The loud and the quiet: searching for radio counterparts of two
  radio-weak BL Lac candidates with VLBI
  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. (2017). Both sources were selected as candidate low-energy
counterparts of unassociated Fermi {\gamma}-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 milli-arcsecond (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 < ~10 mas region. J1410+7405 is
therefore identified as a radio-loud jetted active galactic nucleus, and its
brightness temperature exceeds ~10^9 K. Its properties are similar to those of
other {\gamma}-ray detected BL Lac objects. On the other hand, the second
target, J0644+6031, remains undetected with the EVN with a 6-{\sigma}
brightness upper limit of 0.12 mJy/beam. This source is thus radio-quiet,
confirming its peculiarity, or possibly questioning its BL Lac nature.
