The synchrotron emission from the M87 jet

Abstract

We show that the intensity changes and the spectral evolution along the M 87 jet are consistent with adiabatic changes to the particle momentum distribution function and the magnetic field. We use the deprojection of a 2 cm VLA intensity map by Sparks, Biretta, & Macchetto and spectral data by Neumann, Meisenheimer, & Röser to derive limits on the magnetic field and the total pressure. To explain the weakness of synchrotron cooling along the jet, the magnetic field strength must lie in the range 20 − 40 µG, departing from equipartition by a factor ≈1.5–5. Bulk Lorentz factors Γ _jet in the range 2–5 and inclination angles ϑ _LOS≲25° lead to pressure estimates close to the ISM pressure. Extrapolation of our model back towards the core shows that particle acceleration to the observed spectral shape likely occurs at distances as far as 10 pc away from the core.