Plasma microlensing dynamic spectrum probing fine structures in the ionized interstellar medium

Abstract

Gravitational microlensing has become a mature technique for discovering small gravitational lenses in the Universe which are otherwise beyond our detection limits. Similarly, plasma microlensing can help us explore cosmic plasma lenses. Both pulsar scintillation and extreme scattering events of compact radio sources suggest the existence of $\sim$ AU scale plasma lenses in the ionized interstellar medium (IISM), whose astrophysical correspondence remains a mystery. We demonstrate that plasma microlensing events by these plasma lenses recorded in the form of wide-band dynamic spectra are a powerful probe of their nature. Using the recently developed Picard-Lefschetz integrator for the Kirchhoff-Fresnel integral, we simulate such dynamic spectra for a well-motivated family of single-variable plasma lenses. We demonstrate that the size, strength, and shape of the plasma lens can be measured from the location of the cusp point and the shape of spectral caustics respectively, with a combination of distances and the effective velocity known a priori or measured from the widths of the interference pattern. Future wide-band observations of pulsars, whose plasma microlensing events may be predictable from parabolic arc monitoring, are the most promising ground to apply our results for a deeper insight into the fine structures in the IISM.