Pulsar Double-lensing Sheds Light on the Origin of Extreme Scattering Events

TL;DR

This paper presents a novel observation and modeling of a double-lensing event in a pulsar, revealing that elongated plasma lenses can cause extreme scattering events without requiring high overpressure, thus explaining their longevity.

Contribution

The study introduces a new phase-retrieval technique and a two-screen model to explain extreme scattering events, demonstrating that elongated plasma sheets can produce long-lived lenses.

Findings

Double-lensing event observed in pulsar PSR B0834+06.

Elongated plasma lenses can cause extreme scattering without high overpressure.

Lenses are slow-moving and highly elongated, consistent with edge-on plasma sheets.

Abstract

In extreme scattering events, the brightness of a compact radio source drops significantly, as light is refracted out of the line of sight by foreground plasma lenses. Despite recent efforts, the nature of these lenses has remained a puzzle, because any roughly round lens would be so highly overpressurized relative to the interstellar medium that it could only exist for about a year. This, combined with a lack of constraints on distances and velocities, has led to a plethora of theoretical models. We present observations of a dramatic double-lensing event in pulsar PSR~B0834+06 and use a novel phase-retrieval technique to show that the data can be reproduced remarkably well with a two-screen model: one screen with many small lenses and another with a single, strong one. We further show that the latter lens is so strong that it would inevitably cause extreme scattering events. Our observations show that the lens moves slowly and is highly elongated on the sky. If similarly elongated along the line of sight, as would arise naturally from a sheet of plasma viewed nearly edge-on, no large over-pressure is required and hence the lens could be long-lived. Our new technique opens up the possibility of probing interstellar plasma structures in detail, leading to understanding crucial for high-precision pulsar timing and the subsequent detection of gravitational waves.