2026A&A...706A.176M

Context. We investigate the origin of inter-band continuum time delays in active galactic nuclei (AGNs) in order to study the structure and properties of their accretion disks.
Aims. We aim to measure the inter-band continuum time delays through photometric monitoring of Seyfert galaxy Fairall 9 to construct the lag-spectrum. Additionally, we explain the observed features in the Fairall 9 lag-spectrum and discuss the potential drivers behind them based on our newly collected data from the Obserwatorium Cerro Murphy (OCM) telescope.
Methods. We initiated a long-term, continuous AGN photometric monitoring program in 2024 titled Hubble constant constraints through AGN Light curve Observations (HALO) using intermediate and broadband filters. Here, we present the first results from HALO, focusing on photometric light curves and continuum time-delay measurements for Fairall 9. To complement these observations and extend the wavelength coverage of the lag-spectrum, we also reanalyzed archival Swift light curves and spectroscopic data available in the literature.
Results. Using HALO and Swift light curves, we measured inter-band continuum delays to construct the lag-spectrum of Fairall 9. Excess lags appear in the u and U bands (Balmer continuum contamination) and in the I band (Paschen jump and dust emission from the torus). Overall, the lag-spectrum deviates significantly from standard disk model predictions.
Conclusions. We find that inter-band delays deviate from the power law, τ_λ ∝ λ^β^, due to broad-line region scattering, reprocessing, and dust contributions at longer wavelengths. Power-law fits are therefore not well suited to characterizing the nature of the time delays.