Variation of Inner Radius of Dust Torus in NGC4151

Abstract

The long-term optical and near infrared monitoring observations for a type 1 act ive galactic nucleus NGC 4151 were carried out for six years from 2001 to 2006 b y using the MAGNUM telescope, and delayed response of flux variations in the $K(2.2\mu m)$ band to those in the $V(0.55\mu m)$ band was clearly detected. Based on cross correlation analysis, we precisely measured a lag time $\Delta t$ for eight separate periods, and we found that $\Delta t$ is not constant changing be tween 30 and 70 days during the monitoring period. Since $\Delta t$ is the ligh t travel time from the central energy source out to the surrounding dust torus, this is the first convincing evidence that the inner radius of dust torus did ch ange in an individual AGN. In order to relate such a change of $\Delta t$ with a change of AGN luminosity $L$, we presented a method of taking an average of th e observed $V$-band fluxes that corresponds to the measured value of $\Delta t$, and we found that the time-changing track of NGC 4151 in the $\Delta t$ versus $L$ diagram during the monitoring period deviates from the relation of $\Delta t\propto L^{0.5}$ expected from dust reverberation. This result, combined with t he elapsed time from period to period for which $\Delta t$ was measured, indicat es that the timescale of dust formation is about one year, which should be taken into account as a new constraint in future studies of dust evolution in AGNs.

5 pages, 3 figures, To appear in the ApJ Letter

References in corpus (2)

• Long-term variability of the optical spectra of NGC 4151: I. Light curves and flux correlations
• Multiple Regression Analysis of the Variable Component in the Near-Infrared Region for Type 1 AGN MCG+08-11-011

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• On the difference of torus geometry between hidden and non-hidden broad line active galactic nuclei
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• Reverberation Measurements of the Inner Radii of the Dust Tori in Quasars
• A New Method for Measuring Extragalactic Distances
• Dust Reverberation Mapping in Distant Quasars from Optical and Mid-Infrared Imaging Surveys
• Infrared Spectral Energy Distribution and Variability of Active Galactic Nuclei: Clues to the Structure of Circumnuclear Material
• A New Black Hole Mass Estimate for Obscured Active Galactic Nuclei
• Spitzer Space Telescope Measurements of Dust Reverberation Lags in the Seyfert 1 Galaxy NGC 6418
• X-ray Constraint on Location of AGN Torus in Circinus Galaxy
• Monitoring the temperature and reverberation delay of the circumnuclear hot dust in NGC 4151
• The nuclear architecture of NGC 4151: on the path toward a universal outflow mechanism in light of NGC 1068
• The dust sublimation region of the Type 1 AGN NGC4151 at a hundred micro-arcsecond scale as resolved by the CHARA Array interferometer
• The Relative Wavelength Independence of IR Time Lags in NGC4151 During the Years 2010-2015
• A complex dust morphology in the high-luminosity AGN Mrk 876
• Hot graphite dust in the inner regime of NGC 4151
• Detection of eccentric close-binary supermassive black holes with incomplete interferometric data
• Infrared lags in the light curves of AGN measured using a deep survey