  A few years after its discovery as a magnetar, SGR J1935+2154 started a new
burst-active phase on 2020 April 27, accompanied by a large enhancement of its
X-ray persistent emission. Radio single bursts were detected during this
activation, strengthening the connection between magnetars and fast radio
bursts. We report on the X-ray monitoring of SGR J1935+2154 from ~3 days prior
to ~3 weeks after its reactivation, using Swift, NuSTAR, and NICER. We detected
X-ray pulsations in the NICER and NuSTAR observations, and constrained the spin
period derivative to |Pdot| < 3e-11 s/s (3 sigma c.l.). The pulse profile
showed a variable shape switching between single and double-peaked as a
function of time and energy. The pulsed fraction decreased from ~34% to ~11%
(5-10 keV) over ~10 days. The X-ray spectrum was well fit by an absorbed
blackbody model with temperature decreasing from kT ~ 1.6 to 0.45-0.6 keV, plus
a non-thermal component (Gamma ~ 1.2) observed up to ~25 keV with NuSTAR. The
0.3-10 keV X-ray luminosity (at 6.6 kpc) increased in less than four days from
~ 6e33 erg/s to about 3e35 erg/s and then decreased again to 2.5e34 erg/s over
the following three weeks of the outburst. We also detected several X-ray
bursts, with properties typical of short magnetar bursts.
