Changing-Look AGNs or Short-Lived Radio Sources?

Aleksandra Wołowska,Magdalena Kunert-Bajraszewska,Kunal Mooley,Gregg Hallinan

doi:10.3389/fspas.2017.00038

Abstract

The evolution of extragalactic radio sources has been a fundamental problem in the study of active galactic nuclei for many years. A standard evolutionary model has been created based on observations of a wide range of radio sources. In the general scenario of the evolution, the younger and smaller Gigahertz-Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) sources become large-scale FRI and FRII objects. However, a growing number of observations of low power radio sources suggests that the model cannot explain all their properties and there are still some aspects of the evolutionary path that remain unclear. There are indications, that some sources may be short-lived objects on timescales of $10^4$ - $10^5$ years. Those objects represent a new population of active galaxies. Here, we present the discovery of several radio transient sources on timescales of 5-20 years, largely associated with renewed AGN (Active Galactic Nucleus) activity. These changing-look AGNs possibly represent behaviour typical for many active galaxies.

Highlights

The radio emission of extragalactic sources is usually explained by the existence of strong jets emitting non-thermal synchrotron radiation
This indicates the temporary existence of many weak CSS and GPS sources with compact or slightly resolved radio morphologies similar to that observed in radio-quiet objects (Giroletti and Panessa, 2009; Sadler et al, 2014)
Studies of interactions between the young radio source and interstellar medium can provide information about the energy that is deposited into the ISM by the expanding radio source

Summary

INTRODUCTION

The radio emission of extragalactic sources is usually explained by the existence of strong jets emitting non-thermal synchrotron radiation. This indicates the temporary existence of many weak CSS and GPS sources with compact or slightly resolved radio morphologies (core-jet) similar to that observed in radio-quiet objects (Giroletti and Panessa, 2009; Sadler et al, 2014). They are called short-lived radio objects on timescales 104–105 years. The multi-year Caltech-NRAO Stripe 82 Survey (CNSS), revealed twelve transient sources on timescales of 5–20 yrs, largely associated with renewed AGN activity The rates of such AGN possibly imply episodes of enhanced accretion and jet activity occurring once every 40,000 years in these galaxies. The results from an initial pilot survey of a ∼50 deg of Stripe 82 has been already published by Mooley et al (2016)

NEW RADIO TRANSIENTS

SUMMARY