The Most Distant Blazar

The authors present Director's Discretionary Time multi-frequency observations obtained with the Jansky Very Large Array (VLA) 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 milli- arcsecond angular resolution of the 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, embedded in a more diffuse and faint radio emission, which enshrouds them in a continuous jet structure. The authors derive limits on some physical parameters directly from the observable quantities, such as viewing angle, Lorentz and Doppler factors. If PSO J0309+27 is a genuine blazar, as suggested by its X-ray properties, then its bulk Lorentz factor must be relatively low (less than 5). Such values would favor a scenario currently proposed to reconcile the paucity of high-z blazars with respect to current predictions. Nevertheless, the authors cannot exclude that PSO J0309+27 is seen under a larger viewing angle, which would imply that the X-ray emission must be enhanced, e.g., by inverse Compton with the Cosmic Microwave Background. More stringent constraints on the bulk Lorentz factor in PSO J0309+27 and the other high-z blazars are necessary to test whether their properties are intrinsically different with respect to the low-z blazar population.

Image Caption: [Left to Right ] VLBA images of the z=6.1 blazar at 1.5 GHz, 5 GHz, and 8.4 GHz, down to 2 milli-arcsecond resolution

Publication: Cristiana Spingola (Bologna, INAF) et al. Parsec-scale Properties of the Radio Brightest Jetted AGN at z > 6, Astronomy & Astrophysics, 643, L12 (November 2020).

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