2022A&A...659A..13F

Aims. We present the results of an intensive six-month optical continuum reverberation mapping campaign of the Seyfert 1 galaxy PG 2130+099 at redshift z = 0.063. The ground-based photometric monitoring was conducted on a daily basis with the robotic 46 cm telescope of the WISE observatory located in Israel. Specially designed narrowband filters were used to observe the central engine of the active galactic nucleus (AGN), avoiding line contamination from the broad-line region (BLR). We aim to measure inter-band continuum time lags across the optical range and determine the size-wavelength relation for this system. Methods. We used two methods, the traditional point-spread function photometry and the recently developed proper image subtraction technique, to independently perform the extraction of the continuum light curves. The inter-band time lags are measured with several methods, including the interpolated cross-correlation function, the z-transformed discrete correlation function, a von Neumann estimator, JAVELIN (in spectroscopic mode), and MICA. Results. PG 2130+099 displays correlated variability across the optical range, and we successfully detect significant time lags of up to ∼3 days between the multiband light curves. We find that the wavelength-dependent lags, τ(λ), generally follow the relation τ(λ)∝λ^4/3, as expected for the temperature radial profile T ∝ R^–3/4 of an optically thick, geometrically thin accretion disk. Despite that, the derived time lags can also be fitted by τ(λ)∝λ², implying the possibility of a slim, rather than thin, accretion disk. Using the flux variation gradient method, we determined the AGN's host-galaxy-subtracted rest frame 5100 Å luminosity at the time of our monitoring campaign with an uncertainty of ∼18% (λL₅₁₀₀ = (2.40 ± 0.42)x10⁴⁴ erg s^–1). While a continuum reprocessing model can fit the data reasonably well, our derived disk sizes are a factor of ∼2 - 6 larger than the theoretical disk sizes predicted from the AGN luminosity estimate of PG 2130+099. This result is in agreement with previous studies of AGN/quasars and suggests that the standard Shakura-Sunyaev disk theory has limitations in describing AGN accretion disks.