Very high energy ( VHE ; > 100 GeV ) \gamma -ray observations of PG 1553+113 were made with the High Energy Stereoscopic System ( HESS ) in 2005 and 2006 .
A strong signal , \sim 10 standard deviations , is detected by HESS during the 2 years of observations ( 24.8 hours live time ) .
The time-averaged energy spectrum , measured between 225 GeV to \sim 1.3 TeV , is characterized by a very steep power law ( photon index of \Gamma = 4.5 \pm 0.3 _ { stat } \pm 0.1 _ { syst } ) .
The integral flux above 300 GeV is \sim 3.4 % of the Crab Nebula flux and shows no evidence for any variations , on any time scale .
H+K ( 1.45 - 2.45 \mu m ) spectroscopy of PG 1553+113 was performed in March 2006 with SINFONI , an integral field spectrometer of the ESO Very Large Telescope ( VLT ) in Chile .
The redshift of PG 1553+113 is still unknown , as no absorption or emission lines were found .