X-ray Properties of Optically Variable Low-mass AGN Candidates

TL;DR

This study investigates the X-ray properties of optically variable low-mass AGN candidates, confirming AGN activity in several cases and highlighting the effectiveness of variability-based selection methods in low-mass galaxies.

Contribution

It provides the first X-ray analysis of optically variable low-mass AGN candidates, demonstrating the success of variability in identifying AGN in low-mass galaxies.

Findings

Four AGN candidates show nuclear X-ray emission.

X-ray luminosities exceed those from X-ray binaries.

Variability-based selection effectively identifies low-mass AGN.

Abstract

We present an X-ray analysis of fourteen nearby (z < 0.044) AGN in low mass galaxies (M_* <= 5*10^9 Msun) selected based on their optical variability (Baldassare et al. 2020). Comparing and contrasting different AGN selection techniques in low-mass galaxies is essential for obtaining an accurate estimate of the active fraction in this regime. We use both new and archival observations from the Chandra X-ray Observatory to search for X-ray point sources consistent with AGN. Four objects have detected nuclear X-ray emission with luminosities ranging from L_0.5-7 ~ 3*10^40 to 9*10^42 erg s^-1 with two more marginal detections. All of the detected galaxies have luminosities exceeding those anticipated from X-ray binaries, and all sources are nuclear, suggesting the X-ray emission in most sources is due to an AGN. These observations demonstrate the success of variability at identifying AGN in low-mass galaxies. We also explore emission line diagnostics and discuss the differences in the results of these methods for AGN selection, in particular regarding low-mass and low-metallicity systems.