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
- Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 (United States)
We present a systematic X-ray study, the third in a series, of 49 active galactic nuclei with intermediate-mass black holes (IMBH; {approx}10{sup 5}-10{sup 6} M{sub Sun }) using Chandra observations. We detect 42 out of 49 targets with a 0.5-2 keV X-ray luminosity 10{sup 41}-10{sup 43} erg s{sup -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 ({alpha}{sub ox}) of our IMBH sample is systematically flatter than in active galaxies with more massive black holes, consistent with the well-known correlation between {alpha}{sub ox} and UV luminosity. Thanks to the wide dynamic range of our sample, we find evidence that {alpha}{sub ox} increases with decreasing M{sub BH} as expected from accretion disk models, where the UV emission systematically decreases as M{sub BH} decreases and the disk temperature increases. We also find a long tail toward low {alpha}{sub ox} values. While some of these sources may be obscured, given the high L{sub bol}/L{sub 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.
- OSTI ID:
- 22140028
- Journal Information:
- Astrophysical Journal, Vol. 761, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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