Analysis of the Synchrotron Emission from the M87 Jet

We propose that the intensity changes and spectral evolution along the M87 jet can be explained by adiabatic changes to the particle momentum distribution function and the magnetic field. This is supported by the lack of any significant variation in the radio-to-optical spectral index along the jet and by the moderate changes in radio brightness. Assuming a simple scaling law between magnetic field and density, we use the deprojection of a 2 cm VLA intensity map by Sparks, Biretta, & Macchetto to predict the spectral evolution along the jet. We derive limits for the magnetic field and the total pressure by comparing our results with the spatially resolved fit to the spectral data by Neumann, Meisenheimer, & Röser of a model spectrum that cuts off at ≈ 10 15 Hz. To explain the weakness of synchrotron cooling along the jet, the magnetic field strength must lie below the equipartition value. Although the inferred pressure in the limit of nonrelativistic bulk flow lies far above the estimated pressure of the interstellar matter in the center of M87, bulk Lorentz factors Γ jet in the range of 3-5 and inclination angles θ LOS ≲ 25° lead to pressure estimates close to the interstellar medium pressure. The average best-fit magnetic fields we derive fall in the range of 20-40 μG, departing from equipartition by a factor ≈ 1.5-5. This model is consistent with the proposal by Bicknell & Begelman that the knots in the M87 jet are weak, oblique shocks. First-order Fermi acceleration will then have a minimal effect on the slope of the radio-to-optical spectrum while possibly accounting for the X-ray spectrum.