About Me

I'm a graduate student in the Department of Astronomy and Astrophysics at the University of Toronto, where I study pulsar scintillation and the structures in the interstellar medium that cause it with my advisor, Ue-Li Pen , and the rest of the University of Toronto Scintillometry Group. My work focusses on understanding the effects that interstellar plasma screens have on pulsar radiation as it travels towards us. This involves both delving into new ways to extract information on the interstellar screens from pulsar dynamic spectra, or the intensity pattern in time and frequency that we see at Earth due to the interference of the scattered pulsar radiation, and developing and testing predictive models of pulsar scintillation based on possible realizations of the interstellar screens.

I'm also involved in the revitalization of the Algonquin Radio Observatory 46-m radio telescope in Ontario, Canada. You can check out a public talk I gave about using ARO for global pulsar VLBI as part of Astronomy on Tap T.O.

My Research

Reconstructing scattering screens

In order to study pulsar scintillation, we first need to be able to reconstruct the scattered image of the pulsar. Pulsars themselves would appear as unresolved points on the sky if there were no intervening plasma structures in the way, and even when scattered the images are very small so that traditional imaging techniques are not sufficient. Part of my research is developing the techniques that can be used to reconstruct the image, even if the scattering environment is complicated, with multiple different scattering screens between us and the pulsar. To the left is a reconstructed image of PSR B0834+06 scattered by the interstellar medium that I created based on VLBI observations taken by Brisken et al. (2010).

Models of pulsar scintillation

As pulsar radiation travels towards us, it is scattered and lensed by plasma structures in the interstellar medium. Recently, structure has been noted in scintillation patterns that suggests highly anisotropic scattering at one or more screens along our line-of-sight to the pulsar. Along with my advisor, Ue-Li Pen, I developed a model of pulsar scintillation based on the theory that scintillation is due to grazing refraction lensing of the pulsar emission by plasma sheets in the interstellar medium. In this picture, a plasma screen with a slightly different index of refraction than the surrounding interstellar medium is very closely aligned with the line-of-sight to the pulsar, so that only small perturbations on the screen are needed to bend the pulsar light, as shown in the cartoon to the right. Models of this process make clear predictions about the evolution of the scattering with time and frequency. By comparing these predictions to observations of pulsars, we can start to narrow down the causes of pulsar scintillation. Because these models are predictive, they may also be integral in removing the effects of scattering plasma screens in low-frequency observations of pulsars. You can check out a talk I gave on this research here.

The scintillation of PSR B0834+06

In the fall of 2016, we undertook a global VLBI campaign in order to track the evolution of the scintillation pattern of PSR B0834+06 over 6 weeks. The image on the left shows the secondary spectrum (the power spectrum of the observed interference pattern) over the observation, constructed from the data recorded at Arecibo. We see individual reverse arclets appear in the left hand side of the secondary spectrum and then traverse through the spectrum. This indicates that small, compact regions of the screen are moving in front of the pulsar. The motion of these can be compared to expectations from models, including from the plasma sheet model. We also see a persistent, diffuse parabolic arc of power, which suggests that compact regions aren't the full story of how the plasma sheet scatters the pulsar emission.

Contact Me

  • Address

    MP1318
    McLennan Physics Building
  • Mailing Address

    Canadian Institute for Theoretical Astrophysics
    60 Saint George St., 14th floor
    Toronto, ON, M5S 3H8
    Canada
  • Email

    simard@astro.utoronto.ca