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

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 4.0σlevel in these observations (7.6 hours live time). Three alternative, lower-threshold analyses yield >5σexcesses. The observed integral flux above 200 GeV is $(4.8\pm1.3_{\rm stat}\pm1.0_{\rm syst})\times10^{-12}$cm-2s-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 γ-ray absorption by the Extragalactic Background Light, and is used to place an upper limit on the redshift of $z<0.74$.