Dual-component plasma lens models

Abstract

In contrast to the converging, achromatic behaviour of axisymmetric gravitational lenses, diverging frequency-dependent lensing occurs from refraction due to a distribution of over-dense axisymmetric plasma along an observer's line of sight. Such plasma lenses are particularly interesting from the point of view of astronomical observations because they can both magnify and dim the appearance of background sources as a function of frequency. Plasma lensing is believed to be involved in a number of separate phenomena involving the scintillation of radio pulsars, extreme scattering events of background radio sources and may also play a role in the generation of fast radio bursts. These lensing phenomena are believed to occur in dense environments, in which there may be many density perturbations between an observer and background source. In this work we generalize individual plasma lens models to produce dual component lenses using families of plasma lens models previously studied in the literature, namely the exponential and softened power-law lenses. Similar to binary gravitational lens models, these dual component plasma lenses feature a rich and complex critical and caustic morphology, as well as generate more complicated light curves. We map the number of criticals formed for a given component separation and angular size, and highlight a relevant degeneracy between two particular models. This work provides an argument in favor of close monitoring of extreme scattering events in progress in order to break such model degeneracies.

Two families of elliptical plasma lenses

“…For lenses with h < 2 the magnification at the origin vanishes and the lens has an exclusion region. For lenses with h > 2, the magnification at the origin approaches unity, producing a W-shaped light curve, as opposed to a U-type light curve consistent with the result derived in Er & Rogers (2018) for symmetric exponential lenses and equivalent to Rogers & Er (2019) for a binary lens. We first show the Young diagram of exponential models for h = 1, 2, 3 in Fig.…”

“…3, there is a light green region at the origin, where the magnification is close to unity. It has been observed that such a spike can cause model degeneracy (Rogers & Er 2019). A corresponding magnification map on the source plane is shown in Fig.…”

“…For θc > θcrit, the power-law lens cannot form critical curve. The detail about the core size can be found in Rogers & Er (2019). The characteristic angular radius θ0 indicates the lens strength for the circularly symmetric lens.…”

“…In contrast, for plasma lenses, only simple symmetrical models for the electron density have been largely considered. Model degeneracy has been found between symmetric dual-component plasma lens distributions and an asymmetric model used to fit the ESE in PKS 1939-315 Rogers & Er 2019). The parametric model shows that this distribution is one of a larger family in which two lenses act on a source.…”

The Effects of Plasma Lensing on the Inferred Dispersion Measures of Fast Radiobursts

“…The strength of the lens can be characterized by θ 0 . The relationship between θ 0 and σ determines the number of caustic curves in the source plane that separate areas of different image multiplicity (Rogers & Er 2019). We note that θ 0 depends on the frequency of observation, the number density of electrons as well as the distance of the lens and the source.…”