Photometric reverberation mapping of 3C 120^⋆

¹ Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany
e-mail: francisco.pozon@gmail.com
² Instituto de Astronomia, Universidad Católica del Norte, Avenida Angamos 0610, Casilla 1280, Antofagasta, Chile
³ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

Received: 24 February 2012
Accepted: 30 July 2012

Abstract

We present the results of a five month monitoring campaign of the local active galactic nuclei (AGN) 3C 120. Observations with a median sampling of two days were conducted with the robotic 15 cm telescope VYSOS-6 located near Cerro Armazones in Chile. Broad band (B, V) and narrow band (NB) filters were used in order to measure fluxes of the AGN and the Hβ broad line region (BLR) emission line. The NB flux is constituted by about 50% continuum and 50% Hβ emission line. To disentangle line and continuum flux, a synthetic Hβ light curve was created by subtracting a scaled V-band light curve from the NB light curve. Here we show that the Hβ emission line responds to continuum variations with a rest frame lag of 23.6 ± 1.69 days. We estimate a virial mass of the central black hole M_BH = 57 ± 27 × 10⁶ M_⊙, by combining the obtained lag with the velocity dispersion of a single contemporaneous spectrum. Using the flux variation gradient method, we determined the host galaxy subtracted rest frame 5100 Å  luminosity at the time of our monitoring campaign with an uncertainty of 10% (L_AGN = (6.94 ± 0.71) × 10⁴³ erg s^-1). Compared with recent spectroscopic reverberation results, 3C 120 shifts in the R_BLR − L_AGN diagram remarkably close to the theoretically expected relation of R ∝ L^0.5. Our results demonstrate the performance of photometric AGN reverberation mapping, in particular for efficiently determining the BLR size and the AGN luminosity.