The loud and the quiet: searching for radio counterparts of two radio-weak BL Lac candidates with VLBI

Abstract

ABSTRACT BL Lac objects are known to have compact jets inclined to our line of sight at a small angle, showing prominent radio emission. Two radio-weak BL Lac candidates with no counterparts in current radio surveys were recently reported by Massaro et al. Both sources were selected as candidate low-energy counterparts of unassociated Fermi γ-ray sources. We carried out very long baseline interferometry (VLBI) observations with the European VLBI Network (EVN) at 5 GHz to explore their radio properties at the milliarcsecond (mas) scale. One target, J1410+7405, is clearly detected with the EVN. Its measured 5-GHz flux density, 2.4 mJy, is consistent with recent interferometric measurements with the Karl G. Jansky Very Large Array, suggesting that the radio emission is confined to the inner ${\lesssim}10$-mas region. J1410+7405 is therefore identified as a radio-loud jetted active galactic nucleus, and its brightness temperature exceeds ∼109 K. Its properties are similar to those of other γ-ray-detected BL Lac objects. On the other hand, the second target, J0644+6031, remains undetected with the EVN with a 6σ brightness upper limit of 0.12 mJy beam−1. This source is thus radio-quiet, confirming its peculiarity, or possibly questioning its BL Lac nature.