1996A&A...307...61M

We present imaging photometry of the jet in M87 in three optical wavebands, B, R and I. From this data base we derive the optical jet flux and the run of the optical spectral index along the jet axis with an effective resolution of 1.3". At this resolution no significant variations of the optical spectral index are found perpendicular to the axis. We have reanalysed a single dish measurement of M87 at 1.3mm in order to isolate the jet flux. Combining these data with published flux measurements between 5GHz and 10¹⁶Hz we derive the overall synchrotron spectrum of the brightest parts (i.e. the knot complex A+B+C) which is characterized by a medium flat radio-to-optical spectral index α_ro=-0.65, a steep cutoff at frequencies beyond a critical value ν_c>2x10¹⁵Hz, and some flattening below ν=10¹⁰Hz. The X-ray emission from the jet seems to be emitted by a separate component which is either due to very energetic synchrotron emitting particles (γ=E/mc²>10⁷) or a thermal bremsstrahlung emitting plasma. The astonishingly smooth variations of both the optical spectral index α_opt and the radio-optical spectral index α_ro along the jet can be understood in terms of an almost constant cutoff frequency 8x10¹⁴Hz<ν_c<2.5x10¹⁵Hz. Taking into account magnetic field variations as derived from a minimum energy estimate this implies that the maximum Lorentz factor γ_c = E_c/m_e c² = 9x10⁵ of the underlying electron energy distribution is held constant within ±15%. Likewise no significant variations of the radio-optical power-law slope α_PL=-0.654 can be found arguing for a universal electron spectrum being maintained throughout the entire jet. Most clearly, this can be demonstrated by means of an ad hoc model which is able to explain the observed variations of the optical spectral index and brightness due to changes of the local magnetic field strength alone. From the smoothness of our photometric data and the comparison between optical and radio polarization maps we conclude that the universality of the electron spectrum holds at least to scales of 0.1" (10pc). There is strong evidence that the high electron energies persist into the brightest parts of the lobes. Our results rule out the standard model in which particle acceleration occurs at a few strong shocks within the knots. The nature of the permanent re-acceleration mechanism which is required to maintain the universal spectrum is unclear. We derive, however, some general properties of the acceleration process. They require that the acceleration efficiency is approximately proportional to the local magnetic field energy density.