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HESS J1943+213: a candidate extreme BL Lacertae object
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2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 529, A49- p.Article in journal (Refereed) Published
Abstract [en]

Context. The H. E. S. S. Cherenkov telescope array has been surveying the Galactic plane for new VHE (>100 GeV) gamma-ray sources. Aims. We report on a newly detected point-like source, HESS J1943+213. This source coincides with an unidentified hard X-ray source IGR J19443+2117, which was proposed to have radio and infrared counterparts. Methods. We combine new H. E. S. S., Fermi/LAT and Nancay Radio Telescope observations with pre-existing non-simultaneous multi-wavelength observations of IGR J19443+2117 and discuss the likely source associations as well as the interpretation as an active galactic nucleus, a gamma-ray binary or a pulsar wind nebula. Results. HESS J1943+213 is detected at the significance level of 7.9 sigma (post-trials) at RA(J2000) = 19(h)43(m)55(s) +/- 1(stat)(s) +/- 1(sys)(s), Dec(J2000) = +21 degrees 18'8 '' +/- 17(stat)'' +/- 20(sys)''. The source has a soft spectrum with photon index Gamma = 3.1 +/- 0.3(stat) +/- 0.2(sys) and a flux above 470 GeV of (1.3 +/- 0.2(stat) +/- 0.3(sys)) x 10(-12) cm(-2) s(-1). There is no Fermi/LAT counterpart down to a flux limit of 6 x 10(-9) cm(-2) s(-1) in the 0.1-100 GeV energy range (95% confidence upper limit calculated for an assumed power-law model with a photon index Gamma = 2.0). The data from radio to VHE gamma-rays do not show any significant variability. Conclusions. The lack of a massive stellar counterpart disfavors the binary hypothesis, while the soft VHE spectrum would be very unusual in case of a pulsar wind nebula. In addition, the distance estimates for Galactic counterparts places them outside of the Milky Way. All available observations favor an interpretation as an extreme, high-frequency peaked BL Lac object with a redshift z > 0.14. This would be the first time a blazar is detected serendipitously from ground-based VHE observations, and the first VHE AGN detected in the Galactic Plane.

Place, publisher, year, edition, pages
2011. Vol. 529, A49- p.
Keyword [en]
galaxies: active, BL Lacertae objects: individual: IGR J19443+2117, gamma rays: general
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Physical Sciences
URN: urn:nbn:se:su:diva-68547DOI: 10.1051/0004-6361/201116545ISI: 000289557200057OAI: diva2:472707
authorCount :201Available from: 2012-01-04 Created: 2012-01-04 Last updated: 2012-01-04Bibliographically approved

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Conrad, JanDickinson, Hugh J.Ripken, Joachim
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Department of PhysicsThe Oskar Klein Centre for Cosmo Particle Physics (OKC)
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