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The existence of accreting supermassive black holes up to billions of solarmasses at early cosmological epochs (in the context of this work, redshiftsz>=6) requires very fast growth rates which is challenging to explain. Thepresence of a relativistic jet can be a direct indication of activity andaccretion status in active galactic nuclei (AGN), constraining the radiativeproperties of these extreme objects. However, known jetted AGN beyond z~6 arestill very rare. The radio-emitting AGN J2331+1129 has recently been claimed asa candidate BL Lac object at redshift z=6.57, based on itssynchrotron-dominated emission spectrum and the lack of ultraviolet/opticalemission lines. It is a promising candidate for the highest-redshift blazarknown to date. The aim of the observations described here was to support orrefute the blazar classification of this peculiar source. We performedhigh-resolution radio interferometric imaging observations of J2331+1129 usingthe Very Long Baseline Array at 1.6 and 4.9 GHz in 2022 Feb. The imagesrevealed a compact but slightly resolved, flat-spectrum core feature at bothfrequencies, indicating that the total radio emission is produced by a compactjet and originates from within a central 10-pc scale region. While these areconsistent with the radio properties of a BL Lac object, the inferredbrightness temperatures are at least an order of magnitude lower than expectedfrom a Doppler-boosted radio jet, leaving the high-redshift BL Lacidentification still an open question.

Milliarcsecond-scale radio structure of the most distant BL Lac object candidate at redshift 6.57