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 momentu m distribution function and the magnetic field. This is supported by t he lack of any significant variation in the radio-to-optical spectral index along the jet and by the moderate changes in radio brightness. A ssuming a simple scaling law between magnetic field and density, we us e the deprojection of a 2 cm VLA intensity map by Sparks, Biretta, & M acchetto to predict the spectral evolution along the jet. We derive li mits for the magnetic field and the total pressure by comparing our re sults with the spatially resolved fit to the spectral data by Neumann, Meisenheimer, & Roser of a model spectrum that cuts off at approximat e to 10(15) Hz. To explain the weakness of synchrotron cooling along t he jet, the magnetic held strength must lie below the equipartition va lue. Although the inferred pressure in the limit of nonrelativistic bu lk flow lies far above the estimated pressure of the interstellar matt er in the center of M87, bulk Lorentz factors Gamma(jet) in the range of 3-5 and inclination angles theta(LOS) less than or similar to 25 de grees lead to pressure estimates close to the interstellar medium pres sure. The average best-fit magnetic fields we derive fall in the range of 20-40 mu G, departing from equipartition by a factor approximate t o 1.5-5. This model is consistent with the proposal by Bicknell & Bege lman that the knots in the M87 jet are weak, oblique shocks. First-ord er 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.