Amit Kumar Mandal (NCAC, Warsaw)
based on https://arxiv.org/abs/2512.13296 from Madal et al. (Dec 2025). We have initiated a dedicated photometric AGN monitoring program, Hubble Constant Constraint through AGN Light-curve Observations (HALO), at the Obserwatorium Cerro Murphy (OCM) in Chile. The primary goal of HALO is to estimate the Hubble constant by modeling AGN continuum light curves using a physically motivated framework that combines a standard accretion disk, an optional inner hot flow, lamp-post irradiation, and a radiation-pressure-regulated broad-line region (BLR). By performing simultaneous lag-spectrum and spectral energy distribution (SED) fitting, with the accretion rate and luminosity distance treated as free parameters, this approach enables an independent determination of the Hubble constant. As part of HALO, we have completed observations and lag analysis for the first target in our sample, Fairall 9. In this journal club, I will present the construction of the continuum lag-spectrum and discuss the physical origins of the observed inter-band time delays. We find that the observed lag-spectrum of Fairall 9 significantly deviates from the predictions of a standard Shakura-Sunyaev disk, primarily due to the combined effects of BLR scattering, reprocessing, and dust emission at longer wavelengths. This measured lag-spectrum will serve as a key input for the forthcoming Hubble constant estimation in the next phase of the HALO program.
