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

The high-frequency peaked BL Lac PG1553+ 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 +/- 1. 3(stat) +/- 1.0(syst)) x 10(-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 PG1553+ 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.