# Spica and the annual cycle of PKS B1322-110 scintillations

## TL;DR

This study tracks the scintillation of the quasar PKS B1322-110 over a year, revealing an annual cycle consistent with plasma microstructure scattering related to the nearby star Spica, and constrains the plasma's kinematic properties.

## Contribution

It provides the first detailed model fitting of plasma microstructure scattering near Spica, linking scintillation patterns to anisotropic plasma filaments around the star.

## Key findings

- Detected a strong annual cycle in scintillation rates.
- Constrained plasma velocity components consistent with filamentary scattering model.
- Estimated the plasma microstructure orientation and kinematics near Spica.

## Abstract

PKS B1322-110 is a radio quasar that is located only 8.5' in angular separation from the bright B star Spica. It exhibits intra-day variability in its flux density at GHz frequencies attributed to scintillations from plasma inhomogeneities. We have tracked the rate of scintillation of this source for over a year with the Australia Telescope Compact Array, recording a strong annual cycle that includes a near-standstill in August and another in December. The cycle is consistent with scattering by highly anisotropic plasma microstructure, and we fit our data to that model in order to determine the kinematic parameters of the plasma. Because of the low ecliptic latitude of PKS B1322-110, the orientation of the plasma microstructure is poorly constrained. Nonetheless at each possible orientation our data single out a narrow range of the corresponding velocity component, leading to a one-dimensional constraint in a two-dimensional parameter space. The constrained region is consistent with a published model in which the scattering material is associated with Spica and consists of filaments that are radially oriented around the star. This result has a 1% probability of arising by chance.