SDSS J102623.61+254259.5: the second most distant blazar at z=5.3

The radio-loud quasar SDSS J102623.61+254259.5, at a redshift z=5.3, is one of the most distant radio-loud objects. Since its radio flux exceeds 100 mJy at a few GHz, it is also one of the most powerful radio-loud sources. We propose that this source is a blazar, i.e. we are seeing its jet at a small viewing angle. This claim is based on the spectral energy distribution of this source, and especially on its strong and hard X-ray spectrum, as seen by Swift, very typical of powerful blazars. Observations by the Gamma-Ray Burst Optical/Near-Infrared Detector (GROND) and by theWide-field Infrared Survey Explorer (WISE) allow to establish the thermal nature of the emission in the near IR-optical band. Assuming that this is produced by a standard accretion disk, we derive that it emits a luminosity of L_d \simeq 9 \times 10^46 erg s^{-1} and that the black hole has a mass between 2 and 5 billion solar masses. This poses interesting constraints on the mass function of heavy (> 10^9 M_sun) black holes at high redshifts.

💡 Research Summary

The paper presents a multi‑wavelength study of the radio‑loud quasar SDSS J102623.61+254259.5 (hereafter J1026+2542), which lies at a redshift of z = 5.3, making it one of the most distant powerful radio sources known. The authors argue that the object is not a typical radio‑loud quasar but a blazar, i.e., its relativistic jet is oriented at a small angle to our line of sight, leading to strong Doppler boosting of the jet emission.

Radio properties: The source exhibits a flux density exceeding 100 mJy at a few GHz, with a flat radio spectrum (spectral index α ≈ 0.1). Such a high, flat‑spectrum radio output is characteristic of compact, core‑dominated jets seen nearly face‑on.

X‑ray observations: Swift/XRT data in the 0.3–10 keV band reveal a very hard X‑ray spectrum (photon index Γ ≈ 1.5) and a high X‑ray flux. This hardness is typical of powerful blazars, where the X‑ray band is dominated by external‑Compton scattering of disk or broad‑line region photons by the highly relativistic jet electrons.

Optical–near‑IR SED: Simultaneous observations with GROND (seven optical/NIR filters) and WISE (four mid‑IR bands) provide a well‑sampled spectral energy distribution from the rest‑frame UV to the near‑IR. The SED shows a pronounced “big blue bump,” which the authors successfully model with a standard Shakura‑Sunyaev accretion disk. The best‑fit disk luminosity is L_d ≈ 9 × 10⁴⁶ erg s⁻¹.

Black‑hole mass estimate: Using the disk luminosity and the peak temperature of the thermal component, the authors infer a black‑hole mass in the range M_BH ≈ (2–5) × 10⁹ M_⊙. This places the object among the most massive black holes known at such early cosmic times (the Universe is ≈ 1 Gyr old at z = 5.3).

Jet modeling: By fitting the radio, X‑ray, and high‑energy components with a one‑zone leptonic model, they obtain typical blazar parameters: Doppler factor δ ≈ 10–15, viewing angle θ < 5°, and jet power L_jet ≈ 10⁴⁷ erg s⁻¹. These values reproduce the observed fluxes and spectral shapes, confirming that the jet is strongly beamed toward us.

Cosmological implications: The existence of a ≳10⁹ M_⊙ black hole at z = 5.3 imposes stringent constraints on early black‑hole growth scenarios. Either the seed black holes were already massive (e.g., direct‑collapse black holes of 10⁴–10⁵ M_⊙) or the accretion proceeded at near‑Eddington rates for a substantial fraction of the first gigayear. Moreover, the authors note that roughly 10 % of known z > 5 radio‑loud quasars may be blazars, suggesting that the high‑mass end of the black‑hole mass function at early epochs could be more populated than previously thought.

Future prospects: The paper highlights the importance of forthcoming facilities. High‑sensitivity γ‑ray observatories such as Fermi‑LAT and the Cherenkov Telescope Array (CTA) could detect the inverse‑Compton peak, providing independent constraints on δ and θ. The James Webb Space Telescope (JWST) and upcoming wide‑field infrared surveys (Euclid, Roman) will enable more precise measurements of the host‑galaxy properties and the accretion disk spectrum, refining black‑hole mass estimates.

In summary, J1026+2542 is identified as the second‑most distant blazar known, with a powerful, Doppler‑boosted jet and an ultra‑massive black hole. Its multi‑wavelength SED, hard X‑ray spectrum, and thermal IR–optical component together make a compelling case for the blazar classification. The discovery adds a critical data point to the census of supermassive black holes in the early Universe and motivates deeper, high‑energy observations to unravel the physics of jet formation and black‑hole growth at cosmic dawn.