A few years after its discovery as a magnetar , SGR 1935+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 1935+2154 from \sim 3 days prior to \sim 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 | \dot { P } | < 6 \times 10 ^ { -11 } s s ^ { -1 } ( 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 \sim 34 % to \sim 11 % ( 5–10 keV ) over \sim 10 days .
The X-ray spectrum was well fit by an absorbed blackbody model with temperature decreasing from kT _ { BB } \sim 1.6 to 0.6–0.7 keV , plus a non-thermal component ( \Gamma \sim 1.2 ) observed up to \sim 25 keV with NuSTAR .
The 0.3–10 keV X-ray luminosity increased in less than four days from \sim 4 \times 10 ^ { 33 } erg s ^ { -1 } to about 2.5 \times 10 ^ { 35 } erg s ^ { -1 } and then decreased again to 1.4 \times 10 ^ { 34 } erg s ^ { -1 } over the following three weeks of the outburst .
We also detected several X-ray bursts , with properties typical of short magnetar bursts .