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

TL;DR

This study used VLBI observations to investigate two radio-weak BL Lac candidates, confirming one as a radio-loud active galactic nucleus and the other as radio-quiet, thus providing insights into their radio properties and classifications.

Contribution

First VLBI observations of two radio-weak BL Lac candidates, revealing one as a radio-loud AGN and the other as radio-quiet, clarifying their nature.

Findings

J1410+7405 is a radio-loud AGN with compact jets and high brightness temperature.

J0644+6031 remains undetected, indicating it is radio-quiet or atypical.

Results support the diversity in radio properties among BL Lac candidates.

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. (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.