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Type: Journal article
Title: HESS J1943+213: a candidate extreme BL Lacertae object
Author: Maxted, N.
Nicholas, B.
Rowell, G.
Citation: Astronomy and Astrophysics, 2011; 529:1-10
Publisher: E D P Sciences
Issue Date: 2011
ISSN: 0004-6361
Statement of
H.E.S.S. Collaboration... A. Abramowski... N. Maxted... B. Nicholas... G. Rowell... et al.
Abstract: 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 Nançay 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 gammaray binary or a pulsar wind nebula. Results: HESS J1943+213 is detected at the significance level of 7.9σ (post-trials) at RA(J2000) = 19h43m55s ± 1s stat ± 1s sys , Dec(J2000)=+21 18_8__ ± 17__ stat ± 20__ sys. The source has a soft spectrum with photon index Γ = 3.1 ± 0.3stat ± 0.2sys and a flux above 470 GeV of (1.3±0.2stat±0.3sys)×10−12 cm−2 s−1. There is no Fermi/LAT counterpart down to a flux limit of 6×10−9 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 Γ = 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.
Keywords: Galaxies; active; BL Lacertae objects; individual; IGR J19443+2117; gamma rays; general
Rights: Copyright ESO 2011
RMID: 0020105689
DOI: 10.1051/0004-6361/201116545
Appears in Collections:Physics publications

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