the transformation parameters to observational datasets. We utilize the Ferret evolutionary algorithm to search for the transformation and dust parameters that minimize chi-squared for both the intensity and polarization position angle maps. The code automatically discovers the trade-offs between these two objectives to find the families of models that balance both. We have applied PolCat to multiple datasets from the SCUPOL Legacy Catalogue resulting in a variety of candidate field geometries with polarization patterns in good agreement with the data. PolCat models are not in magnetostatic equilibrium, in general. However, a recent offshoot of PolCat builds equilibrium magnetohydrostatic equilibrium models, which we generate using a generalized version of the self-consistent field method that has been optimized for computational efficiency. The Chandrasekhar-Fermi (CF) method has often been used to estimate magnetic field strengths. The underlying idea is that strong magnetic fields are more rigid than weaker fields, and therefore less perturbed by passing Alfven waves; thus, a measurement of the polarization dispersion can provide an estimate of the field strength. I discuss new calculations that apply the CF method to a turbulent spectrum of Alfven waves that mimic Kolmogorov turbulence. These calculations improve estimates of field strength and their errors. Work is underway to superimpose turbulent fields on PolCat equilibrium and non-equilibrium models to provide additional constraints on the strength of the magnetic field.

Magnetic Field in and around an Intermediate-Velocity Plasma Filament in the Nearby Interstellar Medium

Jeroen Stil (Calgary)

Some fraction—if not most—of the intermediate-velocity (IV) gas in the Milky Way is associated with recent injection of bulk kinetic energy into the interstellar medium, for example by stellar winds or recent supernova explosions. Some of this energy is eventually converted into magnetic field energy through a dynamo process that is still poorly understood. The magnetic field structure in and around IV gas is therefore directly relevant for the small-scale physics of the Galactic dynamo. In this presentation I discuss a nearby extended ionized IV filament and I present a critical look at what we can learn about the astrophysical importance of the magnetic field from measurements of Faraday rotation. New data from the GALFACTS survey allows us to combine Faraday rotation and polarization of diffuse synchrotron emission associated with the ionized filament as recently reported by the Planck foreground collaboration. Although quantitative analysis of the GALFACTS data is just beginning, it reveals a wealth of intriguing detail relating to the magnetic field in and around this nearby intermediate-velocity gas