Disentangling interstellar plasma screens with pulsar VLBI: combining auto- and cross-correlations

Abstract

Pulsar scintillation allows a glimpse into small-scale plasma structures in the interstellar medium, if we can infer their properties from the observed scintillation pattern. With Very Long Baseline Interferometry, and working in delay-delay rate space (after a Fourier transform of the dynamic spectra) where the contributions of pairs of images to the interference pattern become localized, the scattering geometry and distribution of scattered images on the sky can be determined if a single, highly-anisotropic scattering screen is responsible for the scintillation. However, many pulsars are subject to much more complex scattering environments where this method cannot be used. We present a novel technique to reconstruct the scattered flux of the pulsar and solve for the scattering geometry in these complex cases by combining interferometric visibilities with cross-correlations of single-station intensities. This takes advantage of the fact that, considering a single image pair in delay-delay rate space, the visibilities are sensitive to the sum of the image angular displacements, while the cross-correlated intensities are sensitive to the difference, so that their combination can be used to localize both images of the pair. We show that this technique is able to reconstruct the previously published scattering geometry of PSR B0834+06, then apply it to simulations of more complicated scattering systems, where we find that it can distinguish features from different scattering screens even when the presence of multiple screens is not obvious in the Fourier transform of the visibilities. This technique will allow us to both better understand the distribution of scattering within our local interstellar medium and to apply current scintillometry techniques, such as modelling scintillation and constraining the location of pulsar emission, to sources for which a current lack of understanding of the scattering environment precludes the use of these techniques.

Scintillation of PSR B1508+55 -- the view from a 10,000-km baseline

“…Simard et al (2019a) employ intensity dynamic spectra correlations to the data of Brisken et al (2010) to break the above degeneracy and proceed along similar lines to measure distances, velocities and perform astrometry. More importantly, Simard et al (2019b) combine the techniques of Brisken et al (2010) and Simard et al (2019a), both of which rely on simultaneous multi-station observations of scintillation, to disentangle multiple screens.…”

“…The primary obstacle in following an analysis similar to Brisken et al (2010) and Simard et al (2019a,b) is the lack of confidence in attributing a reliable curvature to the parabola with which the detected phase gradient is likely to align. Multiple baseline measurements would have allowed us to proceed along the lines of Simard et al (2019b), estimating the screen distance as well as the orientation 𝛼 𝑠 of the scattering axis.…”

Measuring Interstellar Delays of PSR J0613-0200 over 7 years, using the Large European Array for Pulsars

“…Such analysis would be greatly improved with precise, quantitative measurements of the arc curvature. Additionally, one could instantaneously measure the scattering screen's properties using the multiple telescopes of LEAP, either using the visibilities (Brisken et al 2010) or more simply using the inter-station time delays of the dynamic spectra (Simard et al 2019a, while the method of combining of visibilities and intensites is outlined in Simard et al 2019b). This is being investigated, and will be the subject of future work.…”

Plasma lensing in comparison to gravitational lensing -- Formalism and degeneracies

“…As observational evidence suggests, see e.g. Liu et al (2016) or Simard et al (2019), the approach may also be extended to thick lenses or multiple lens plane, which has already been studied in gravitational lensing as well and found to be highly degenerate Wagner (2018), Schneider (2019.…”