A powerful (and likely young) radio-loud quasar at z = 5.3

¹ INAF – Osservatorio Astronomico di Brera, Via Brera, 28, 20121 Milano, Italy
e-mail: silvia.belladitta@inaf.it
² DiSAT, Università degli Studi dell’Insubria, Via Valleggio 11, 22100 Como, Italy
³ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁴ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁵ INAF – Fundación Galileo Galilei, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
⁶ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica (IASF), Via A. Corti 12, 20133 Milano, Italy

Received: 24 April 2022
Accepted: 20 October 2022

Abstract

We present the discovery of PSO J191.05696+86.43172 (hereafter PSO J191+86), a new powerful radio-loud quasar (QSO) in the early Universe (z = 5.32). We discovered it by cross-matching the NRAO VLA Sky Survey (NVSS) radio catalog at 1.4 GHz with the first data release of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS PS1) in the optical. With a NVSS flux density of 74.2 mJy, PSO J191+86 is one of the brightest radio QSO discovered at z ∼ 5. The intensity of its radio emission is also confirmed by the very high value of radio loudness (R > 300). The observed radio spectrum of PSO J191+86 shows a possible turnover around ∼1 GHz (i.e., ∼6 GHz in the rest frame), making it a gigahertz-peaked spectrum (GPS) source. However, variability could affect the real shape of the radio spectrum, because the data in hand were taken ∼25 years apart. By assuming a peak in the observed radio spectrum between 1 and 2 GHz (i.e., ∼6 and 13 GHz in the rest-frame) we find a linear size of the source of ∼10–30 pc and a corresponding kinetic age of 150–460 yr. This would make PSO J191+86 a newly born radio source. However, the large X-ray luminosity (5.3×10⁴⁵ erg s⁻¹), the flat X-ray photon index (Γ_X = 1.32), and the optical–X-ray spectral index (α_ox̃ = 1.329) are typical of blazars. This could indicate that the nonthermal emission of PSO J191+86 is Doppler boosted. Further radio observations (both on arcsec and parsec scales) are necessary to better investigate the nature of this powerful radio QSO.