# VLBI observations of four radio quasars at $z>4$: blazars or not?

## TL;DR

This study used VLBI imaging to investigate four high-redshift radio quasars, confirming one as a blazar and revealing diverse jet orientations, highlighting the importance of high-resolution imaging in classifying AGN types.

## Contribution

The paper provides high-resolution VLBI observations of four $z>4$ radio sources, clarifying their blazar nature and jet orientations, which challenges previous classifications based on spectral energy distributions.

## Key findings

- J2134$-$0419 is confirmed as a blazar with relativistic beaming.
- J0839+5112 shows a compact, flat-spectrum core but no Doppler boosting.
- Two candidates exhibit large-inclination, double-lobed structures contrary to blazar expectations.

## Abstract

Blazars are active galactic nuclei (AGN) whose relativistic jets point nearly to the line of sight. Their compact radio structure can be imaged with very long baseline interferometry (VLBI) on parsec scales. Blazars at extremely high redshifts provide a unique insight into the AGN phenomena in the early Universe. We observed four radio sources at redshift $z>4$ with the European VLBI Network (EVN) at 1.7 and 5 GHz. These objects were previously classified as blazar candidates based on X-ray observations. One of them, J2134$-$0419 is firmly confirmed as a blazar with our VLBI observations, due to its relativistically beamed radio emission. Its radio jet extended to $\sim$10 milli-arcsec scale makes this source a promising target for follow-up VLBI observations to reveal any apparent proper motion. Another target, J0839+5112 shows a compact radio structure typical of quasars. There is evidence for flux density variability and its radio "core" has a flat spectrum. However, the EVN data suggest that its emission is not Doppler-boosted. The remaining two blazar candidates (J1420+1205 and J2220+0025) show radio properties totally unexpected from radio AGN with small-inclination jet. Their emission extends to arcsec scales and the Doppler factors of the central components are well below 1. Their structures resemble that of double-lobed radio AGN with large inclination to the line of sight. This is in contrast with the blazar-type modeling of their multi-band spectral energy distributions. Our work underlines the importance of high-resolution VLBI imaging in confirming the blazar nature of high-redshift radio sources.