Photometric AGN reverberation mapping – an efficient tool for BLR sizes, black hole masses, and host-subtracted AGN luminosities

¹ Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany
e-mail: haas@astro.rub.de
² Instituto de Astronomia, Universidad Católica del Norte, Avenida Angamos 0610, Casilla 1280 Antofagasta, Chile

Received: 23 May 2011
Accepted: 8 September 2011

Abstract

Photometric reverberation mapping employs a wide band pass to measure the AGN continuum variations and a suitable narrow band to trace the echo of an emission line in the broad line region (BLR). The narrow band catches both the emission line and the underlying continuum, and one needs to extract the pure emission line light curve. We performed a test on two local AGNs, PG0003+199 and Ark120, by observing well-sampled broad- (B, V) and narrow-band light curves with the robotic 15 cm telescope VYSOS-6 on Cerro Armazones, Chile. We find that, as long as the emission line contributes 50% to the band pass, the pure emission line light curve can be reconstructed from photometric monitoring data so that the time lag τ can be measured. For both objects the lags are consistent with spectroscopic reverberation results. We calculated virial black hole masses in agreement with literature values, by combining the BLR size R_BLR (τ) from photometric monitoring with the velocity dispersion of a single contemporaneous spectrum. Applying the flux variation gradient method, we estimate the host galaxy contribution in the apertures used and the host-subtracted restframe 5100 Å luminosity L_AGN. Our L_AGN differs significantly from previous estimates, placing both sources  ~50% closer to the R_BLR − L_AGN relation. This suggests that the scatter in the current R_BLR − L_AGN relation is largely caused by uncertainties in R_BLR due to undersampled light curves and by uncertainties in the host-subtracted AGN luminosities inferred so far. If the scatter can be reduced, then two quasar samples matching in R_BLR should also match in intrinsic L_AGN, independent of redshift, thus offering the prospect of probing cosmological models. Photometric reverberation mapping opens the door to efficiently measuring hundreds of BLR sizes and host-subtracted AGN luminosities even with small telescopes, but also routinely with upcoming large survey telescopes like the LSST.