The X‐Ray Emissions from the M87 Jet: Diagnostics and Physical Interpretation

We reanalyze the deep Chandra observations of the M87 jet, first examined by Wilson & Yang (2002). By employing an analysis chain that includes image deconvolution, knots HST-1 and I are fully separated from adjacent emission. We find slight but significant variations in the spectral shape, with values of \αₓ ranging from \~ 1.2-1.6. We use VLA radio observations, as well as HST imaging and polarimetry data, to examine the jet's broad-band spectrum and inquire as to the nature of particle acceleration in the jet. As shown in previous papers, a simple continuous injection model for synchrotron-emitting knots, in which both the filling factor, facc, of regions within which particles are accelerated and the energy spectrum of the injected particles are constant, cannot account for the X-ray flux or spectrum. Instead, we propose that facc is a function of position and energy and find that in the inner jet, facc \∝ E_\γ-0.4 \± 0.2 \∝ Eₑ-0.2 \± 0.1, and in knots A and B, facc \∝ E_\γ-0.7 \± 0.2 \∝ Eₑ-0.35 \± 0.1, where E_\γ is the emitted photon energy and and Eₑ is the emitting electron energy. In this model, the index p of the injected electron energy spectrum (n(Eₑ) \∝ Eₑ⁻ᵖ) is $p=2.2$ at all locations in the jet, as predicted by models of cosmic ray acceleration by ultrarelativistic shocks. 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 which 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.