RCW 103

RCW 103 is the remnant of a supernova that exploded around 12000 years ago. At the center of the remnant is a neutron star blasting out X-ray radiation, the core of the star that exploded.

A peculiar characteristic of this neutron star is that it only rotates once every 6.7 hours. That sounds fast, but typical neutron stars rotate much faster, at a rate of many times a second.

One possible answer for the mystery is that the original star that detonated wasn't alone. It might have had a much lower-mass companion which still remains. It was the magnetic field interaction between the neutron star and the low-mass companion that slowed down its rotation.