HESS J1943+213: a candidate extreme BL Lacertae object
Klochkov, D.; Santangelo, A.; Quirrenbach, A.; Hill, A. B.; Ripken, J.; Burnett, T. H.; Carrigan, S.; Conrad, J.; Farnier, C.; Reimer, A.; Reimer, O.; Ryde, F.; Tibolla, O.; Venter, C.; Schlickeiser, R.; Petrucci, P. -O.; Giebels, B.; Stawarz, Ł.; Sikora, M.; Fontaine, G.; Daniel, M. K.; Hauser, M.; Boisson, C.; Aharonian, F.; Akhperjanian, A. G.; Bazer-Bachi, A. R.; Bernlöhr, K.; Chadwick, P. M.; Chounet, L. -M.; Degrange, B.; Djannati-Ataï, A.; Dubus, G.; Espigat, P.; Feinstein, F.; Gallant, Y. A.; Glicenstein, J. F.; Goret, P.; Heinzelmann, G.; Henri, G.; Hermann, G.; Hinton, J. A.; Hofmann, W.; Horns, D.; de Jager, O. C.; Khélifi, B.; Komin, Nu.; Lohse, T.; Marcowith, A.; McComb, T. J. L.; de Naurois, M.; Nolan, S. J.; Panter, M.; Pelletier, G.; Pita, S.; Pühlhofer, G.; Punch, M.; Raue, M.; Rayner, S. M.; Rob, L.; Rowell, G.; Sahakian, V.; Schwanke, U.; Sol, H.; Steenkamp, R.; Stegmann, C.; Tavernet, J. -P.; Terrier, R.; Tluczykont, M.; Vasileiadis, G.; Vincent, P.; Völk, H. J.; Wagner, S. J.; Ward, M.; Heinz, S.; Kossakowski, R.; Lamanna, G.; Vialle, J. P.; Acero, F.; Chaves, R. C. G.; Schulz, A.; Renaud, M.; Cerruti, M.; Barres de Almeida, U.; de Oña Wilhelmi, E.; Pedaletti, G.; Anton, G.; Becherini, Y.; Behera, B.; Bochow, A.; Bolmont, J.; Borrel, V.; Brucker, J.; Brun, F.; Brun, P.; Bulik, T.; Büsching, I.; Charbonnier, A.; Cheesebrough, A.; Clapson, A. C.; Coignet, G.; Dalton, M.; Davids, I. D.; Deil, C.; Dickinson, H. J.; Domainko, W.; Dubois, F.; Dyks, J.; Dyrda, M.; Egberts, K.; Eger, P.; Fallon, L.; Fegan, S.; Fiasson, A.; Förster, A.; Füßling, M.; Gérard, L.; Gerbig, D.; Glück, B.; Göring, D.; Hoffmann, A.; Hofverberg, P.; Jacholkowska, A.; Jahn, C.; Jung, I.; Katarzyński, K.; Katz, U.; Kaufmann, S.; Khangulyan, D.; Keogh, D.; Kluźniak, W.; Kneiske, T.; Kosack, K.; Marandon, V.; Masbou, J.; Maurin, D.; Medina, M. C.; Méhault, J.; Moderski, R.; Moulin, E.; Naumann-Godo, M.; Nedbal, D.; Nekrassov, D.; Nicholas, B.; Niemiec, J.; Ohm, S.; Olive, J. -F.; Ostrowski, M.; Paz Arribas, M.; Rieger, F.; Rosier-Lees, S.; Rudak, B.; Rulten, C. B.; Ruppel, J.; Schöck, F. M.; Schönwald, A.; Schwarzburg, S.; Schwemmer, S.; Shalchi, A.; Sushch, I.; Skilton, J. L.; Stinzing, F.; Szostek, A.; van Eldik, C.; Vivier, M.; Volpe, F.; Vorobiov, S.; Zdziarski, A. A.; Zech, A.; H. E. S. S. Collaboration; Abramowski, A.; Balzer, A.; Barnacka, A.; Bordas, P.; Casanova, S.; Fernandes, M. V.; Häffner, S.; Holler, M.; Jamrozy, M.; Kastendieck, M. A.; Laffon, H.; Lopatin, A.; Lu, C. -C.; Maxted, N.; Opitz, B.; de los Reyes, R.; Spengler, G.; Stycz, K.; Valerius, K.; Viana, A.; Vorster, M.; Wierzcholska, A.; Zechlin, H. -S.; Becker, J.; Gast, H.; Hague, J. D.; Hampf, D.; Lennarz, D.; Naumann, C. L.; Nguyen, N.; Zajczyk, A.; Drury, L. O. 'C.; Colom, P.
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) = 19h 43m 55s ± 1^s_stat ± 1^s_sys, Dec(J2000) = +21° 18' 8'' ± 17''_stat ± 20 arcsec_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.