We present the X-ray timing and spectral evolution of the Galactic Center magnetar SGR J1745 - 2900 for the first \sim 4 months post-discovery using data obtained with the Nuclear Spectroscopic Telescope Array ( NuSTAR ) and Swift observatories .
Our timing analysis reveals a large increase in the magnetar spin-down rate by a factor of 2.60 \pm 0.07 over our data span .
We further show that the change in spin evolution was likely coincident with a bright X-ray burst observed in 2013 June by Swift , and if so , there was no accompanying discontinuity in the frequency .
We find that the source 3–10 keV flux has declined monotonically by a factor of \sim 2 over an 80-day period post-outburst accompanied by a \sim 20 % decrease in the source ’ s blackbody temperature , although there is evidence for both flux and kT having levelled off .
We argue that the torque variations are likely to be magnetospheric in nature and will dominate over any dynamical signatures of orbital motion around Sgr A* .