  We present results from the archival Chandra observations of the 0.3 s X-ray
pulsar PSR J1846-0258 associated with the supernova remnant (SNR) Kes 75. The
pulsar has the highest spin-down luminosity (Edot = 8.3e36 erg/s) among all the
high magnetic field pulsars (HBPs) and has been classified as a Crab-like
pulsar despite its magnetic field (5e13 G) being above the quantum critical
field. It is the only HBP described by a non-thermal Crab-like spectrum,
powering a bright pulsar wind nebula (PWN). Our spectroscopic study shows
evidence of spectral softening (photon index = 1.32 to 1.97) and temporal
brightening (unabsorbed flux = 4.3e-12 to 2.7e-11 erg/cm^2/s) of the pulsar by
~6 times from 2000 to 2006. The 0.5-10 keV luminosity of the pulsar at the
revised distance of 6 kpc has also increased from 1.85e34 to 1.16e35 erg/s, and
the X-ray efficiency increased from 0.2% to 1.4%. The observed X-ray
brightening and softening of the pulsar suggests for the first time that this
HBP is revealing itself as a magnetar.
