A New Sample of Candidate Intermediate-Mass Black Holes Selected by X-ray Variability

TL;DR

This study identifies and analyzes 15 candidate intermediate-mass black holes using X-ray variability, revealing their properties and suggesting some are actively growing supermassive black holes with low masses and high accretion rates.

Contribution

The paper introduces a new sample of candidate intermediate-mass black holes selected via X-ray variability, with detailed spectral analysis and correlations between X-ray properties and accretion rates.

Findings

Seven sources have black hole masses below 2 million solar masses.

X-ray photon indices range from 0.57 to 2.57, with some resembling narrow-line Seyfert 1s.

A correlation between X-ray photon indices and Eddington ratios is established.

Abstract

We present the results of X-ray variability and spectral analysis of a sample of 15 new candidates for active galactic nuclei with relatively low-mass black holes (BHs). They are selected from the Second XMM-Newton Serendipitous Source Catalogue based on strong variability quantified by normalized excess variances. Their BH masses are estimated to be 1.1-6.6x10^6 M_solar by using a correlation between excess variance and BH mass. Seven sources have estimated BH masses smaller than 2x10^6 M_solar, which are in the range for intermediate-mass black holes. Eddington ratios of sources with known redshifts range from 0.07 to 0.46 and the mean Eddington ratio is 0.24. These results imply that some of our sources are growing supermassive black holes, which are expected to have relatively low masses with high Eddington ratios. X-ray photon indices of the 15 sources are in the range of ~0.57-2.57, and 5 among them have steep (>2) photon indices, which are the range for narrow-line Seyfert 1s. Soft X-ray excess is seen in 12 sources, and is expressed by a blackbody model with kT~83-294 eV. We derive a correlation between X-ray photon indices and Eddington ratios, and find that the X-ray photon indices of about a half of our sources are flatter than the positive correlation suggested previously.