Astronomy and Astrophysics – Astrophysics
Scientific paper
2006-01-24
Astron.Astrophys. 448 (2006) L19-L23
Astronomy and Astrophysics
Astrophysics
5 pages, 3 figures; accepted (Jan. 19, 2006) for publication in Astronomy & Astrophysics
Scientific paper
10.1051/0004-6361:200600010
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.
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