Akiyama, KazunoriKazunoriAkiyamaStawarz, ŁukaszŁukaszStawarzTanaka, Yasuyuki T.Yasuyuki T.TanakaNagai, HiroshiHiroshiNagaiGIROLETTI, MARCELLOMARCELLOGIROLETTIHonma, MarekiMarekiHonma2020-05-272020-05-2720162041-8205http://hdl.handle.net/20.500.12386/25221We report on the 1.6 GHz (18 cm) very long baseline interferometry (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 ≳ 1.8× {10}<SUP>9</SUP> K, which is typical for low-luminosity blazars in general. Overall, the 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 because HESS J1943+213 does not reveal any optical or infrared signatures of the active galactic nucleus activity, it would never be recognized and identified as a BL Lac object if not for its location close to the Galactic plane where the High Energy Stereoscopic System surveyed for 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.STAMPAenArticle10.3847/2041-8205/823/2/L262-s2.0-84975068896000377031700005https://iopscience.iop.org/article/10.3847/2041-8205/823/2/L262016ApJ...823L..26AFIS/05 - ASTRONOMIA E ASTROFISICA