  Observations with the Newton X-ray Multimirror Mission (XMM) show a strong
periodic modulation at 6.67+/-0.03 hours of the X-ray source at the centre of
the 2,000-year-old supernova remnant RCW 103. No fast pulsations are visible.
If genetically tied to the supernova remnant, the source could either be an
X-ray binary, comprising a compact object and a low-mass star in an eccentric
orbit, or an isolated neutron star. In the latter case, its age-period
combination would point to a peculiar magnetar, dramatically slowed-down,
possibly by a supernova debris disc. Both scenarios require non-standard
assumptions on the formation and evolution of compact objects in supernova
explosions.
