Spitzer space telescope measurements of dust reverberation lags in the Seyfert 1 galaxy NGC 6418
- Physics Department, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623-5603 (United States)
- SUPA, School of Physics and Astronomy, The University of St Andrews, North Haugh, St Andrews KY169SS (United Kingdom)
- Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210-1173 (United States)
- Department of Physics, Southwestern University, Georgetown, TX 78626 (United States)
- Department of Physics and Astronomy, Bucknell University, 1 Dent Drive, Lewisburg, PA 17837 (United States)
- Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506 (United States)
- School of Physics and Astronomy, Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv (Israel)
- Departamento de Astronomia, Instituto de Fsica, Universidade Federal do Rio Grande do Sul, Campus do Vale, Av. Bento Goncalves 9500, 91501-970 Porto Alegre, RS (Brazil)
- Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019, Sesto Fiorentino (Firenze) (Italy)
- INAF—Osservatorio Astronomico di Torino, Strada Osservatorio 20, I-10025 Pino Torinese (Italy)
- Department of Physics and Space Sciences, Florida Institute of Technology, 150 W. University Blvd, Melbourne, FL 32901 (United States)
- University of Melbourne, 1100 Grattan Street, Parkville, Victoria 3010 (Australia)
- INAF—Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127, Bologna (Italy)
- Max Planck Institute for Astronomy, Knigstuhl 17, D-69117 Heidelberg (Germany)
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{sub −2.2}{sup +2.4} days and 47.1{sub −3.1}{sup +3.1} days, respectively. We report a cross-correlation time lag between the 4.5 and 3.6 μm flux of 13.9{sub −0.1}{sup +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∝L{sup 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.
- OSTI ID:
- 22883301
- Journal Information:
- Astrophysical Journal, Vol. 801, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); Since 2009, the country of publication for this journal is the UK.; ISSN 0004-637X
- Country of Publication:
- United Kingdom
- Language:
- English
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