Low-Mass Active Galactic Nuclei with Rapid X-Ray Variability

TL;DR

This study investigates 12 low-mass active galactic nuclei with rapid X-ray variability, revealing high accretion rates, low black hole masses, and ongoing star formation, using optical spectroscopy and proposing a new star formation rate estimation method.

Contribution

It provides detailed optical spectroscopic analysis of low-mass AGNs selected by X-ray variability, confirming their high accretion rates and proposing a new method to estimate star formation rates in such systems.

Findings

All sources show broad Hα emission enabling black hole mass estimates.

Median black hole mass is 1.2×10^6 solar masses with high Eddington ratios.

Sample follows the M_BH–σ_* relation with significant scatter.

Abstract

We present a detailed study of the optical spectroscopic properties of 12 active galactic nuclei (AGNs) with candidate low-mass black holes (BHs) selected by Kamizasa et al. through rapid X-ray variability. The high-quality, echellette Magellan spectra reveal broad H$\alpha$ emission in all the sources, allowing us to estimate robust viral BH masses and Eddington ratios for this unique sample. We confirm that the sample contains low-mass BHs accreting at high rates: the median $M_{\rm BH} = 1.2\times 10^6M_\odot$ and median $L_{\rm bol}/L_{\rm Edd}=0.44$. The sample follows the $M_{\rm BH}-\sigma_*$ relation, within the considerable scatter typical of pseudobulges, the probable hosts of these low-mass AGNs. Various lines of evidence suggest that ongoing star formation is prevalent in these systems. We propose a new strategy to estimate star formation rates in AGNs hosted by low-mass, low-metallicity galaxies, based on modification of an existing method using the strength of [O II] $\lambda 3727$, [O III] $\lambda 5007$, and X-rays.