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HESS J1943+213: a candidate extreme BL Lacertae object
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.ORCID iD: 0000-0002-9769-8016
2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 529, no A49Article in journal (Refereed) Published
Abstract [en]

ontext. 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 gamma-ray binary or a pulsar wind nebula.

Results. HESS J1943+213 is detected at the significance level of 7.9σ (post-trials) at RA(J2000) = , Dec(J2000) = . 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.

Place, publisher, year, edition, pages
2011. Vol. 529, no A49
Keyword [en]
galaxies, active / BL Lacertae objects, individual, IGR J19443+2117 / gamma rays, general
National Category
Astronomy, Astrophysics and Cosmology
URN: urn:nbn:se:kth:diva-33387DOI: 10.1051/0004-6361/201116545ISI: 000289557200057ScopusID: 2-s2.0-79953211908OAI: diva2:414960
QC 20110505Available from: 2011-05-05 Created: 2011-05-05Bibliographically approved

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