The authors investigate the refractive lensing effects of ionized, cool (T ∼104 K) gas cloudlets in the circumgalactic medium (CGM) of galaxies. They show that if the CGM comprises a mist of sub-parsec cloudlets with column densities of order 1017 cm−2, as predicted by previous models, then FRBs whose sightlines pass within a virial radius of a CGM halo will be lensed into tens of refractive images with a ∼10 ms scattering timescale.
When these lensed images are formed, they will be resolved by scintillating screens in the Milky Way interstellar medium (ISM), suppressing the observed scintillation. The authors argue that positive detections of FRB scintillation may constrain the properties of these cool-gas cloudlets, with current scintillation observations weakly disfavoring the cloudlet model. They propose that sheet-like geometries for the cool gas in the CGM can reconcile quasar absorption measurements and the lack of lensing signals observed thus far.
The authors first provide a quantitative analysis of how an additional lens that resolves the scintillation screen can suppress the observed scintillation. They then apply this framework to model the refractive lensing properties of the CGM cloudlets, deriving scaling relations for the number of lensed images and the associated scattering timescales. Based on these results, the authors argue that if the CGM is ubiquitously populated by sub-parsec cloudlets, any FRB with a sightline passing within a galactic virial radius will not exhibit significant scintillation, or will do so very weakly.
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by Dylan L. Jow... at arxiv.org 10-01-2024
https://arxiv.org/pdf/2309.07256.pdfDeeper Inquiries