The γ-Ray-emitting BL Lac 4FGL J1219.0+3653 in the Radio

Abstract We collect radio data of 4FGL J1219.0+3653, the first γ -ray BL Lac candidate known above redshift 3. We analyze total flux density measurements covering a wide frequency range (0.33–5 GHz) as well as the only available very long baseline interferometry (VLBI) data. The radio spectrum is flat. Based on the radio data, there is no evidence that the compact jet emission is Doppler-enhanced, at odds with the BL Lac classification. The Gaia-VLBI optical-radio positional offset is ~9 mas which appears larger than usual in a blazar-like source but is still within 3 σ uncertainties.