  We report on a new NuSTAR observation and on the ongoing Swift XRT monitoring
campaign of the peculiar source 1E 161348-5055, located at the centre of the
supernova remnant RCW 103, which is recovering from its last outburst in June
2016. The X-ray spectrum at the epoch of the NuSTAR observation can be
described by either two absorbed blackbodies ($kT_{BB_1}$ ~ 0.5 keV,
$kT_{BB_2}$ ~ 1.2 keV) or an absorbed blackbody plus a power law ($kT_{BB_1}$ ~
0.6 keV, $\Gamma$ ~ 3.9). The observed flux was ~ 9 $\times$ 10$^{-12}$ erg
s$^{-1}$ cm$^{-2}$, ~ 3 times lower than what observed at the outburst onset,
but about one order of magnitude higher than the historical quiescent level. A
periodic modulation was detected at the known 6.67 hr periodicity. The spectral
decomposition and evolution along the outburst decay are consistent with 1E
161348-5055 being a magnetar, the slowest ever detected.
