A two-zone synchrotron model for the knots in the M87 jet

Abstract

Abstract The flux and the spectral index in the X-ray energy band from the knots of the M87 jet as observed by Chandra indicate a possible synchrotron origin but cannot be explained by simple one-zone models with continuous injection of non-thermal electrons. In this Letter we propose a two-zone model to explain the observed spectra of the knots of the M87 jet. We consider the synchrotron emission from a region with a tangled magnetic field where relativistic non-thermal electrons are continuously injected in from an associated acceleration region. The acceleration region is assumed to be a compact zone possibly around a shock front. A power-law distribution of electrons is injected into the acceleration region and accelerated to a maximum energy determined by the acceleration time-scale and the loss processes. With the present model we are able to explain the overall broad-band features of the knots of the M87 jet. Also the present model predicts a change in spectral index at ultraviolet energies, and future observations at these energies, can be used to constrain the parameters involved in the model.