Spitzer Space Telescope Measurements of Dust Reverberation Lags in the Seyfert 1 Galaxy NGC 6418
Journal
Date Issued
2015
Author(s)
Vazquez, Billy
•
Galianni, Pasquale
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Richmond, Michael
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Robinson, Andrew
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Axon, David J.
•
Horne, Keith
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Almeyda, Triana
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Fausnaugh, Michael
•
Peterson, Bradley M.
•
Bottorff, Mark
•
Gallimore, Jack
•
Eltizur, Moshe
•
Netzer, Hagai
•
Storchi-Bergmann, Thaisa
•
•
•
Batcheldor, Dan
•
Buchanan, Catherine
•
•
Kishimoto, Makoto
•
Packham, Christopher
•
Perez, Enrique
•
Tadhunter, Clive
•
Upton, John
•
Estrada-Carpenter, Vicente
Description
We dedicate this paper to the memory of our great friend, colleague and mentor David Axon, who initiated this project and brought the collaboration together. We would like to thank Davide Lena for improvements to the manuscript and discussions on spectroscopic data reduction. S.U. thanks Lex Shaw for a generous instrumentation donation. This work is based (in part) on observations made with the Spitzer Space Telescope , which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech, GO-80120. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France, and of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III web site is http://sdss3.org/ SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofísica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University. The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias with financial support from the UK Science and Technology Facilities Council. The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). BMP is supported by NSF grant AST-1008882.
Abstract
We present results from a 15 month campaign of high-cadence (̃3 days) mid-infrared Spitzer and optical (B and V) monitoring of the Seyfert 1 galaxy NGC 6418, with the objective of determining the characteristic size of the dusty torus in this active galactic nucleus (AGN). We find that the 3.6 and 4.5 μm flux variations lag behind those of the optical continuum by 37.2-2.2+2.4 days and 47.1-3.1+3.1 days, respectively. We report a cross-correlation time lag between the 4.5 and 3.6 μm flux of 13.9-0.1+0.5 days. The lags indicate that the dust emitting at 3.6 and 4.5 μm is located at a distance ≈ 1 light-month (≈ 0.03 pc) from the source of the AGN UV-optical continuum. The reverberation radii are consistent with the inferred lower limit to the sublimation radius for pure graphite grains at 1800 K, but smaller by a factor of ̃2 than the corresponding lower limit for silicate grains; this is similar to what has been found for near-infrared (K-band) lags in other AGNs. The 3.6 and 4.5 μm reverberation radii fall above the K-band τ \propto {{L}0.5} size-luminosity relationship by factors ≲ 2.7 and ≲ 3.4, respectively, while the 4.5 μm reverberation radius is only 27% larger than the 3.6 μm radius. This is broadly consistent with clumpy torus models, in which individual optically thick clouds emit strongly over a broad wavelength range.
Volume
801
Issue
2
Start page
127
Issn Identifier
0004-637X
Ads BibCode
2015ApJ...801..127V
Rights
open.access
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