On the Cusp of Cusps: A Universal Model for Extreme Scattering Events in the ISM

The scattering structures in the ISM responsible for so-called “extremescattering events" (ESEs), observed in quasars and pulsars, remain enigmatic.Current models struggle to explain the high-frequency light curves of ESEs, anda recent analysis of a double lensing event in PSR B0834+06 reveals featuresof ESEs that may also be challenging to accommodate via existing models. Wepropose that these features arise naturally when the lens has a cusp-likeprofile, described by the elementary A_3 cusp catastrophe. This is anextension of previous work describing pulsar scintillation as arising fromA_2 fold catastrophes in thin, corrugated plasma sheets along the line ofsight. We call this framework of describing the lens potentials via elementarycatastrophes “doubly catastrophic lensing", as catastrophes (e.g. folds andcusps) have long been used to describe universal features in the light curvesof lensing events that generically manifest, regardless of the precise detailsof the lens. Here, we argue that the lenses themselves may be described bythese same elementary structures. If correct, the doubly catastrophic lensingframework would provide a unified description of scintillation and ESEs, wherethe lenses responsible for these scattering phenomena are universal and can befully described by a small number of unfolding parameters. This could enabletheir application as giant cosmic lenses for precision measurements of coherentsources, including FRBs and pulsars.