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|a Reynolds, Christopher S.
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|a MIT Kavli Institute for Astrophysics and Space Research
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|a Nowak, Michael A.
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|a Nowak, Michael A.
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|a Markoff, Sera B.
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|a Tueller, Jack
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|a Wilms, Joern
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|a Young, Andrew J.
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|a PROBING THE ACCRETION DISK AND CENTRAL ENGINE STRUCTURE OF NGC 4258 WITH SUZAKU AND XMM-NEWTON OBSERVATIONS
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|b Institute of Physics/American Astronomical Society,
|c 2015-03-26T20:40:11Z.
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|z Get fulltext
|u http://hdl.handle.net/1721.1/96205
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|a We present an X-ray study of the low-luminosity active galactic nucleus (AGN) in NGC 4258 using data from Suzaku, XMM-Newton, and the Swift/Burst Alert Telescope survey. We find that signatures of X-ray reprocessing by cold gas are very weak in the spectrum of this Seyfert-2 galaxy; a weak, narrow fluorescent Kα emission line of cold iron is robustly detected in both the Suzaku and XMM-Newton spectra but at a level much below that of most other Seyfert-2 galaxies. We conclude that the circumnuclear environment of this AGN is very "clean" and lacks the Compton-thick obscuring torus of unified Seyfert schemes. From the narrowness of the iron line, together with evidence of line flux variability between the Suzaku and XMM-Newton observations, we constrain the line emitting region to be between 3 × 10[superscript 3] rg and 4 × 10[superscript 4] rg from the black hole. We show that the observed properties of the iron line can be explained if the line originates from the surface layers of a warped accretion disk. In particular, we present explicit calculations of the expected iron line from a disk warped by Lens-Thirring precession from a misaligned central black hole. Finally, the Suzaku data reveal clear evidence of large amplitude 2-10 keV variability on timescales of 50 ksec and smaller amplitude flares on timescales as short as 5-10 ksec. If associated with accretion disk processes, such rapid variability requires an origin in the innermost regions of the disk (r ≈ 10rg or less). Analysis of the difference spectrum between a high- and low-flux states suggests that the variable component of the X-ray emission is steeper and more absorbed than the average AGN emission, suggesting that the primary X-ray source and absorbing screen have a spatial structure on comparable scales. We note the remarkable similarity between the circumnuclear environment of NGC 4258 and another well studied low-luminosity AGN, M81*.
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|a United States. National Aeronautics and Space Administration (NASA Grant NNX06A135G, Suzaku guest observer program)
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|a United States. National Aeronautics and Space Administration (XMM-Newton guest observer program, NNX07AE97G)
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|a Article
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|t Astrophysical Journal
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