Dual frequency VSOP imaging of the jet in S 5 0836 + 710

The luminous high-redshift (z = 2.17) quasar S5 0836+710 has been observed in October 1997 with the VSOP at 1.6GHz and 5GHz. We report here a previously unpublished image made from the data at 1.6GHz and compare the structure of a relativistic jet in this quasazr at the two frequencies. We present a spectral index image tracing spectral properties of the jet up to ∼ 40 milliarcsecond distance from the nucleus. The curved jet ridge line observed in the images and the spectral index distribution can be described by Kelvin-Helmholtz instability developing in a relativistic outflow with a Mach number of ∼ 6. In this description, the overall ridge line of the jet is formed by the helical surface mode of Kelvin-Helmholtz instability, while areas of flatter spectral index embedded into the flow correspond to pressure enhancements produced by the elliptical surface mode of the instability. An alternative explanation involving a sequence of slowly dissipating shocks cannot be ruled out at this point.