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Recent Very Long Baseline Interferometry observations of the relativistic jetin the M87 radio galaxy at 43 GHz show gradual relativistic acceleration of theplasma and suggest a linear dependence of Lorentz factor on jet radius atscales up to 8 marcsec (0.65 pc) from the core (2.5 marcsec in projection).General analysis of integrals of motion being unaltered along the jet andreflecting fundamental conservation laws shows that the above dependenceimplies a polytropic equation of state with index 4/3. The inferred value ofthe polytropic index appears independent of the exact nature of forcessustaining the transverse balance of the jet and indicates exact conservationof the longitudinal electric current and hence the existence of a stableinternal electromagnetic structure at the scales under consideration. At thisindex the flow is hot and corresponds to relativistic thermodynamic motion ofparticles. Considerable weakening of the acceleration efficiency after 8marcsec with the jet form being unchanged can be related to the plasma cooling,when the polytropic index becomes 5/3. Such a sharp change in the index withoutintermediate delay at 1.44 during cooling favours the existence of anelectron-positron plasma and requires at least partial participation of theBlandford-Znajek mechanism in the launching of the M87 jet.

monthly notices of the royal astronomical society letters

Relativistic polytrope from the collimation and acceleration profiles of  the M87 jet at subparsec scales and thermodynamic evidence for the  Blandford-Znajek mechanism