X-ray Spectral and Variability Properties of Low-Mass AGN

TL;DR

This study analyzes the X-ray spectral and variability properties of 14 low-mass AGN, revealing soft X-ray excesses, rapid variability, and deviations from the expected mass-variability relation, providing insights into their accretion processes.

Contribution

It significantly increases the sample of low-mass AGN with detailed X-ray analysis and compares their properties to more massive AGN, highlighting differences in variability behavior.

Findings

All low-mass AGN show soft X-ray excess.

Rapid short-term and long-term variability observed.

Deviation from the linear mass-variability relation at low masses.

Abstract

We study the X-ray properties of a sample of 14 optically-selected low-mass AGN whose masses lie within the range 1E5 -2E6 M(solar) with XMM-Newton. Only six of these low-mass AGN have previously been studied with sufficient quality X-ray data, thus, we more than double the number of low-mass AGN observed by XMM-Newton with the addition of our sample. We analyze their X-ray spectral properties and variability and compare the results to their more massive counterparts. The presence of a soft X-ray excess is detectable in all five objects which were not background dominated at 2-3 keV. Combined with previous studies, this gives a total of 8 low-mass AGN with a soft excess. The low-mass AGN exhibit rapid, short-term variability (hundreds to thousands of seconds) as well as long-term variability (months to years). There is a well-known anti-correlation between black hole mass and variability amplitude (normalized excess variance). Comparing our sample of low-mass AGN with this relation we find that all of our sample lie below an extrapolation of the linear relation. Such a flattening of the relation at low masses (below about 1E6 M(solar)) is expected if the variability in all AGN follows the same shape power spectrum with a break frequency that is dependent on mass. Finally, we also found two objects that show significant absorption in their X-ray spectrum, indicative of type 2 objects, although they are classified as type 1 AGN based on optical spectra.