VERITAS and VLBA Observations of HESS J1943+213

TL;DR

This paper presents deep VERITAS and VLBA observations confirming HESS J1943+213 as an extreme blazar with stable VHE gamma-ray emission, extended jet structure, and typical blazar spectral indices, contributing to its classification.

Contribution

First VLBA observations of HESS J1943+213 at multiple frequencies revealing extended jet structure and core-jet spectral indices, supporting its classification as an extreme blazar.

Findings

Detected the source with ~20 sigma significance in 38 hours of VERITAS data.

Confirmed extended jet-like structure and core variability with VLBA at multiple frequencies.

HESS J1943+213 exhibits stable VHE gamma-ray flux and spectrum, typical for blazars.

Abstract

HESS J1943+213 is a very high energy (VHE; 100 GeV) {\gamma}-ray source in the direction of the Galactic plane. 38 hours of deep VERITAS observations taken over two seasons detect the source with ~20 {\sigma} significance. Monitoring observations of HESS J1943+213 show a remarkably stable flux and spectrum in VHE {\gamma}-rays. Studies exploring the classification of HESS J1943+213 are converging towards accepting the source as an extreme synchrotron BL Lac object. Specifically, overall SED characteristics of the source, the detection of a potential host galaxy in near-IR imaging, and VLBI observations of the HESS J1943+213 radio counterpart showing extended jet-like emission at milliarcsecond scale and core flux density variability establish the source as a blazar. Recent Very Long Baseline Array (VLBA) observations of the source (shown here for the first time) confirm the extended structure found in the 1.6 GHz band and detect the jet-like component in the 4.6 GHz and the 7.3 GHz bands. The spectral indices of the core and the jet-like components derived from the VLBA observations are in a range typical for blazars. HESS J1943+213 fits the extreme HBL description; however, with variability detected only at milliarcseconds scales in radio, it appears to be an abnormally stable VHE {\gamma}-ray blazar.