  The accretion-induced pulse-period changes of the Be/X-ray binary pulsar X
Persei were investigated over a period of 1996 January to 2017 September. This
study utilized the monitoring data acquired with the RXTE/ASM in 1.5$-$12 keV
and MAXI/GSC in 2$-$20 keV. The source intensity changed by a factor of 5$-$6
over this period. The pulsar was spinning down for 1996$-$2003, and has been
spinning up since 2003, as already reported. The spin up/down rate and the
3$-$12 keV flux, determined every 250 d, showed a clear negative correlation,
which can be successfully explained by the accretion torque model proposed by
Ghosh & Lamb (1979). When the mass, radius and distance of the neutron star are
allowed to vary over a range of 1.0$-$2.4 solar masses, 9.5$-$15 km, and
0.77$-$0.85 kpc, respectively, the magnetic field strength of $B=(4-25)
\times10^{13}\ \rm G$ gave the best fits to the observation. In contrast, the
observed results cannot be explained by the values of $B\sim10^{12}\ \rm G$
previously suggested for X Persei, as long as the mass, radius, and distance
are required to take reasonable values. Assuming a distance of $0.81\pm0.04$
kpc as indicated by optical astrometry, the mass of the neutron star is
estimated as $M=2.03\pm0.17$ solar masses.
