Simultaneous H alpha and dust reverberation mapping of 3C120: Testing the bowl-shaped torus geometry

A&A 620, A137 (2018) At the Universitaetssternwarte Bochum near Cerro Armazones we have monitored the Seyfert-1 galaxy 3C 120 between September 2014 and March 2015 in BVRI and a narrow band filter covering the redshifted H alpha line; in addition we obtained a single con-temporary spectrum with 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 H alpha BLR echo. We investigate how far this supports an optically thick bowl-shaped BLR and dust torus geometry as proposed by Kawaguchi & Mori (2010) and Goad et al. (2012). 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 half covering angle 0 deg < theta < 40 deg (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 deg < theta < 45 deg). Then, for small inclinations as for 3C120, the top dust edge forms a ring which 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. This leads to luminosity dependent foreshortening effects of the lag. We discuss implications and possible corrections of the foreshortening for the black hole mass determination and consequences for the lag (size) - luminosity relationships and the difference to interferometric torus sizes.