HALO, Fairall 9? How are you?
The HALO project revealed that the accretion disk of the AGN Fairall 9 is significantly more expansive and physically complex than standard theoretical models predict. This was done by tracking time delays between light captured in different passbands, the method known as photometric continuum reverberation mapping.
HALO I: Photometric continuum reverberation mapping of Fairall 9
A.K. Mandal, F. Pozo Nuñez, V.K. Jaiswal, M.H. Naddaf, B. Czerny, S. Panda, P. Karczmarek, G. Pietrzyński, S. Pandey, B.M. Peterson, M. Zajaček, M. Dovčiak, V. Karas, W. Narloch, M. Kicia, M. Górski, M. Kałuszyński, G. Hajdu, P. Wielgórski, B. Zgirski, C. Gałan, W. Pych, R. Smolec, K. Bąkowska, W. Gieren, P. Kervella
Fairall 9 is a Type I Seyfert galaxy with an Active Galactic Nuclei (AGN) in its core. It was chosen as the first target in the HALO project (Hubble constant constraints through AGN Light curve Observations), because of its hight brightness and small distance (roughly z = 0.047), relatively simple structure, which makes its central accretion disk clearer to observe; significant flux variability across multiple optical and UV bands, which is essential for tracking time delays between different continuum emission bands; well characterized parameters (such as a black hole mass of approximately ) thanks to decades of monitoring across X-ray, UV, optical, and infrared wavelengths; and presence of 1.8-year daily monitoring campaign from the Swift space satellite, allowing the comparison between ground-based and space-based measurements.
The data were collected at OCM using our 0.6-meter Wojtek Krzemiński telescope (WK06) over an intensive 146-day campaign. Monitoring Fairall 9 with an exceptionally dense 0.3-day cadence across five specific spectral windows (the Strömgren u, v, b, y and Johnson-Cousins I filters) allowed the authors to track fine light variations that broad-band surveys easily overlook. Operating continuously on a single telescope provided remarkably uniform data. This high degree of homogeneity greatly enhanced the precision of the results, demonstrating the unique advantage of dedicated observing facilities
The authors utilized the continuum reverberation mapping method. By tracking the time delays between different colors of light, they mathematically reconstructed the size and temperature profile of an accretion disk. The time it takes for a change in the inner, hotter disk (UV) to be “echoed” by the outer, cooler disk (optical) serves as a cosmic ruler.
The traditional “Standard Thin Accretion Disk” model predicts a smooth power-law relationship where the delay is proportional to the wavelength with a predicted slope of 4/3. However, the high-precision data from OCM revealed significant deviations for Fairall 9. The authors report negative delays in the u, v, and b bands – a major red flag that the standard model cannot account for these echoes on its own.
Overall, the findings confirm a disk-size anomaly: the accretion disk of Fairall 9 appears 1.5 to 3.6 times larger than the theory predicts. This discrepancy likely arises from two sources: the scattering and reprocessing of light within the BLR, or a significant vertical extent of the X-ray corona, which would increase the travel distance for the illuminating radiation.
The detailed measurements of Fairall 9 are a cornerstone for the HALO project’s ultimate mission: resolving the Hubble tension. Modern cosmology is currently divided over the value of the Hubble constant, the rate at which the universe is expanding. Different measurement methods yield conflicting results, and the HALO project aims to carve out an independent route to measure cosmic distances, potentially providing the clarity needed to resolve the debate.
By using simultaneous modeling of the lag-spectrum and the spectral energy distribution (SED), we can determine the absolute luminosity of an AGN and, by extension, its distance. The determination of the Hubble constant will be the focal point of the next HALO paper.
