2005ApJ...627..140P

We reanalyze the deep Chandra observations of the M87 jet, first examined by Wilson & Yang in 2002. By employing an analysis chain that also includes image deconvolution, knots HST-1 and I are fully separated from adjacent emission. We derive the spatially resolved X-ray spectrum of the jet using the most recent response functions and find slight but significant variations in the spectral shape, with values of α_X(S_ν∝ν^–α) ranging from ∼1.2-1.4 (in the nucleus and knots HST-1, D, and C) to ∼1.6 (in knots F, A, and B). We make use of VLA radio observations, as well as HST imaging and polarimetry data, to examine the jet's broadband spectrum and inquire as to the nature of particle acceleration in the jet. As shown in previous papers, a simple continuous injection model for the synchrotron-emitting knots, in which one holds constant both the filling factor f_accof the regions within which particles are accelerated and the energy spectrum of the injected particles, cannot account for the flux or spectrum of the X-ray emission. Instead, we propose that f_accis a function of both position and energy and find that in the inner jet, f_acc∝E^–0.4±0.2_γ∝E^–0.2±0.1_e, and in knots A and B, f_acc∝E^–0.7±0.2_γ∝E^–0.35±0.1_e, where E_γis the energy of the emitted photon and E_e_is the energy of the emitting electron. In this model, the index p of the relativistic electron energy spectrum at injection [n(E_e)∝E^–p_e] is p=2.2 at all energies and all locations along the jet, in excellent agreement with the predictions of models of cosmic-ray acceleration by ultrarelativistic shocks (p=2.23). There is a strong correlation between the peaks of X-ray emission and minima of optical percentage polarization, i.e., regions where the jet magnetic field is not ordered. We suggest that the X-ray peaks coincide with shock waves that accelerate the X-ray-emitting electrons and cause changes in the direction of the magnetic field; the polarization is thus small because of beam averaging.