  We report on the detection of a bright, short, structured X-ray burst coming
from the supernova remnant RCW 103 on 2016 June 22 caught by the Swift/BAT
monitor, and on the follow-up campaign made with Swift/XRT, Swift/UVOT and the
optical/NIR GROND detector. The characteristics of this flash, such as
duration, and spectral shape, are consistent with typical short bursts observed
from soft gamma repeaters. The BAT error circle at 68 per cent confidence range
encloses the point-like X-ray source at the centre of the nebula,
1E161348-5055. Its nature has been long debated due to a periodicity of 6.67 hr
in X-rays, which could indicate either an extremely slow pulsating neutron
star, or the orbital period of a very compact X-ray binary system. We found
that 20 min before the BAT trigger, the soft X-ray emission of 1E161348-5055
was a factor of ~100 higher than measured 2 yr earlier, indicating that an
outburst had already started. By comparing the spectral and timing
characteristics of the source in the two years before the outburst and after
the BAT event, we find that, besides a change in luminosity and spectral shape,
also the 6.67 hr pulsed profile has significantly changed with a clear phase
shift with respect to its low-flux profile. The UV/optical/NIR observations did
not reveal any counterpart at the position of 1E161348-5055. Based on these
findings, we associate the BAT burst with 1E161348-5055, we classify it as a
magnetar, and pinpoint the 6.67 hr periodicity as the magnetar spin period.
