Scintillometry of fast radio bursts

Fast radio bursts (FRBs) exhibit scintillation and scattering, which are often attributed to interactions with plasma screens in the Milky Way and the host galaxy. When these two screens appear “point-like” to each other, two scales of scintillation can be observed with sufficient frequency resolution. A screen perceives a second screen as extended or resolved when the angular size of the latter is smaller than the angular resolution of the former. The ratio of these two quantities is defined as the resolution power (RP). Previous observational studies have argued that, in the resolving regime, scintillations disappear, assuming that a screen resolving another screen is equivalent to a screen resolving an incoherent emission region. In this theoretical and simulation-based study of resolving effects in two-screen scenarios, we argue that resolving quenches only the relatively broad-scale scintillation and that this quenching is a gradual process. We present qualitative and quantitative predictions for dynamic spectra, spectral autocorrelation functions (ACFs), and modulation indices in resolved and unresolved regimes of two-screen systems. We show that the spectral ACFs of a two-screen system has a product term in addition to the sum of individual screen contributions, causing the total modulation index to rise to $ \sqrt{3} $ in the unresolved regime. To aid in discovering resolving systems, we also present observable trends in multi-frequency observations of a screen resolving another screen or incoherent emission. Additionally, we introduce a new formula to estimate the distance between the FRB and the screen in its host galaxy. We also show that this formula, as with previous ones in the literature, is only applicable to screens that are two-dimensional in the plane of the sky.