Year-timescale changes in AGN radio luminosity as seen by the ASKAP Variables and Slow Transients Survey

Abstract

A few dozen previously radio-quiet active galactic nuclei (AGN) have been observed to transition to radio-loud at 1-3 GHz frequencies over timescales of more than a decade, and this has has been interpreted to be due to newly launched jets. We identified 101 compact radio sources out of a sample of 64,972 non-blazar AGN which increased in flux density by 80-1800% over 1-6 years in the 887.5 MHz Australian SKA Pathfinder Variables and Slow Transients survey. We obtained optical spectra and radio SEDs using new observations and archival survey data. We determined 60 sources were consistent with extrinsic variability due to refractive interstellar scintillation and 41 were variable due to intrinsic causes, with 26 continuously brightening and two transitioning from radio-quiet to radio-loud. We concluded that young radio jets launched by either tidal disruption events or changes in the accretion properties were responsible for the continuously brightening AGN. These sources were non-variable at higher frequencies over the same time period, as expected for an expanding emission region. Fourteen sources had inverted or peaked SEDs initially which either flattened below the turnover or evolved into steep SEDs, consistent with young, expanding jets. Twelve sources had non-variable steep or gigahertz-peaked SEDs, which suggested these hosted more slowly evolving jets. We investigated the previously discovered AGN with newly launched jets, and found several have faded at multiple frequencies, which suggested the observed radio-loudness was temporary, rather than the onset of a sustained period of radio activity.