Black hole mass estimation through accretion disk spectral fitting for high-redshift blazars

¹ Department of Physics, National and Kapodistrian University of Athens, University Campus Zografos, GR 15784 Athens, Greece
² Institute of Accelerating Systems & Applications, University Campus Zografos, Athens, Greece

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 September 2025
Accepted: 16 January 2026

Abstract

Context. High-redshift (z > 2) blazars, with relativistic jets aligned toward us, probe the most powerful end of the active galactic nuclei (AGNs) population.

Aims. We aimed to determine the black hole masses and mass accretion rates of high-z blazars in a common framework that utilizes a Markov chain Monte Carlo (MCMC) fitting method and the Shakura-Sunayev multi-temperature accretion disk model, accounting also for attenuation due to neutral hydrogen gas in the intergalactic medium (IGM).

Methods. We compiled a sample of 23 high-redshift blazars from the literature with publicly available infrared-to-ultraviolet photometric data. We performed a Bayesian fit to the spectral energy distribution (SED) of the accretion disk, accounting for upper limits, and determined the black hole masses and mass accretion rates with their uncertainties. We also examined the impact of optical-ultraviolet attenuation due to gas in the IGM.

Results. We find that neglecting IGM attenuation in SED fits leads to systematically larger black hole mass estimates and correspondingly lower Eddington ratios, with the bias becoming more severe at higher redshift. Our MCMC fits yield median black hole masses in the range ∼(10⁸ − 10¹⁰) M_⊙ and a broad distribution of median Eddington ratios (λ_Edd ∼ 0.04 up to ∼1), with several objects accreting at λ_Edd ≳ 0.4 both at z ∼ 2 − 3 and at z ≳ 5. Comparison with previous literature shows no clear method-dependent systematic offsets, although individual mass estimates can differ by up to a factor of a few. We also demonstrate that assumptions about black hole spin introduce a systematic degeneracy, changing the inferred M_BH and Ṁ by factors up to five.

Conclusions. This work is, to our knowledge, the first systematic study to model the accretion-disk emission of a large sample of high-z blazars within a single, consistent statistical framework. Our results emphasize the importance of accounting for IGM attenuation and of using uniform fitting methods when comparing disk-based black hole estimates across samples.