AGN in Gaia Alerts: from flares to Changing Look Quasars

TL;DR

This paper reports on Gaia Alert detections of variable AGN and quasars, confirming 64 new candidates, analyzing over 200 known AGN, and revealing phenomena like flares, TDEs, and Changing Look Quasars, highlighting variability as a key detection tool.

Contribution

It provides the first spectroscopic confirmation of Gaia-identified variable AGN and quasars, and characterizes the diverse phenomena causing their variability, including a significant fraction of Changing Look Quasars.

Findings

Confirmed 64 new Gaia AGN/quasar candidates.

Identified 56 secure and 23 probable Changing Look Quasars.

Observed diverse phenomena including flares, TDEs, and microlensing events.

Abstract

The Gaia Alerts system is providing alerts on a variety of objects displaying a significant photometric change detected by the Gaia satellite from one passage to the next one over the same region of the sky. Among the over 22000 alerts published until the end of 2022, around 13 percent concern AGN or quasar candidates. We have embarked on a spectroscopic ground-based follow-up of some of those (including some selected by a different method specifically in galactic nuclei), to establish their true nature, and reveal the various phenomena leading to a change in magnitude of those AGN. The present paper deals with radio-quiet objects, while the radio-loud ones will be described in a companion paper. We confirm, on one hand, the AGN/quasar nature of 64 new candidates alerted by Gaia, and, on the other hand, obtained second-epoch spectra of over 200 known AGN also alerted for large photometric variations. The observed phenomena show a large variety: from Flares to Tidal Disruption Events (TDEs) and a large number of Changing Look Quasars (CLQs, 56 secure ones, plus 23 probable ones), not forgetting some rarer events like SNe, microlensing events or Extreme Coronal Line Emitters. This sample shows that variability is an excellent tool to detect new quasars, especially radio-quiet ones that otherwise would be undetected, and that a significant fraction of variable AGN/quasars, around 10 percent, presents the CLQ phenomenon. Some of the new CLQs are followed-up to monitor further changes and measure time scales.