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Motivated by the measured velocity profile of the M87 jet using the KVN andVERA Array (KaVA) by Park et al. indicating that the starting position of thejet acceleration is farther from the central engine of the jet than predictedin general relativistic magnetohydrodynamic simulations, we explore how tomitigate the apparent discrepancy between the simulations and the KaVAobservation. We use a semi-analytic jet model proposed by Tomimatsu andTakahashi. consistently solving the trans-magnetic field structure butneglecting any dissipation effects. By comparing the jet model with theobserved M87 jet velocity profile, we find that the model can reproduce thelogarithmic feature of the velocity profile, and can fit the observed data whenchoosing $c/(100r_{g}) \lesssim \Omega_{F} \lesssim c/(70r_{g})$ where $r_{g}$is the gravitational radius. While a total specific energy (${\cal E}$) of thejet changes the terminal bulk Lorentz factor of the jet, a slower angularvelocity of the black hole magnetosphere (funnel region) ($\Omega_{F}$) makes alight-cylinder radius ($r_{\rm lc}$) larger and it consequently pushes out alocation of a starting point of the jet acceleration. Using the estimated$\Omega_{F}$ we further estimate the magnetic field strength on the eventhorizon scale in M87 by assuming Blandford-Znajek (BZ) process is in action.The corresponding magnetic flux threading the event horizon of M87 is in goodagreement with a magnetically arrested disc (MAD) regime.

Implications from the velocity profile of the M87 jet: a possibility of a slowly rotating black hole magnetosphere