Program: GN-2011A-Q-28

This proposal aims at obtaining a reliable redshift determination for the BL Lac object PG1553+113. It is motivated by the recent detection of this blazar at very high energy (VHE, i.e. E >100 GeV) gamma-rays by H.E.S.S. and MAGIC. The redshift of PG1553+113 is uncertain. Upper limits z<0.4 to z<0.8 have been poroposed based on its detection at VHE gamma-rays, which would be otherwise absorbed by infrared photons of the extragalactic background light (EBL), however, these redshift values strongly depend on the adopted spectral energy distribution for the blazar at TeV frequencies. A lower limit z > 0.25, in turn, is set by the non-detection of its host galaxy. A high redshift value would make PG1553+113 the most distant TeV source detected so far, thus raising questions on its emission properties and on the EBL absorption models. On the other hand, a large magnitude difference AGN-host would imply that PG1553+113 is a highly luminous and/or beamed blazar (or, alternatively, that its host is anomalously underluminous). A reliable redshift determination for PG1553+113 would thus provide a solid ground to understand the emission properties of this object at gamma-ray frequencies, while better constraining the absorption mechanism of gamma-rays by the EBL.