HALO

I. Photometric continuum reverberation mapping of Fairall 9

¹ Center for Theoretical Physics, Polish Academy of Sciences Al. Lotników 32/46 02-668 Warsaw, Poland
² Astroinformatics, Heidelberg Institute for Theoretical Studies Schloss-Wolfsbrunnenweg 35 69118 Heidelberg, Germany
³ Institut d’Astrophysique et de Géophysique Université de Liège Allée du six août 19c B-4000 Liège (Sart-Tilman), Belgium
⁴ International Gemini Observatory/NSF NOIRLab Casilla 603 La Serena, Chile
⁵ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences Bartycka 18 00-716 Warszawa, Poland
⁶ Universidad de Concepción, Departamento de Astronomía Casilla 160 – C Concepción, Chile
⁷ Aryabhatta Research Institute of Observational Sciences Nainital 263001 Uttarakhand, India
⁸ c/o Tracy L. Turner 205 South Prospect Street Granville OH 43023, USA
⁹ Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University Kotlářská 2 611 37 Brno, Czech Republic
¹⁰ Astronomical Institute of the Czech Academy of Sciences Boční II 1401 CZ-14100 Prague, Czech Republic
¹¹ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University ul. Grudziądzka 5 87-100 Toruń, Poland
¹² Millenium Institute of Astrophysics Avenue Libertador Bernardo O’Higgins 340 Casa Central Santiago, Chile
¹³ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS 92190 Meudon, France
¹⁴ French-Chilean Laboratory for Astronomy, IRL 3386, CNRS and U. de Chile Casilla 36-D Santiago, Chile
¹⁵ Department of Applied Physics/Physics, Mahatma Jyotiba Phule Rohilkhand University Bareilly 243006, India

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Received: 22 October 2025
Accepted: 14 December 2025

Abstract

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.