VERITAS Observations of the Unidentified Point Source HESS J1943+213

TL;DR

This paper reports the VERITAS detection and analysis of the unidentified point source HESS J1943+213, likely a distant BL Lac object, using high-elevation observations to clarify its nature through spectral and variability studies.

Contribution

It provides the most significant VHE detection of HESS J1943+213 and uses multi-wavelength data to support its classification as a BL Lac object behind the Galactic plane.

Findings

Most significant VHE detection above 200 GeV with 18σ significance.

Spectral analysis shows a steep spectrum consistent with a BL Lac object.

Variability analysis supports the active galactic nucleus hypothesis.

Abstract

The H.E.S.S. Galactic plane scan has revealed a large population of Galactic very high energy (VHE; E > 100 GeV) emitters. The majority of the galactic sources are extended and can typically be associated with pulsar wind nebulae (35%) and supernova remnants (21%), while some of the sources remain unidentified (31%). A much smaller fraction of point-like sources (5 in total, corresponding to 4%) are identified as gamma-ray binaries. Active galactic nuclei located behind the Galactic plane are also a potential source class. An active galaxy could be identified in the VHE regime by a point-like appearance, a high variability amplitude (up to a factor of 100) and a typically soft spectrum (due to absorption by the extra-galactic background light). Here we report on VERITAS observations of HESS J1943+213, an unidentified point source discovered to emit above 470 GeV during the extended H.E.S.S. Galactic plane scan. This source is thought to be a distant BL Lac object behind the Galactic plane and, though it exhibits a steep spectrum it is a weak GeV source, only recently detected using 5 years of Fermi-LAT data. Deep VERITAS observations at high elevations result in the most significant VHE detection of this object so far, with an excess above 200 GeV of more than 18 standard deviations. We use variability and spectral analyses of VERITAS data on HESS J1943+213 in a multi-wavelength context to address the source classification.