2009MNRAS.398L..49S

The high-resolution image of the jet of the BL Lac object MKN501 in radio shows a limb-brightened feature. An explanation of this feature as an outcome of the differential Doppler boosting of jet spine and jet boundary due to transverse velocity structure of the jet requires large viewing angle. However, this inference contradicts with the constraints derived from the high-energy γ-ray studies unless the jets bend over a large angle immediately after the γ-ray zone (close to the central engine). In this Letter, we propose an alternate explanation to the limb-brightened feature of MKN501 by considering the diffusion of electrons accelerated at the boundary shear layer into the jet medium, and this consideration does not require large viewing angle. Also, the observed difference in the spectral index at the jet boundary and jet spine can be understood within the framework of shear acceleration.