Evidence for VHE gamma-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 4.0$\sigma$ 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.