1E 161348-5055 (1E 1613), the source at the center of the supernova remnant RCW 103, has defied any easy classification since its discovery, owing to its long-term variability (a factor of ~ 100 in flux on time scales from months to years) and a periodicity of 6.67 hr with a variable light curve profile across different flux levels. On June 2016, 1E 1613 emitted a magnetar-like millisecond burst of hard X-rays accompanied with a factor ~ 100 brightening in the persistent soft X-ray emission. The duration and spectral decomposition of the burst, the discovery of a hard X-ray tail in the spectrum, and the long-term outburst history suggest that 1E 1613 is an isolated magnetar and the periodicity of 6.67 hr is the rotational spin period, making 1E 1613 the slowest neutron star ever detected.