The Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey II: The First Four Epochs

TL;DR

This study analyzes radio source variability caused by interstellar scintillation, revealing correlations with the ionized medium and effects of redshift, providing insights into source compactness and cosmological implications.

Contribution

First large-scale multi-epoch survey linking radio variability to interstellar medium properties and redshift effects on source compactness.

Findings

Over half of sources showed 2-10% rms variability.

Variability correlates with ionized medium emission measure.

Variability decreases with higher redshift sources.

Abstract

We report on the variability of 443 flat spectrum, compact radio sources monitored using the VLA for 3 days in 4 epochs at ~ 4 month intervals at 5 GHz as part of the Micro-Arcsecond Scintillation-Induced Variability (MASIV) survey. Over half of these sources exhibited 2-10% rms variations on timescales over 2 days. We analyzed the variations by two independent methods, and find that the rms variability amplitudes of the sources correlate with the emission measure in the ionized Interstellar Medium along their respective lines of sight. We thus link the variations with interstellar scintillation of components of these sources, with some (unknown) fraction of the total flux density contained within a compact region of angular diameter in the range 10-50 micro-arcseconds. We also find that the variations decrease for high mean flux density sources and, most importantly, for high redshift sources. The decrease in variability is probably due either to an increase in the apparent diameter of the source, or a decrease in the flux density of the compact fraction beyond z ~ 2. Here we present a statistical analysis of these results, and a future paper will the discuss the cosmological implications in detail.