Pulsar Observations of Extreme Scattering Events

TL;DR

This paper reports pulsar observations of extreme scattering events (ESEs), revealing fully turbulent plasma lens structures on AU scales and providing insights into their size, turbulence, and possible extended nature along the line of sight.

Contribution

It presents detailed pulsar-based observations of ESEs, including models of plasma lens turbulence and structure, advancing understanding of their physical characteristics.

Findings

ESE structures are fully turbulent on AU scales.

Observations support the idea of extended scattering shells.

Dispersion measurements reveal AU scale electron density variations.

Abstract

Extreme scattering events (ESEs) in the interstellar medium (ISM) were first observed in regular flux measurements of compact extragalactic sources. They are characterized by a flux variation over a period of weeks, suggesting the passage of a "diverging plasma lens" across the line of sight. Modeling the refraction of such a lens indicates that the structure size must be of order AU and the electron density of order 10s of cm^{-3}. Similar structures have been observed in measurements of pulsar intensity scintillation and group delay. Here we report observations of two ESEs showing increases in both intensity scintillation and dispersion made with the Parkes Pulsar Timing Array (PPTA). These allow us to make more complete models of the ESE, including an estimate of the "outer-scale" of the turbulence in the plasma lens. These observations show clearly that the ESE structure is fully turbulent on an AU scale. They provide some support for the idea that the structures are extended along the line of sight, such as would be the case for a scattering shell. The dispersion measurements also show a variety of AU scale structures which would not be called ESEs, yet involve electron density variations typical of ESEs and likely have the same origin.