Fermi Gamma-ray Space Telescope

VERITAS observations of the unidentified point source HESS J1943+213

Karsten Berger
on behalf of the VERITAS Collaboration

Abstract:

The H.E.S.S. Galactic plane scan has revealed a large population of Galactic very high energy (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 hiding behind the Galactic plane are also a potential source class. An active galaxy could be identified by a point like extension, high variability amplitude (up to a factor of 100) and typically steep 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 500 GeV during the extended H.E.S.S. Galactic plane scan. This source is thought to be a distant blazar behind the Galactic plane and, although the source exhibits a steep spectrum, it has not been detected by the Fermi LAT in the 2FGL and 1FHL source catalogs. Deep VERITAS observations at high elevation resulted in the most significant detection of this object so far, with an excess above 100 GeV of more than 20 standard deviations. The lower energy threshold of the VERITAS observations allows the connection of the energy spectrum to the energy range accessible by Fermi LAT and to search for curvature in the spectrum. We also use the VERITAS variability and spectral analysis in a multi-wavelength context to discuss the possible source classification.