X-ray Analysis of AGN from the GALEX Time Domain Survey

TL;DR

This study investigates the X-ray properties of ultraviolet-variable AGN candidates, revealing diverse optical signatures and highlighting the effectiveness of variability-based selection in identifying AGN regardless of host galaxy obscuration or stellar emission.

Contribution

It provides the first X-ray analysis of UV-variable AGN candidates from the GALEX survey, demonstrating the diversity of their optical properties and the limitations of traditional spectroscopic identification.

Findings

11 out of 23 nuclei are X-ray detected.

X-ray luminosities exceed those expected from star formation by at least an order of magnitude.

Some objects fall outside typical quasar luminosity relations.

Abstract

We analyze the X-ray properties for a sample of 23 high probability AGN candidates with ultraviolet variability identified in Wasleske et al. (2022). Using data from the Chandra X-ray Observatory and the XMM-Newton Observatory, we find 11/23 nuclei are X-ray detected. We use SED modeling to compute star formation rates and show that the X-ray luminosities are typically in excess of the X-ray emission expected from star formation by at least an order of magnitude. Interestingly, this sample shows a diversity of optical spectroscopic properties. We explore possible reasons for why some objects lack optical spectroscopic signatures of black hole activity while still being UV variable and X-ray bright. We find that host galaxy stellar emission and obscuration from gas and dust are all potential factors. We study where this sample falls on relationships such as $\alpha_{\rm OX}-L_{2500}$ and $L_{X}-L_{IR}$ and find that some of the sample falls outside the typical scatter for these relations, indicating they differ from the standard quasar population. With the diversity of optical spectroscopic signatures and varying impacts of dust and stellar emissions on our sample, these results emphasizes the strength of variability in selecting the most complete set of AGN, regardless of other host galaxy properties.