Refractive lensing of scintillating FRBs by sub-parsec cloudlets in the multi-phase CGM

Abstract

We consider the refractive lensing effects of ionized cool ($T \sim 10^4\,{\rm K}$) gas cloudlets in the circumgalactic medium (CGM) of galaxies. In particular, we discuss the combined effects of lensing from these cloudlets and scintillation from plasma screens in the Milky Way interstellar medium (ISM). We show that, if the CGM comprises a mist of sub-parsec cloudlets with column densities of order $10^{17}\,{\rm cm}^{-2}$ (as predicted by McCourt et al. 2018), then FRBs whose sightlines pass within a virial radius of a CGM halo will may be lensed into tens of refractive images with a $\sim 10\,{\rm ms}$ scattering timescale. When these images are formed, they will be resolved by scintillating screens in the Milky Way ISM, and will suppress the observed scintillation. We illustrate this effect in refractive lensing and argue that positive detections of FRB scintillation may constrain the properties of these cool-gas cloudlets, with current scintillation observation weakly disfavouring the cloudlet model. We propose that sheet-like geometries for the cool gas in the CGM can reconcile quasar absorption measurements (from which we infer the presence of the cool gas with structure on sub-parsec scales) and the unexpected lack of lensing signals from this gas thus far observed.