NVSS J151002+570243: Accretion and spin of a z > 4 Fermi-detected blazar

¹ Dipartimento di Fisica “Aldo Pontremoli”, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
² Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy
³ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, (LC), Italy

^⋆ Corresponding author: alzati.2200689@studenti.uniroma1.it

Received: 20 December 2024
Accepted: 13 May 2025

Abstract

Active galactic nuclei (AGN) at high redshifts feature two of the most peculiar characteristics related to key open questions in AGN physics: they are overly massive, which is not explainable via sub-Eddington accretion, and most of their population hosts relativistic jets that are commonly connected to spinning black holes and are hence less effective in accreting mass onto the black hole. The formation and evolution of such massive objects is currently an open puzzle. Being the most distant blazar consistently detected by Fermi/LATNVSS, J151002+570243 is part of this population, thereby hosting a powerful jet. We tested the hypothesis of a super-Eddington accretion process for this source by modeling its big blue bump with a set of accretion disk emission models. We first tested a standard, geometrically thin, optically thick α-disk, obtaining a mass of Log M/M_⊙ = 8.65 ± 0.19 consistent with virial-based results and a significantly sub-Eddington accretion rate of λ = 0.02 ± 0.01. We then focused on the analytic approximations of two numerical models (KERBB and SLIMBH) that take into account the general relativity effects of a spinning black hole (reasonably due to the presence of a jet) and a close-to- or super-Eddington accretion rate . Despite the focus on super-critical accretion, these models confirm a surprisingly low Eddington ratio on the order of 3%. The hypothesis of a continuous accretion at this measured rate is unrealistic, since it would imply a seed black hole mass of ∼10⁶ − 10⁸ M_⊙ at redshift z = 20. Hence, we explore the possibility of a continuous super-critical accretion starting from a ∼10² M_⊙ seed that would spin up the black hole and eventually contribute to launching the relativistic jet. The measured low accretion rate would thus only occur once the jet is active. This idea would reconcile the existence at z > 4 of extremely massive black holes accreting at somewhat slow rates, with the need for an extremely fast evolution by allowing the formation of stellar-sized black hole seeds even as late as at z ∼ 8.