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.

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 × 10 6 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) × 10 43 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.

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Using robotic telescopes of the Universitätssternwarte Bochum near Cerro Armazones in Chile, we monitored the z = 0.0377 Seyfert-1 galaxy WPVS48 (2MASX J09594263-3112581) in the optical (B and R) and near-infrared (NIR, J and K s ) with a cadence of two days. The light curves show unprecedented variability details. The NIR variation features of WPVS48 are consistent with the corresponding optical variations, but the features appear sharper in the NIR than in the optical, suggesting that the optical photons undergo multiple scatterings. The J and K s emission, tracing the hot (∼1600 K) dust echo, lags the B and R variations by on average τ = 64 ± 4 days and 71 ± 5 days, respectively (restframe). WPVS48 lies on the known τ − M V relationship. However, the observed lag τ is about three times shorter than expected from the dust sublimation radius r sub inferred from the optical-UV luminosity, and explanations for this common discrepancy are searched for. The sharp NIR echos argue for a face-on torus geometry and allow us to put forward two potential scenarios: 1) as previously proposed, in the equatorial plane of the accretion disk the inner region of the torus is flattened and may come closer to the accretion disk. 2) The dust torus with inner radius r sub is geometrically and optically thick, so that the observer only sees the facing rim of the torus wall, which lies closer to the observer than the torus equatorial plane and therefore leads to an observed foreshortened lag. Both scenarios are able to explain the factor three discrepancy between τ and r sub . Longer-wavelength dust reverberation data might enable one to distinguish between the scenarios.

We are monitoring a 6• wide stripe along the southern Galactic disk simultaneously in the r and i bands, using a robotic 15-cm twin telescope of the Universitätsternwarte Bochum near Cerro Armazones in Chile. Utilising the telescope's 2.7• field of view, the survey aims at observing a mosaic of 268 fields once per month and to monitor dedicated fields once per night. The survey reaches a sensitivity from 10 m down to 18 m (AB system), with a completeness limit of r ∼ 15.5 m and i ∼ 14.5 m which -due to the instrumental pixel size of 2. 4 -refers to stars separated by >3 . This brightness range is ideally suited to examine the intermediately bright stellar population supposed to be saturated in deep variability surveys with large telescopes. To connect to deep surveys or to explore faint long term variables, coadded images of several nights reach a depth of ∼ 20 m . The astrometric accuracy is better than 1 , as determined from the overlap of neighbouring fields. We describe the survey design, the data properties and our procedures to derive the light curves and to extract variable stars. We present a list of ∼2200 variable stars identified in 50 square degrees with 50-80 observations between May and October 2011. For bright stars the variability amplitude A reaches down to A ∼ 0.05 m , while at the faint end variations of A > 1 m are detected. About 200 stars were known to be variable, and their amplitudes and periods -as far as determinable from our six month monitoring -agree with literature values, demonstrating the performance of the Bochum Galactic Disk Survey.