Discovery of the Hard Spectrum VHE γ-Ray Source HESS J1641-463
Klochkov, D.; Santangelo, A.; Quirrenbach, A.; Bregeon, J.; Carrigan, S.; Conrad, J.; Farnier, C.; Giavitto, G.; Grondin, M. -H.; Hadasch, D.; Lemoine-Goumard, M.; Reimer, A.; Reimer, O.; Venter, C.; Schlickeiser, R.; Petrucci, P. -O.; Colafrancesco, S.; Giebels, B.; Stawarz, Ł.; Fontaine, G.; Hervet, O.; Vink, J.; Boisson, C.; Aharonian, F.; Akhperjanian, A. G.; Berge, D.; Bernlöhr, K.; Chadwick, P. M.; Degrange, B.; Djannati-Ataï, A.; O'C. Drury, L.; Dubus, G.; Espigat, P.; Feinstein, F.; Gallant, Y. A.; Glicenstein, J. F.; Heinzelmann, G.; Henri, G.; Hermann, G.; Hinton, J. A.; Hofmann, W.; Horns, D.; Khélifi, B.; Komin, Nu.; Lemière, A.; Lohse, T.; Marcowith, A.; McComb, T. J. L.; de Naurois, M.; Nolan, S. J.; Panter, M.; Pelletier, G.; Pita, S.; Pühlhofer, G.; Punch, M.; 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.; Böttcher, M.; Fukui, Y.; Meyer, M.; Lamanna, G.; Chaves, R. C. G.; Mayer, M.; Schulz, A.; Wagner, R. M.; Renaud, M.; Reichardt, I.; Becherini, Y.; Bolmont, J.; Brun, F.; Brun, P.; Bulik, T.; Dalton, M.; Davids, I. D.; Deil, C.; Domainko, W.; Dyks, J.; Dyrda, M.; Egberts, K.; Eger, P.; Fegan, S.; Fiasson, A.; Förster, A.; Füßling, M.; Gabici, S.; Hofverberg, P.; Jacholkowska, A.; Jahn, C.; Katarzyński, K.; Katz, U.; Kaufmann, S.; Kluźniak, W.; Kosack, K.; Lenain, J. -P.; Marandon, V.; Méhault, J.; Moderski, R.; Moulin, E.; Niemiec, J.; Ohm, S.; Ostrowski, M.; Paz Arribas, M.; Rieger, F.; Rosier-Lees, S.; Rudak, B.; Rulten, C. B.; Schwarzburg, S.; Schwemmer, S.; Sushch, I.; Stinzing, F.; van Eldik, C.; Volpe, F.; Zdziarski, A. A.; Zech, A.; H. E. S. S. Collaboration; Abramowski, A.; Ait Benkhali, F.; Angüner, E. O.; Backes, M.; Balenderan, S.; Balzer, A.; Barnacka, A.; Becker Tjus, J.; Bernhard, S.; Birsin, E.; Biteau, J.; Bordas, P.; Bryan, M.; Casanova, S.; Chakraborty, N.; Chalme-Calvet, R.; Chrétien, M.; Cologna, G.; Couturier, C.; Cui, Y.; deWilt, P.; Donath, A.; Dutson, K.; Edwards, T.; Fernandes, M. V.; Fernandez, D.; Gajdus, M.; Garrigoux, T.; Gottschall, D.; Grudzińska, M.; Häffner, S.; Hahn, J.; Harris, J.; Hillert, A.; Holler, M.; Ivascenko, A.; Jamrozy, M.; Janiak, M.; Jankowsky, F.; Jung-Richardt, I.; Kastendieck, M. A.; Kieffer, M.; Klepser, S.; Kolitzus, D.; Krakau, S.; Krayzel, F.; Krüger, P. P.; Laffon, H.; Lau, J.; Lefaucheur, J.; Lefranc, V.; Lopatin, A.; Lu, C. -C.; Marx, R.; Maurin, G.; Maxted, N.; Meintjes, P. J.; Menzler, U.; Mitchell, A. M. W.; Mohamed, M.; Morå, K.; Murach, T.; Oakes, L.; Odaka, H.; Opitz, B.; Oya, I.; Parsons, R. D.; Pekeur, N. W.; Peyaud, B.; Poon, H.; Raab, S.; de los Reyes, R.; Romoli, C.; Salek, D.; Sanchez, D. A.; Schüssler, F.; Spanier, F.; Spengler, G.; Spies, F.; Stycz, K.; Tavernier, T.; Taylor, A. M.; Trichard, C.; Valerius, K.; van Soelen, B.; Veh, J.; Viana, A.; Vorster, M.; Vuillaume, T.; Wagner, P.; Weidinger, M.; Weitzel, Q.; White, R.; Wierzcholska, A.; Willmann, P.; Wörnlein, A.; Wouters, D.; Yang, R.; Zabalza, V.; Zaborov, D.; Zacharias, M.; Zechlin, H. -S.; Sano, H.; Fukuda, T.; Yoshiike, S.
Germany, Ireland, Armenia, Namibia, United Kingdom, Netherlands, Poland, United States, Sweden, Austria, France, South Africa, Australia, Japan
Abstract
This Letter reports the discovery of a remarkably hard spectrum source, HESS J1641-463, by the High Energy Stereoscopic System (H.E.S.S.) in the very high energy (VHE) domain. HESS J1641-463 remained unnoticed by the usual analysis techniques due to confusion with the bright nearby source HESS J1640-465. It emerged at a significance level of 8.5 standard deviations after restricting the analysis to events with energies above 4 TeV. It shows a moderate flux level of phi(E>1 TeV) = (3.64 ± 0.44stat ± 0.73sys) × 10-13 cm-2 s-1, corresponding to 1.8% of the Crab Nebula flux above the same energy, and a hard spectrum with a photon index of Γ = 2.07 ± 0.11stat ± 0.20sys. It is a point-like source, although an extension up to a Gaussian width of σ = 3 arcmin cannot be discounted due to uncertainties in the H.E.S.S. point-spread function. The VHE γ-ray flux of HESS J1641-463 is found to be constant over the observed period when checking time binnings from the year-by-year to the 28 minute exposure timescales. HESS J1641-463 is positionally coincident with the radio supernova remnant SNR G338.5+0.1. No X-ray candidate stands out as a clear association; however, Chandra and XMM-Newton data reveal some potential weak counterparts. Various VHE γ-ray production scenarios are discussed. If the emission from HESS J1641-463 is produced by cosmic ray protons colliding with the ambient gas, then their spectrum must extend close to 1 PeV. This object may represent a source population contributing significantly to the galactic cosmic ray flux around the knee.