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Evidence for VHE gamma-ray emission from the distant BL Lac PG 1553+113

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posted on 24.10.2012, 08:53 by F. Aharonian, A. G. Akhperjanian, A. R. Bazer-Bachi, M. Beilicke, W. Benbow, D. Berge, K. Bernlohr, C. Boisson, O. Bolz, V. Borrel, I. Braun, J. Ripken, L. Rob, L. Rolland, G. Rowell, V. Sahakian, L. Sauge, S. Schlenker, R. Schlickeiser, C. Schuster, U. Schwanke, M. Siewert, H. Sol, D. Spangler, R. Steenkamp, C. Stegmann, G. Superina, J. P. Tavernet, R. Terrier, C. G. Theoret, M. Tluczykont, van Eldik C., G. Vasileiadis, C. Venter, P. Vincent, H. J. Volk, S. J. Wagner, M. Ward, F. Breitling, A. M. Brown, R. Buhler, S. Carrigan, P. M. Chadwick, L. M. Chounet, R. Cornils, L. Costamante, B. Degrange, H. J. Dickinson, A. Djannati-Atai, L. O. Drury, G. Dubus, K. Egberts, D. Emmanoulopoulos, P. Espigat, F. Feinstein, G. Fontaine, S. Funk, Y. A. Gallant, B. Giebels, J. F. Glicenstein, P. Goret, C. Hadjichristidis, D. Hauser, M. Hauser, G. Heinzelmann, G. Henri, G. Hermann, J. A. Hinton, W. Hofmann, M. Holleran, D. Horns, A. Jacholkowska, de Jager O. C., B. Khelifi, N. Komin, A. Konopelko, I. J. Latham, Le Gallou R., A. Lemiere, M. Lemoine-Goumard, T. Lohse, J. M. Martin, O. Martineau-Huynh, A. Marcowith, C. Masterson, T. J. L. McComb, de Naurois M., D. Nedbal, S. J. Nolan, A. Noutsos, K. J. Orford, J. L. Osborne, M. Ouchrif, M. Panter, G. Pelletier, S. Pita, G. Puhlhofer, M. Punch, B. C. Raubenheimer, M. Raue, S. M. Rayner, A. Reimer, O. Reimer
The high-frequency peaked BL Lac PG 1553+113 was observed in 2005 with the HESS stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. Using the HESS standard analysis, an excess was measured at the 4.0$\sigma$ level in these observations (7.6 hours live time). Three alternative, lower-threshold analyses yield >5$\sigma$ excesses. The observed integral flux above 200 GeV is $(4.8\pm1.3_{\rm stat}\pm1.0_{\rm syst})\times10^{-12}$ cm-2 s-1, and shows no evidence for variability. The measured energy spectrum is characterized by a very soft power law (photon index of $\Gamma=4.0\pm0.6$). Although the redshift of PG 1553+113 is unknown, there are strong indications that it is greater than z=0.25 and possibly larger than z=0.78. The observed spectrum is interpreted in the context of VHE $\gamma$-ray absorption by the Extragalactic Background Light, and is used to place an upper limit on the redshift of z<0.74.



Astronomy & Astrophysics, 2006, 448 (2), pp. L19-L23


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