HESS observations and VLT spectroscopy of PG1553+113

Aims.The properties of the very high energy (VHE; E > 100 GeV) $\gamma$-ray emission from the high-frequency peaked BL Lac PG 1553+113 are investigated. An attempt is made to measure the currently unknown redshift of this object. Methods.VHE Observations of PG 1553+113 were made with the High Energy Stereoscopic System (HESS) in 2005 and 2006. 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. Results.A VHE signal, ~10 standard deviations, is detected by HESS during the 2 years of observations (24.8 h live time). The integral flux above 300 GeV is $(4.6 \pm 0.6_{\rm stat} \pm 0.9_{\rm syst})$ $\times$ 10-12 cm-2 s-1, corresponding to ~3.4% of the flux from the Crab Nebula above the same threshold. The time-averaged energy spectrum is measured from 225 GeV to ~1.3 TeV, and is characterized by a very soft power law (photon index of $\Gamma = 4.5 \pm 0.3_{\rm stat} \pm 0.1_{\rm syst}$). No evidence for any flux or spectral variations is found on any sampled time scale within the VHE data. The redshift of PG 1553+113 could not be determined. Indeed, even though the measured SINFONI spectrum is the most sensitive ever reported for this object at near infrared wavelengths, and the sensitivity is comparable to the best spectroscopy at other wavelengths, no absorption or emission lines were found in the H+K spectrum presented here.