Identifying tidal disruption events among radio transient galaxies

Abstract

We present the optical and infrared properties of a sample of 24 radio transient sources discovered in the Very Large Array Sky Survey (VLASS). Previous studies of their radio emission showed that these sources resemble young gigahertz-peaked spectrum (GPS) radio sources, but are less powerful and characterized by low-power jets. The bursts of radio activity in most cases are likely due to intrinsic changes in the accretion processes. However, for a few sources in this sample, we cannot rule out the possibility that their radio variability results from a tidal disruption event (TDE). In this work, we extend our analysis to the optical and infrared regimes, confirming that our sample of radio transients is not homogeneous in terms of their optical and infrared properties either. The host galaxies of most of these sources are massive ellipticals with emission dominated by active galactic nuclei (AGN). They host supermassive black holes (SMBHs) with masses typical of radio-loud AGNs ($\rm >10^7\,M_{\odot}$), but exhibit very low accretion activity. In contrast, the sources for which a TDE origin is suspected are either pure star-forming galaxies or show significant ongoing star formation, similar to radio-selected optically-detected TDEs. Additionally, two of them exhibit infrared flares characteristic of TDEs, while the remaining sources do not display significant variability outside the radio regime. Moreover, the evolution of their radio brightness in the W3/radio diagnostic diagram, which we employ in our analysis, also sets our TDE candidates apart from the rest of the sample and resembles the radio variability seen in optically discovered TDEs with radio emission. Finally, based on our findings, we hypothesize that the mid-IR/radio relation can serve as a tool to distinguish between radio transients caused by TDEs and those originating from intrinsic AGN variability.