The Total and Polarized Radio Emission from the Innermost Jets of a High-Redshift Quasar and a Candidate at Parsec-Scale Resolution

Journal article, 2026

High-frequency very long baseline interferometry (VLBI) polarimetry probes synchrotron-emitting plasma closer to the central engines of radio-loud active galactic nuclei (AGNs), but observations above 43 GHz are technically demanding. We present 22-GHz European VLBI Network observations of the (Formula presented.) quasar J1510+5702 and J1606+3124, whose published spectroscopic redshift, (Formula presented.), is uncertain; a photometric estimate gives (Formula presented.). For the published (Formula presented.) redshifts, 22 GHz corresponds to rest-frame frequencies above 118 GHz. Polarized emission is detected in J1510+5702, and a low-level polarized signal is recovered from the brightest feature of J1606+3124. Adopting (Formula presented.), that feature has a brightness temperature of (Formula presented.) K, allowing a mildly Doppler-boosted interpretation, while the young compact-source scenario also remains viable. The core of J1510+5702 has (Formula presented.) K, implying a Doppler factor of (Formula presented.) under the equipartition assumption. This component has a (Formula presented.) % fractional polarization. These observations show that cm-wavelength VLBI can access rest-frame millimeter-band polarization in bright (Formula presented.) jets.