(F. D'Ammando, M. Giroletti on behalf of the Fermi-LAT collaboration, and D.Dallacasa, G. Giovannini)
High redshift blazars are among the most powerful objects in the Universe. Although they represent a significant fraction of the extragalactic hard X-ray sky, high redshift objects represent less than 10 per cent of the AGN population observed by Fermi so far, and gamma-ray flaring activity from these sources is even more uncommon. We present results on a multi-band campaign of the high-redshift quasar S5 0836+710 during a high activity period detected by the Fermi Large Area Telescope (LAT). The sub-daily gamma-ray light curve indicates significant flux variation on a 3-hr time scale, suggesting that the gamma-ray emitting region is located within the broad line region, in agreement with the lack of correlated variability observed in the radio band. Multi-epoch full-polarization radio observations with pc-scale resolution point out changes in the polarized emission and allow us to set constraints on the structure of the magnetic field in the jet of this source. In this contribution we discuss the multi-band properties of the high-redshift blazar S5 0836+710 as inferred from Fermi-LAT, Swift and Very Long Baseline Array observations. In particular, we focus on the high energy emitting region and the characteristics of the relativistic jet.