HESS J1943+213: A candidate extreme BL Lacertae object

A. Abramowski, F. Acero, F. Aharonian, A. G. Akhperjanian, G. Anton, A. Balzer, A. Barnacka, U. Barres De Almeida, A. R. Bazer-Bachi, Y. Becherini, J. Becker, B. Behera, K. Bernlöhr, A. Bochow, C. Boisson, J. Bolmont, P. Bordas, V. Borrel, J. Brucker, F. BrunP. Brun, T. Bulik, I. Büsching, S. Carrigan, S. Casanova, M. Cerruti, P. M. Chadwick, A. Charbonnier, R. C G Chaves, A. Cheesebrough, L. M. Chounet, A. C. Clapson, G. Coignet, P. Colom, J. Conrad, M. Dalton, M. K. Daniel, I. D. Davids, B. Degrange, C. Deil, H. J. Dickinson, A. Djannati-Ataï, W. Domainko, L. O C Drury, F. Dubois, G. Dubus, J. Dyks, M. Dyrda, K. Egberts, P. Eger, P. Espigat, L. Fallon, C. Farnier, S. Fegan, F. Feinstein, M. V. Fernandes, A. Fiasson, G. Fontaine, A. Förster, M. Füßling, Y. A. Gallant, H. Gast, L. Gérard, D. Gerbig, B. Giebels, J. F. Glicenstein, B. Glück, P. Goret, D. Göring, S. Häffner, J. D. Hague, D. Hampf, M. Hauser, S. Heinz, G. Heinzelmann, G. Henri, G. Hermann, J. A. Hinton, A. Hoffmann, W. Hofmann, P. Hofverberg, M. Holler, D. Horns, A. Jacholkowska, O. C. De Jager, C. Jahn, M. Jamrozy, I. Jung, M. A. Kastendieck, K. Katarzyński, U. Katz, S. Kaufmann, D. Keogh, D. Khangulyan, B. Khélifi, D. Klochkov, W. Kluźniak, T. Kneiske, Nu Komin, K. Kosack, R. Kossakowski, H. Laffon, G. Lamanna, D. Lennarz, T. Lohse, A. Lopatin, C. C. Lu, V. Marandon, A. Marcowith, J. Masbou, D. Maurin, N. Maxted, T. J L McComb, M. C. Medina, J. Méhault, N. Nguyen, R. Moderski, E. Moulin, C. L. Naumann, M. Naumann-Godo, M. De Naurois, D. Nedbal, D. Nekrassov, B. Nicholas, J. Niemiec, S. J. Nolan, S. Ohm, J. F. Olive, E. De Oña Wilhelmi, B. Opitz, M. Ostrowski, M. Panter, M. Paz Arribas, G. Pedaletti, G. Pelletier, P. O. Petrucci, S. Pita, G. Pühlhofer, M. Punch, A. Quirrenbach, M. Raue, S. M. Rayner, A. Reimer, O. Reimer, M. Renaud, R. De Los Reyes, F. Rieger, J. Ripken, L. Rob, S. Rosier-Lees, G. Rowell, B. Rudak, C. B. Rulten, J. Ruppel, F. Ryde, V. Sahakian, A. Santangelo, R. Schlickeiser, F. M. Schöck, A. Schönwald, A. Schulz, U. Schwanke, S. Schwarzburg, S. Schwemmer, A. Shalchi, M. Sikora, J. L. Skilton, H. Sol, G. Spengler, L. Stawarz, R. Steenkamp, C. Stegmann, F. Stinzing, K. Stycz, I. Sushch, A. Szostek, J. P. Tavernet, R. Terrier, O. Tibolla, M. Tluczykont, K. Valerius, C. Van Eldik, G. Vasileiadis, C. Venter, J. P. Vialle, A. Viana, P. Vincent, M. Vivier, H. J. Völk, F. Volpe, S. Vorobiov, M. Vorster, S. J. Wagner, M. Ward, A. Wierzcholska, A. Zajczyk, A. A. Zdziarski, A. Zech, H. S. Zechlin, T. H. Burnett, A. B. Hill

Research output: Contribution to journalArticlepeer-review

27 Scopus citations

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 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)= 19 h43m55s ± 1s stat ± 1s say, 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.

Original languageEnglish (US)
Article numberA49
JournalAstronomy and Astrophysics
Volume529
DOIs
StatePublished - 2011

Keywords

  • galaxies: active
  • gamma rays: general

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