VLASS-based survey of transition state galaxies and their relationship to compact peaked-spectrum radio sources

TL;DR

This study investigates 24 radio transient sources with convex spectra, suggesting they are young AGN-related objects that may evolve into radio-quiet quasars or low-frequency peaked-spectrum sources, with transient emission likely caused by accretion changes.

Contribution

The paper provides a multi-frequency, high-resolution analysis of radio transients, linking their properties to AGN evolution and proposing a possible origin involving accretion variability and low-power ejecta.

Findings

Sources are young GPS-like AGNs with potential to evolve into RQ quasars.

Transient emission likely caused by accretion rate changes and low-power ejecta.

Some sources may be tidal disruption events (TDEs).

Abstract

We present multi-frequency and high-resolution studies of a sample of 24 radio transients sources discovered by comparing the NRAO VLA Sky Survey (NVSS) and Very Large Array Sky Survey (VLASS) surveys. All of them are characterized by a significant increase in radio flux density over the last two decades.Their convex spectra, small sizes and high brightness temperatures are typical for young gigahertz-peaked spectrum (GPS) radio sources and indicative of an AGN buried in the host galaxy. On the other hand, they are much weaker than the archetypical GPS objects and their parsec-scale radio structures, although indicating the presence of young radio jets, are similar to radio-quiet AGNs like Seyfert and low-ionization nuclear emission-line region (LINER) galaxies. Based on the distribution of these objects in power$-$size ($P - D$) and peak frequency$-$size ($\nu_p - D$) diagrams, we suggest that after stabilizing their radio activity, some of the GHz-peaked radio transients (galaxies and quasars) will develop into radio-intermediate and radio-quiet (RI/RQ) quasars and low-frequency peaked-spectrum (PS) objects. We discuss several possible origins for the transient radio emission in our sources and conclude that changes in the accretion rate combined with low-power radio ejecta are the most probable cause. This is the scenario we also propose for one of our sources, 101841$-$13, which was independently identified as a candidate tidal disruption event (TDE) based on its infrared variability. However, we cannot exclude that 101841$-$13 or other sources in our sample are TDEs.