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

Abstract

We present an X-ray analysis of 14 nearby (z < 0.044) active galactic nuclei (AGNs) in low-mass galaxies (M * ≲ 5 × 109 M ⊙) selected based on their optical variability. 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 AGNs. Four objects have detected nuclear X-ray emission with luminosities ranging from L 0.5−7 ≈ 3 × 1040 to 9 × 1042 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 AGNs 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.