Volume 620, December 2018
|Number of page(s)||16|
|Published online||10 December 2018|
Simultaneous Hα and dust reverberation mapping of 3C 120: Testing the bowl-shaped torus geometry⋆
Astronomisches Institut, Ruhr–Universität Bochum, Universitätsstraße 150, 44801
2 Institut für Astrophysik, Universität Göttingen, Friedrich-Hund Platz 1, 37077 Göttingen, Germany
3 Institute for Astronomy, 640 North A’ohōkū Place, Hilo, HI 96720-2700, USA
4 Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile
Accepted: 8 August 2018
We monitored the Seyfert-1 galaxy 3C 120 between September 2014 and March 2015 at the Universitätssternwarte Bochum near Cerro Armazones in BVRIJK and a narrowband filter covering the redshifted Hα line. In addition we obtained a single contemporary spectrum with the spectrograph FAST at Mt. Hopkins. Compared to earlier epochs 3C 120 is about a factor of three brighter, allowing us to study the shape of the broad line region (BLR) and the dust torus in a high luminosity phase. The analysis of the light curves yields that the dust echo is rather sharp and symmetric in contrast to the more complex broad Hα BLR echo. We investigated how far this supports an optically thick bowl-shaped BLR and dust torus geometry. The comparison with several parameterizations of these models supports the following geometry: The BLR clouds lie inside the bowl closely above the bowl rim up to a halfcovering angle 0° < θ < 40° (measured against the equatorial plane). Then the BLR is spread over many isodelay surfaces, yielding a smeared and structured echo as observed. Furthermore, if the BLR clouds shield the bottom of the bowl rim against radiation from the nucleus, the hot dust emission comes essentially from the top edge of the bowl (40° < θ < 45°). Then, for small inclinations as for 3C120, the top dust edge forms a ring that largely coincides with a narrow range of isodelay surfaces, yielding the observed sharp dust echo. The scale height of the BLR increases with radial distance from the black hole (BH). This leads to luminosity dependent foreshortening effects of the lag. We discuss the implications and possible corrections of the foreshortening for the BH mass determination and consequences for the lag (size)–luminosity relationships and the difference from interferometric torus sizes.
Key words: galaxies: Seyfert / quasars: emission lines / infrared: galaxies / galaxies: nuclei / galaxies: structure / quasars: supermassive black holes
Tables of the lightcurve data are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/620/A137
© ESO 2018
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