Proton Acceleration beyond 100 EeV by an Oblique Shock Wave in the Jet of 3C 273

We estimate the highest energy of a proton diffusively accelerated by a shock in knot A1 of the jet in luminous nearby quasar 3C 273. Referring to the recent polarization measurements using very long baseline interferometry, we consider the shock propagation across magnetic field lines, namely, configuration of the oblique shock. For larger inclination of the field lines, the effects of particle reflection at the shock front are more pronounced, to significantly increase acceleration efficiency. The quasiperpendicular shock turns out to be needed for safely achieving the proton acceleration to the energy above 100 EeV (1020 eV) in a parameter domain reflecting conceivable energy restrictions.