HALO I: Photometric continuum reverberation mapping of Fairall 9

TL;DR

This study measures inter-band continuum time delays in the AGN Fairall 9 using photometric monitoring and archival data, revealing deviations from standard disk models due to BLR and dust effects.

Contribution

First photometric continuum lag spectrum of Fairall 9 combining new and archival data, highlighting complex deviations from standard accretion disk models.

Findings

Excess lags in $u$ and $U$ bands due to Balmer continuum

Excess lag in $I$ band from dust emission and Paschen jump

Lag-spectrum deviates from standard power-law predictions

Abstract

We investigate the origin of inter-band continuum time delays in active galactic nuclei (AGNs) to study the structure and properties of their accretion disks. 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. 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 broad band 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. 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/dust emission from the torus). Overall, the lag-spectrum deviates significantly from standard disk model predictions. We find that inter-band delays deviate from the power-law, $\tau_{\lambda} \propto \lambda^{\beta}$ due to BLR scattering, reprocessing, and dust contributions at longer wavelengths. Power-law fits are therefore not well suited for characterizing the nature of the time delays.