Auteur(s): Aharonian F., Akhperjanian A.G., Bazer-Bachi A.R., Beilicke M., Chounet L.-M., Degrange B., Djannati-Ataï A., Dubus G., Espigat P., Feinstein F., Fontaine G., Gallant Y., Giebels B., Jacholkowska Agnieszka, Khelifi B., Lemiere A., Lemoine-Goumard M., Martineau-Huynh O., Marcowith A., De Naurois M., Ouchrif M., Pita S., Punch M., Rolland L., Tavernet J.-P., Terrier R., Theoret C.G., Tluczykont M., Vasileiadis G., Vincent P.
(Article) Publié:
Astronomy & Astrophysics, vol. 448 p.L19-L23 (2006)
Texte intégral en Openaccess : 
Ref HAL: in2p3-00025548_v1
Ref Arxiv: astro-ph/0601545
DOI: 10.1051/0004-6361:200600010
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
68 citations
Résumé:
The high-frequency peaked BL Lac PG 1553+113 was observed in 2005 with the H.E.S.S. stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. Using the H.E.S.S. 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 +/- 1.3_{stat} +/- 1.0_{syst}) x10^{-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 +/- 0.6). 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.
Commentaires: 5 pages, 3 figures; accepted (Jan. 19, 2006) for publication in Astronomy & Astrophysics