EVN Observations of HESS J1943+213: Evidence for an Extreme TeV BL Lac
  Object
  We report on the 1.6 GHz (18 cm) VLBI observations of the unresolved, steady
TeV source HESS J1943+213 located in the Galactic plane, performed with the
European VLBI Network (EVN) in 2014. Our new observations with a nearly full
EVN array provide the deepest image of HESS J1943+213 at the highest resolution
ever achieved, enabling us to resolve the long-standing issues of the source
identification. The milliarcsecond-scale structure of HESS J1943+213 has a
clear asymmetric morphology, consisting of a compact core and a diffuse
jet-like tail. This is broadly consistent with the previous e-EVN observations
of the source performed in 2011, and re-analyzed in this work. The core
component is characterized by the brightness temperature of $\gtrsim1.8 \times
10^9$ K, which is typical for low-luminosity blazars in general. Overall, radio
properties of HESS J1943+213 are consistent with the source classification as
an "extreme high-frequency-peaked BL Lac object". Remarkably, we note that
since HESS J1943+213 does not reveal any optical or infrared signatures of the
AGN activity, it would never be recognized and identified as a BL Lac object,
if not its location close to the Galactic plane where the High Energy
Stereoscopic System has surveyed, and the follow-up dedicated X-ray and radio
studies triggered by the source detection in the TeV range. Our results suggest
therefore a presence of an unrecognized, possibly very numerous population of
particularly extreme HBLs, and simultaneously demonstrate that the
low-frequency VLBI observations with high-angular resolution are indispensable
for a proper identification of such objects.
