How common are extreme scattering events?: First large sample from the ASKAP VAST survey

Compact radio sources are occasionally observed to undergo extreme scattering events (ESEs): large, chromatic flux-density variations, often by more than a factor of two, over a few months. These were interpreted to be a smooth plasma lensing phenomenon due to AU-scale structures in the ISM. Although ESEs were discovered decades ago, observationally, only a few tens of ESEs have been reported, often from individual sources monitored at very high cadences. As such, the nature of the plasma lenses generating such events remains elusive. In this article, we present the most comprehensive sample of ESEs to date, using the ASKAP Variables and Slow Transients (VAST) survey. We report the discovery of a three-tier sample --- 72 sources that show promising ESE events (gold), 76 sources that are likely ESE-exhibiting (silver), and an additional 88 sources where ESE events are one plausible explanation (bronze). We show that the light curves demonstrate varied morphologies, which can be interpreted in terms of the source size and the strength of the lens in the paradigm of a simple one-dimensional Gaussian lens model. The spatial distribution of ESEs shows an appreciable excess in the direction of the Galactic center and along the Galactic loops. Using this sample, we infer an overall rate of 0.012 such events per year per deg^2 (or 0.006 events per source per year) for our (gold+silver) sample, for sources with mean flux density above 5 mJy at 888 MHz (0.01% of all isolated point sources detected in VAST).