Multiband RadioAstron space VLBI imaging of the jet in quasar S5 0836+710

Detailed studies of relativistic jets in active galactic nuclei (AGN) require high-fidelity imaging at the highest possibleresolution. This can be achieved using very long baseline interferometry (VLBI) at radio frequencies, combining worldwide (global)VLBI arrays of radio telescopes with a space-borne antenna on board a satellite.Aims.We present multiwavelength images made of the radio emission in the powerful quasar S5 0836+710, obtained using a globalVLBI array and the antennaSpektr-Rof theRadioAstronmission of the Russian Space Agency, with the goal of studying the internalstructure and physics of the relativistic jet in this object.Methods.TheRadioAstronobservations at wavelengths of 18 cm, 6 cm, and 1.3 cm are part of the Key Science Program for imagingradio emission in strong AGN. The internal structure of the jet is studied by analyzing transverse intensity profiles and modeling thestructural patterns developing in the flow.Results.TheRadioAstronimages reveal a wealth of structural detail in the jet of S5 0836+710 on angular scales ranging from0.02 mas to 200 mas. Brightness temperatures in excess of 1013K are measured in the jet, requiring Doppler factors of≥100 forreconciling them with the inverse Compton limit. Several oscillatory patterns are identified in the ridge line of the jet and canbe explained in terms of the Kelvin-Helmholtz (KH) instability. The oscillatory patterns are interpreted as the surface and bodywavelengths of the helical mode of the KH instability. The interpretation provides estimates of the jet Mach number and of the ratioof the jet to the ambient density, which are found to beMj≈12 andη≈0.33. The ratio of the jet to the ambient density should beconservatively considered an upper limit because its estimate relies on approximations.