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arxiv: 2606.27195 · v1 · pith:VWDYH4OEnew · submitted 2026-06-25 · 🌌 astro-ph.GA · astro-ph.CO· gr-qc· hep-ph· hep-th

HALO II: Constraining Hubble constant H₀ through continuum delay fitting of Fairall 9

Amit Kumar Mandal , Francisco Pozo Nu\~nez , Vikram Kumar Jaiswal , Mohammad Hassan Naddaf , Bo\.zena Czerny , Swayamtrupta Panda , Paulina Karczmarek , Grzegorz Pietrzy\'nski
show 23 more authors
Shivangi Pandey R. Edelson B. M. Peterson Michal Zaja\v{c}ek Alberto Floris Mary Loli Mart\'inez-Aldama Michal Dov\v{c}iak Vladimir Karas Weronika Narloch Miros{\l}aw Kicia Marek G\'orski Miko{\l}aj Ka{\l}uszy\'nski Gergely Hajdu Piotr Wielg\'orski Bart{\l}omiej Zgirski Cezary Ga{\l}an Wojciech Pych Rados{\l}aw Smolec Ricardo Salinas Henryka Netzel-I{\l}kiewicz Karolina B\k{a}kowska Wolfgang Gieren Pierre Kervella
This is my paper

Pith reviewed 2026-06-26 03:42 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.COgr-qchep-phhep-th
keywords Hubble constantactive galactic nucleicontinuum time delaysaccretion diskFairall 9reverberation mappingcosmology
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The pith

Continuum time delays and SED modeling in Fairall 9 yield H0 = 72.4 km s^{-1} Mpc^{-1} independent of the cosmic distance ladder.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper presents a new method to measure the Hubble constant from time delays between different wavelengths in the continuum emission of an active galaxy's accretion disk. By monitoring Fairall 9 and fitting both the observed lag spectrum and its broadband spectral energy distribution with a radiation reprocessing model, the authors obtain a value of H0 that matches both early- and late-universe determinations within uncertainties. The approach requires no calibration against other distance indicators and achieves roughly 5 percent precision from a single source. This precision is expected to improve below 1 percent when applied to larger samples from upcoming surveys. The result demonstrates that AGN continuum reverberation can serve as a direct cosmological probe.

Core claim

Through the simultaneous modeling of the continuum lag-spectrum and the broadband SED of Fairall 9 using the H0RIZON-AGN model that incorporates radiation reprocessing in the cold accretion disk, a Hubble constant of H0=72.4_{-3.7}^{+3.4} km s^{-1} Mpc^{-1} is derived, showing that this method can deliver independent H0 measurements at approximately 5 percent precision from one object.

What carries the argument

The H0RIZON-AGN model, which predicts continuum time delays by simulating radiation reprocessing in the accretion disk and is fitted jointly to the lag spectrum and SED.

If this is right

  • The method supplies an H0 measurement entirely free from cosmic distance ladder systematics and cross-calibrations.
  • A single source already reaches 5 percent precision, indicating that samples of many AGNs can push uncertainty below 1 percent.
  • Data from the Vera Rubin Observatory are expected to enable this technique as a competitive probe of the Hubble tension.
  • The derived H0 remains consistent with both early- and late-Universe values inside current error bars.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Repeating the analysis on a statistical sample of AGNs would test whether Fairall 9 is typical or whether source-to-source variations affect the inferred H0.
  • Incorporating additional observables such as line reverberation or X-ray lags could tighten the model constraints without changing the core distance inference.
  • If the method scales cleanly, it offers a route to map H0 across redshift using the same class of objects at different look-back times.

Load-bearing premise

The H0RIZON-AGN model correctly captures all dominant effects of radiation reprocessing in the cold accretion disk for Fairall 9 with no significant unmodeled systematics such as disk winds or non-standard temperature profiles.

What would settle it

An independent geometric distance to Fairall 9 that produces an H0 value outside the reported 72.4_{-3.7}^{+3.4} range when combined with the observed lags would falsify the central claim.

Figures

Figures reproduced from arXiv: 2606.27195 by Alberto Floris, Amit Kumar Mandal, Bart{\l}omiej Zgirski, B. M. Peterson, Bo\.zena Czerny, Cezary Ga{\l}an, Francisco Pozo Nu\~nez, Gergely Hajdu, Grzegorz Pietrzy\'nski, Henryka Netzel-I{\l}kiewicz, Karolina B\k{a}kowska, Marek G\'orski, Mary Loli Mart\'inez-Aldama, Michal Dov\v{c}iak, Michal Zaja\v{c}ek, Miko{\l}aj Ka{\l}uszy\'nski, Miros{\l}aw Kicia, Mohammad Hassan Naddaf, Paulina Karczmarek, Pierre Kervella, Piotr Wielg\'orski, Rados{\l}aw Smolec, R. Edelson, Ricardo Salinas, Shivangi Pandey, Swayamtrupta Panda, Vikram Kumar Jaiswal, Vladimir Karas, Weronika Narloch, Wojciech Pych, Wolfgang Gieren.

Figure 1
Figure 1. Figure 1: Fairall 9 SED. Left: Best-fit SED obtained with AGNFitter-RX (Martínez-Ramírez et al. 2024). The contributions from the stellar popu￾lation, cold dust emission, torus (CAT3D, Hönig & Kishimoto 2017), accretion disk, and radio components are shown by the yellow, green, purple, dark-blue, and light-blue solid curves, respectively. The shaded regions represent the ranges spanned by 100 random realizations dra… view at source ↗
Figure 2
Figure 2. Figure 2: A schematic representation of the H0RIZON-AGN framework (not to scale). This diagram illustrates the individual components of AGN included in our modeling approach, which collectively contribute to the observed continuum lag-spectrum and broadband SED. atomic features in AGN lag-spectra, which are naturally ex￾plained by diffuse BLR continuum emission (Korista & Goad 2001; Lawther et al. 2018). Additional … view at source ↗
Figure 3
Figure 3. Figure 3: Representative BLR properties of Fairall 9 from FRADO. Left: 3-D distribution of BLR cloud positions inferred from the FRADO model, assuming MBH = 2.18 × 108 M⊙, ˙m = 0.0316 (one of the three adopted accretion rates), and metallicity Z = 5 in solar unit. The axes are given in units of rg. Middle: 2-D velocity–delay map of Fairall 9 constructed from the FRADO model for an inclination angle of i = 35◦ . The … view at source ↗
Figure 4
Figure 4. Figure 4: Emissivity profiles. BLR emissivity profile (ratio of the repro￾cessed to the incident continuum) of a BLR cloud centered at log r (cm) = 16.64, with an incident ionizing luminosity of log L (erg/s) = 44.97, a hydrogen density of log nH (cm−3 ) = 12, a column density of log NH (cm−2 ) = 23.5, and fBLR = 1, shown by the solid red line computed us￾ing Cloudy. The corresponding dust emissivity profile, assumi… view at source ↗
Figure 5
Figure 5. Figure 5: Transfer functions of the different emitting regions in the central parts of Fairall 9 for the best-fit luminosity distance, DL = 190.6 Mpc. From left to right: (i) the transfer function of the warm corona; (ii) transfer functions of the cold accretion disk at different wavelengths, with the wavelength-integrated disk response function shown by the black line; (iii) the BLR response function obtained from … view at source ↗
Figure 6
Figure 6. Figure 6: Total transfer functions, ψtot(λ, t), at different wavelengths. The color coding follows the wavelength labels. The wavelength-integrated total transfer function, ψtot(t), is plotted as the black curve for illustrative purposes. Parameters used: MBH = 2.18 × 108M⊙, Eddington ratio ˙m = 0.0224, LX = 6.74 × 1044 erg s−1 , h = 11.26 rg, TWC = 4.58 × 106 K, τWC = 22.08, rtransition = 252.38 rg, rout = 10000 rg… view at source ↗
Figure 8
Figure 8. Figure 8: Best fit of the broadband SED of Fairall 9 for luminosity dis￾tance, DL = 190.6 Mpc. The observational data are marked in black, the total model in red, the components of the model are the following: warm corona (orange), cold outer disk (blue), BLR and Fe II (green), dusty torus(peru), and starlight (brown). 4.8. Distance estimation via simultaneous lag-spectrum and SED fitting In our model, we fixed MBH,… view at source ↗
Figure 9
Figure 9. Figure 9: Simultaneous lag-spectrum and SED fitting results for Fairall 9. Top: Best-fit χ 2 /dof as a function of the luminosity distance, DL, marginalized over all other model parameters. Bottom: Same as the top panel, but shown as a function of the inferred Hubble constant, H0. The red dashed and black dashed curves represent spline and quartic fits, respectively, to the data shown as red circular points. The ver… view at source ↗
read the original abstract

The Hubble tension remains one of the most significant unresolved problems in modern cosmology. A key question is whether it may arise from underestimated systematic uncertainties in the different measurement techniques. In this context, new independent methods are of exceptional importance. We therefore pursue a novel approach to determining the Hubble constant, $H_{0}$ based on continuum time delay and spectral energy distribution (SED) modeling in active galactic nuclei (AGNs). Unlike conventional techniques, this method is entirely independent of the cosmic distance ladder and does not require cross-calibration against other distance indicators. As a result, it enables a direct determination of $H_{0}$, free from the arbitrary normalizations that often affect indirect measurements. We conducted a dedicated monitoring campaign of the Seyfert galaxy Fairall 9 and further developed the {\tt H0RIZON-AGN} model to interpret the resulting observations. The model incorporates the effects of radiation reprocessing in the surrounding cold accretion disk, enabling a more realistic description of the observed continuum delays. Through the simultaneous modeling of the continuum lag-spectrum and the broadband SED of Fairall 9, we derived a Hubble constant of $H_{0}=72.4_{-3.7}^{+3.4} \, \rm km \, s^{-1} \, Mpc^{-1}$. Achieving a measurement precision of approximately 5% from a single source demonstrates the considerable potential of this method for independent determinations of the Hubble constant. Our determination of $H_{0}$ is broadly consistent, within the current uncertainties, with both early- and late-Universe measurements. Future applications of the method to larger datasets, particularly those provided by the Vera Rubin Observatory, are expected to reduce the uncertainty to below 1%, thereby establishing this approach as a powerful independent probe of the Hubble tension.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 1 minor

Summary. The manuscript presents a new method to measure the Hubble constant H0 independently of the cosmic distance ladder by jointly fitting the continuum lag spectrum and broadband SED of the Seyfert galaxy Fairall 9 using the H0RIZON-AGN model, which incorporates radiation reprocessing in a cold accretion disk. From a dedicated monitoring campaign, the authors derive H0 = 72.4_{-3.7}^{+3.4} km s^{-1} Mpc^{-1} at approximately 5% precision and note consistency with both early- and late-Universe measurements, with potential for sub-1% precision using larger samples from future surveys.

Significance. If the H0RIZON-AGN model assumptions hold without significant unmodeled systematics, the result provides a genuinely independent cosmological probe at useful precision from a single AGN. The approach avoids cross-calibration issues and could become a valuable addition to Hubble tension studies once applied to larger datasets. The dedicated monitoring and model development are positive steps toward falsifiable, geometry-based distance estimates.

major comments (1)
  1. [H0RIZON-AGN model description and fitting procedure] The central H0 result is obtained by fitting the H0RIZON-AGN model (including luminosity distance via H0) to the observed lags and SED. The model assumes reprocessing occurs only in a standard thin disk with temperature profile and response function fully specified by the free parameters (BH mass, accretion rate, inclination, H0). If Fairall 9 has disk winds, non-standard radial temperature index, or inclination-dependent scattering, the predicted lag spectrum will be offset, biasing the inferred distance (and H0) at a level comparable to the quoted uncertainty. No validation against hydrodynamic simulations or a second AGN with independent geometric distance is reported.
minor comments (1)
  1. [Abstract] The abstract and introduction would benefit from an explicit statement of the key model assumptions (e.g., thin-disk temperature profile, neglect of winds) so readers can immediately assess the scope of the claim.

Simulated Author's Rebuttal

1 responses · 1 unresolved

We thank the referee for their constructive review and for acknowledging the potential of this independent approach to measuring H0. We respond to the major comment below.

read point-by-point responses

  1. Referee: [H0RIZON-AGN model description and fitting procedure] The central H0 result is obtained by fitting the H0RIZON-AGN model (including luminosity distance via H0) to the observed lags and SED. The model assumes reprocessing occurs only in a standard thin disk with temperature profile and response function fully specified by the free parameters (BH mass, accretion rate, inclination, H0). If Fairall 9 has disk winds, non-standard radial temperature index, or inclination-dependent scattering, the predicted lag spectrum will be offset, biasing the inferred distance (and H0) at a level comparable to the quoted uncertainty. No validation against hydrodynamic simulations or a second AGN with independent geometric distance is reported.

    Authors: We agree that the H0RIZON-AGN model is constructed under the standard thin-disk reprocessing assumptions, with the temperature profile and response function set by the free parameters (black-hole mass, accretion rate, inclination, and H0) as described in the manuscript. We acknowledge that unaccounted effects such as disk winds, a non-standard radial temperature index, or inclination-dependent scattering could shift the predicted lag spectrum and thereby bias the inferred luminosity distance and H0 at a level comparable to the reported uncertainty. In the revised manuscript we have added an explicit discussion of these model assumptions and the possible impact of such deviations. Regarding validation, the present work constitutes the first application of the method to a single source. We do not report comparisons against hydrodynamic simulations or against a second AGN possessing an independent geometric distance; such tests lie outside the scope of this initial demonstration and are reserved for future studies with expanded samples. revision: partial

standing simulated objections not resolved
  • Validation of the H0RIZON-AGN model against hydrodynamic simulations or application to a second AGN with an independent geometric distance measurement

Circularity Check

0 steps flagged

No significant circularity; H0 obtained via standard model fit to external data

full rationale

The paper derives H0 by jointly fitting the H0RIZON-AGN model parameters (including distance) to the observed continuum lag spectrum and broadband SED of Fairall 9. This is a direct parameter estimation from independent observational inputs using a physical reprocessing model, not a reduction of the claimed result to its own inputs by construction. No self-definitional equations, fitted subsets renamed as predictions, or load-bearing self-citations appear in the provided text. The method is presented as independent of the distance ladder, with the output H0 value arising from the data fit rather than being presupposed.

Axiom & Free-Parameter Ledger

2 free parameters · 1 axioms · 0 invented entities

Based on the abstract alone, the central claim rests on the validity of the H0RIZON-AGN reprocessing model and the assumption that H0 can be extracted directly from the joint fit without hidden calibrations to other distance indicators. No explicit free parameters beyond the model fit itself are listed.

free parameters (2)
  • H0 = 72.4
    Extracted as the distance parameter that reconciles the physical size implied by lags with the observed flux in the SED fit.
  • accretion-disk model parameters (BH mass, accretion rate, inclination)
    Simultaneously adjusted in the joint lag-spectrum and SED fit.
axioms (1)
  • domain assumption Radiation reprocessing in a cold, standard accretion disk fully accounts for the observed continuum lags and SED shape in Fairall 9.
    Invoked by the H0RIZON-AGN model description in the abstract.

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