Boundary Shear Acceleration in the Jet of MKN501

The high resolution image of the jet of the BL Lac object MKN501 in radio, show a limb-brightened feature. An explanation of this feature as an outcome of 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 $\gamma$-ray studies unless the jets bends over a large angle immediately after the $\gamma$-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 frame work of shear acceleration.