HESS J1943+213: An Extreme Blazar Shining Through The Galactic Plane
  HESS J1943+213 is a very-high-energy (VHE; $>$100 GeV) $\gamma$-ray source in
the direction of the Galactic Plane. Studies exploring the classification of
the source are converging towards its identification as an extreme synchrotron
BL Lac object. Here we present 38 hours of VERITAS observations of HESS
J1943+213 taken over two years. The source is detected with $\sim$20 standard
deviations significance, showing a remarkably stable flux and spectrum in VHE
$\gamma$-rays. Multi-frequency very-long-baseline array (VLBA) observations of
the source confirm the extended, jet-like structure previously found in the 1.6
GHz band with European VLBI Network and detect this component in the 4.6 GHz
and the 7.3 GHz bands. The radio spectral indices of the core and the jet and
the level of polarization derived from the VLBA observations are in a range
typical for blazars. Data from VERITAS, $Fermi$-LAT, $Swift$-XRT, FLWO 48$''$
telescope, and archival infrared and hard X-ray observations are used to
construct and model the spectral energy distribution (SED) of the source with a
synchrotron-self-Compton model. The well-measured $\gamma$-ray peak of the SED
with VERITAS and $Fermi$-LAT provides constraining upper limits on the source
redshift. Possible contribution of secondary $\gamma$-rays from
ultra-high-energy cosmic ray-initiated electromagnetic cascades to the
$\gamma$-ray emission is explored, finding that only a segment of the VHE
spectrum can be accommodated with this process. A variability search is
performed across X-ray and $\gamma$-ray bands. No statistically significant
flux or spectral variability is detected.
