Letter to the Editor

Parsec-scale properties of the radio brightest jetted AGN at z > 6

¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
² INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
e-mail: spingola@ira.inaf.it
³ INAF – Osservatorio Astronomico di Brera, Via Brera, 28, 20121 Milano, Italy
⁴ DiSAT, Università degli Studi dell’Insubria, Via Valleggio 11, 22100 Como, Italy

Received: 17 September 2020
Accepted: 20 October 2020

Abstract

We present Director’s Discretionary Time multi-frequency observations obtained with the Jansky Very Large Array and the Very Long Baseline Array (VLBA) of the blazar PSO J030947.49+271757.31 (hereafter PSO J0309+27) at z = 6.10 ± 0.03. The milliarcsecond angular resolution of our VLBA observations at 1.5, 5, and 8.4 GHz unveils a bright one-sided jet extended for ∼500 parsecs in projection. This high-z radio-loud active galactic nucleus is resolved into multiple compact sub-components that are embedded in a more diffuse and faint radio emission that enshrouds them in a continuous jet structure. We directly derive limits on some physical parameters from observable quantities such as viewing angle and Lorentz and Doppler factors. If PSO J0309+27 is a genuine blazar, as suggested by its X-ray properties, then we find that its bulk Lorentz factor must be relatively low (lower than 5). This value would be in favour of a scenario currently proposed to reconcile the paucity of high-z blazars with current predictions. Nevertheless, we cannot exclude that PSO J0309+27 is seen under a larger viewing angle, which would imply that the X-ray emission must be enhanced, for example, by inverse Compton scattering with the cosmic microwave background. More stringent constraints on the bulk Lorentz factor in PSO J0309+27 and on these factors in the other high-z blazars are necessary to test whether their properties are intrinsically different from those of the low-z blazar population.