X-ray properties of intermediate-mass black holes in active galaxies. III. Spectral energy distribution and possible evidence for intrinsically X-ray-weak active galactic nuclei

Abstract

We present a systematic X-ray study, the third in a series, of 49 active galactic nuclei with intermediate-mass black holes (IMBH; 105-10 6 M ) using Chandra observations. We detect 42 out of 49 targets with a 0.5-2 keV X-ray luminosity 1041-1043 erg s -1. We perform spectral fitting for the 10 objects with enough counts (>200), and they are all well fit by a simple power-law model modified by Galactic absorption, with no sign of significant intrinsic absorption. While we cannot fit the X-ray spectral slope directly for the rest of the sample, we estimate it from the hardness ratio and find a range of photon indices consistent with those seen in more luminous and massive objects. The X-ray-to-optical spectral slope (αox) of our IMBH sample is systematically flatter than in active galaxies with more massive black holes, consistent with the well-known correlation between αox and UV luminosity. Thanks to the wide dynamic range of our sample, we find evidence that αox increases with decreasing M BH as expected from accretion disk models, where the UV emission systematically decreases as M BH decreases and the disk temperature increases. We also find a long tail toward low αox values. While some of these sources may be obscured, given the high L bol/L Edd values in the sample, we argue that some may be intrinsically X-ray-weak, perhaps owing to a rare state that radiates very little coronal emission. © 2012. The American Astronomical Society. All rights reserved..