ASKAP observations of multiple rapid scintillators reveal a degrees-long plasma filament

TL;DR

This study used ASKAP to discover multiple rapid radio scintillators, revealing a large plasma filament and providing new insights into the scattering medium causing intra-hour variability.

Contribution

First detection of multiple scintillators behind the same plasma screen, revealing a large-scale filament and proposing a new model involving a cold tidal stream.

Findings

Discovered six rapid scintillators with extreme intra-hour variability.

Identified a large plasma filament approximately 2 degrees long and 4 parsecs away.

Estimated the plasma filament's physical properties and proposed a new scattering model.

Abstract

We present the results from an Australian Square Kilometre Array Pathfinder search for radio variables on timescales of hours. We conducted an untargeted search over a 30 deg$^2$ field, with multiple 10-hour observations separated by days to months, at a central frequency of 945 MHz. We discovered six rapid scintillators from 15-minute model-subtracted images with sensitivity of $\sim 200\,\mu$Jy/beam; two of them are extreme intra-hour variables with modulation indices up to $\sim 40\%$ and timescales as short as tens of minutes. Five of the variables are in a linear arrangement on the sky with angular width $\sim 1$ arcmin and length $\sim 2$ degrees, revealing the existence of a huge plasma filament in front of them. We derived kinematic models of this plasma from the annual modulation of the scintillation rate of our sources, and we estimated its likely physical properties: a distance of $\sim 4$ pc and length of $\sim 0.1$ pc. The characteristics we observe for the scattering screen are incompatible with published suggestions for the origin of intra-hour variability leading us to propose a new picture in which the underlying phenomenon is a cold tidal stream. This is the first time that multiple scintillators have been detected behind the same plasma screen, giving direct insight into the geometry of the scattering medium responsible for enhanced scintillation.