Imaginary images and Stokes phenomena in the weak plasma lensing of coherent sources. (arXiv:2103.08687v2 [astro-ph.HE] UPDATED)

<a href="http://arxiv.org/find/astro-ph/1/au:+Jow_D/0/1/0/all/0/1">Dylan L. Jow</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lin_F/0/1/0/all/0/1">Fang Xi Lin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tyhurst_E/0/1/0/all/0/1">Emily Tyhurst</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pen_U/0/1/0/all/0/1">Ue-Li Pen</a>

The study of astrophysical plasma lensing, such as in the case of extreme

scattering events, has typically been conducted using the geometric limit of

optics, neglecting wave effects. However, for the lensing of coherent sources

such as pulsars and fast radio bursts (FRBs), wave effects can play an

important role. Asymptotic methods, such as the so-called Eikonal limit, also

known as the stationary phase approximation, have been used to include

first-order wave effects; however, these methods fail at Stokes lines. Stokes

lines are generic features of a variety of lens models, and are regions in

parameter space where imaginary images begin to contribute to the overall

intensity modulation of lensed sources. Using the mathematical framework of

Picard-Lefschetz theory to compute diffraction integrals, we argue that these

imaginary images contain as much information as their geometric counterparts,

and may potentially be observable in data. Thus, weak-lensing events where

these imaginary images are present can be as useful for inferring lens

parameters as strong-lensing events in which multiple geometric images are

present.

The study of astrophysical plasma lensing, such as in the case of extreme

scattering events, has typically been conducted using the geometric limit of

optics, neglecting wave effects. However, for the lensing of coherent sources

such as pulsars and fast radio bursts (FRBs), wave effects can play an

important role. Asymptotic methods, such as the so-called Eikonal limit, also

known as the stationary phase approximation, have been used to include

first-order wave effects; however, these methods fail at Stokes lines. Stokes

lines are generic features of a variety of lens models, and are regions in

parameter space where imaginary images begin to contribute to the overall

intensity modulation of lensed sources. Using the mathematical framework of

Picard-Lefschetz theory to compute diffraction integrals, we argue that these

imaginary images contain as much information as their geometric counterparts,

and may potentially be observable in data. Thus, weak-lensing events where

these imaginary images are present can be as useful for inferring lens

parameters as strong-lensing events in which multiple geometric images are

present.

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