Multiple imaging of the quasar 2005+403 formed by anisotropic scattering

TL;DR

This paper reports the rare observation of multiple imaging of the quasar 2005+403 caused by anisotropic plasma scattering, supported by multi-frequency VLBA data and modeling of an extreme scattering event.

Contribution

It provides the first detailed analysis of multiple imaging caused by anisotropic scattering in a low Galactic latitude quasar, including modeling of a plasma lens and its properties.

Findings

Multiple images are caused by plasma scattering with a wavelength squared dependence.

An extreme scattering event was identified and modeled, revealing properties of the plasma lens.

The scattering screen is located in the Cygnus region, with a size of 0.4 mas and a transverse speed of 37 km/s.

Abstract

We report on the low Galactic latitude ($b=4.3^\circ$) quasar 2005$+$403, the second active galactic nuclei, in which we detected a rare phenomenon of multiple imaging induced by refractive-dominated scattering. The manifestation of this propagation effect is revealed at different frequencies ($\lesssim8$ GHz) and epochs of VLBA observations. The pattern formed by anisotropic scattering is stretched out along the line of constant Galactic latitude with a local $\mathrm{PA}\approx40^\circ$ showing one-two sub-images, often on either side of the core. Analysing the multi-frequency VLBA data ranging from 1.4 to 43.2 GHz, we found that both the angular size of the apparent core component and the separation between the primary and secondary core images follow a wavelength squared dependence, providing convincing evidence for a plasma scattering origin for the multiple imaging. Based on the OVRO long-term monitoring data at 15 GHz obtained for 2005$+$403, we identified the characteristic flux density excursions occurred in April-May 2019 and attributed to an extreme scattering event (ESE) associated with the passage of a plasma lens across the line of sight. Modeling the ESE, we determined that the angular size of the screen is 0.4 mas and it drifts with the proper motion of 4.4 mas yr$^{-1}$. Assuming that the scattering screen is located in the highly turbulent Cygnus region, the transverse linear size and speed of the lens with respect to the observer are 0.7 AU and 37 km s$^{-1}$, respectively.