Plasma lensing modeling of substructures on pulsar scintillation screens

TL;DR

This paper models plasma lensing effects on pulsar scintillation screens to constrain substructure properties, emphasizing the importance of multiepoch and ultrawideband observations for understanding interstellar plasma structures.

Contribution

It introduces a plasma lensing framework with three models to identify observable features constraining substructure properties on scintillation screens.

Findings

Outer caustic location constrains maximum column density gradient.

Inner caustic indicates substructure size, observable in extreme-scattering events.

Brightness of arclets varies concavely with pulsar-lens separation, aiding analysis.

Abstract

Radio pulsars, as highly coherent point sources, serve as powerful probes of the ionized interstellar medium (IISM). Pulsar scintillation observations have revealed inverted arclets on the secondary spectrum, indicating quasilinearly aligned images created by substructures on a scintillation screen. The density profiles of these substructures remain unconstrained but are crucial to identifying their physical nature. This work employs a plasma lensing framework to study observable features of substructure phase screens. Using three lens models, we identify the substructure properties that can be constrained by observables. The outer caustic is the most prominent feature of a lensing substructure, measurable via multiepoch or ultrawideband observations. Its location constrains the maximum column density gradient of the substructure. The inner caustic, though difficult to observe except for substructures capable of producing extreme-scattering events, directly indicates the substructure size. Even when caustic locations are not observed, the minimum span where substructure images exist can be measured and used to place a lower limit on the column density amplitude. The logarithmic brightness of individual arclets forms a concave function of the pulsar-lens angular separation, contrasting with the convex brightness distribution of all substructure images--highlighting the complementarity of individual arclets to statistical studies. These findings reveal the potential of pulsar scintillation to uncover IISM substructure and underscore the need for multiepoch and/or ultrawideband measurements to constrain discrete lensing morphologies and help reveal the nature of interstellar plasma structures.