  The magnetar SGR J1935+2154 underwent a new active episode on 2020 April
27-28, when a forest of hundreds of X-ray bursts and a large enhancement of the
persistent flux were detected. For the first time, a radio burst with
properties similar to those of fast radio bursts and with a X-ray counterpart
was observed from this source, showing that magnetars can power at least a
group of fast radio bursts. In this paper, we report on the X-ray spectral and
timing properties of SGR J1935+2154 based on a long-term monitoring campaign
with Chandra, XMM-Newton, NuSTAR, Swift and NICER covering a time span of ~7
months since the outburst onset. The broadband spectrum exhibited a non-thermal
power-law component (photon index~1.2) extending up to ~20-25 keV throughout
the campaign and a blackbody component with temperature decreasing from ~1.5
keV at the outburst peak to ~0.45 keV in the following months. We found that
the luminosity decay is well described by the sum of two exponential functions,
reflecting the fast decay (~1 d) at the early stage of the outburst followed by
a slower decrease (~30 d). The source reached quiescence about ~80 days after
the outburst onset, releasing an energy of ~6e40 erg during the outburst. We
detected X-ray pulsations in the XMM-Newton data sets and derived an average
spin-down rate of ~3.5e-11 s/s using the spin period measurements derived in
this work and three values reported previously during the same active period.
Moreover, we report on simultaneous radio observations performed with the
Sardinia Radio Telescope. No evidence for periodic or single-pulse radio
emission was found.
