Determination of the intrinsic velocity field in the M87 jet

A new method to estimate the Doppler beaming factor of relativistic large-scale jet regions is presented. It is based on multiwaveband fitting to radio-to-X-ray continua with synchrotron spectrum models. Combining our method with available observational data of proper motions, we derive the intrinsic velocity as well as the viewing angles to the line of sight for eight knotty regions down the M87 jet. The results favor the ’modest beaming’ scenario along the jet, with Doppler factors varying between ∼ 2−5. The inner jet of M87 suffers sharp deceleration, and the intrinsic speed remains roughly constant down the outer jet. The orientation of the inner jet regions is fully consistent with the result of 10◦-19◦to the line of sight suggested by previous Hubble Space Telescope (HST) proper motion studies of the M87 jet. The outer jet, however, shows systematic deflection off the inner jet to much smaller inclination (θ ≪ 10◦). Further calculation of knot A suggests this deflection can be regarded as evidence that the outer jet suffers some departure from equipartition. The nucleus region of the M87 jet should have a viewing angle close to its first knot HST-1, i.e. θ ∼ 15◦, which favors the idea that M87 may be a misaligned blazar. This work provides some hints about the overall dynamics of this famous extragalactic jet.