HESS J1943+213: An Extreme Blazar Shining through the Galactic Plane

The Veritas Collaboration, A. Archer, W. Benbow, R. Bird, R. Brose, M. Buchovecky, V. Bugaev, W. Cui, M. K. Daniel, A. Falcone, Q. Feng, J. P. Finley, A. Flinders, L. Fortson, A. Furniss, G. H. Gillanders, M. Hütten, D. Hanna, O. Hervet, J. HolderG. Hughes, T. B. Humensky, C. A. Johnson, P. Kaaret, P. Kar, N. Kelley-Hoskins, D. Kieda, M. Krause, F. Krennrich, S. Kumar, M. J. Lang, T. T.Y. Lin, S. McArthur, P. Moriarty, R. Mukherjee, D. Nieto, S. O'Brien, R. A. Ong, A. N. Otte, N. Park, A. Petrashyk, M. Pohl, A. Popkow, E. Pueschel, J. Quinn, K. Ragan, P. T. Reynolds, G. T. Richards, E. Roache, C. Rulten, I. Sadeh, G. H. Sembroski, K. Shahinyan, J. Tyler, S. P. Wakely, O. M. Weiner, A. Weinstein, R. M. Wells, P. Wilcox, A. Wilhelm, D. A. Williams, W. F. Brisken, P. Pontrelli

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4 Scopus citations


HESS J1943+213 is a very high energy (VHE; >100 GeV) γ-ray source in the direction of the Galactic plane. Studies exploring the classification of the source are converging toward its identification as an extreme synchrotron BL Lac object. Here we present 38 hr of VERITAS observations of HESS J1943+213 taken over 2 yr. The source is detected with a significance of ∼20 standard deviations, showing a remarkably stable flux and spectrum in VHE γ-rays. Multifrequency Very Long Baseline Array (VLBA) observations of the source confirm the extended, jet-like structure previously found in the 1.6 GHz band with the European VLBI Network and detect this component in the 4.6 and 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, the 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 γ-ray peak of the SED with VERITAS and Fermi-LAT provides constraining upper limits on the source redshift. Possible contribution of secondary γ-rays from ultra-high-energy cosmic-ray-initiated electromagnetic cascades to the γ-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 γ-ray bands. No statistically significant flux or spectral variability is detected.

Original languageEnglish (US)
Article number41
JournalAstrophysical Journal
Issue number1
StatePublished - Jul 20 2018

Bibliographical note

Funding Information:
This research is supported by grants from the U.S. Department of Energy Office of Science, the U.S. National Science Foundation, and the Smithsonian Institution and by NSERC in Canada. We acknowledge the excellent work of the technical support staff at the Fred Lawrence Whipple Observatory and at the collaborating institutions in the construction and operation of the instrument.

Funding Information:
The VLBA is operated by the Long Baseline Observatory. The Long Baseline Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.


  • BL Lacertae objects: individual (HESS J1943+213, VER J1943+213)
  • astroparticle physics
  • galaxies: active
  • galaxies: jets
  • galaxies: nuclei
  • gamma rays: galaxies

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