HESS J1943+213: An Extreme Blazar Shining Through The Galactic Plane

TL;DR

This study confirms HESS J1943+213 as an extreme blazar through extensive multi-wavelength observations, revealing a stable VHE gamma-ray flux, jet-like radio structure, and constraining its redshift and emission mechanisms.

Contribution

It provides the first detailed multi-frequency analysis and modeling of HESS J1943+213, establishing its classification as an extreme blazar with stable emission and constraining its physical properties.

Findings

Detected with ~20 sigma significance in VHE gamma-rays.

Confirmed jet-like radio structure with VLBA observations.

Modeled the spectral energy distribution with a synchrotron-self-Compton model.

Abstract

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.