Original: For example, X-ray pulsars are probably old rotation-powered pulsars that have already lost most of their energy, and have only become visible again after their binary companions expanded and began transferring matter on to the neutron star.
Simplified: For example, w:en:X-ray pulsars are probably old rotation-powered pulsars that have already lost most of their energy, and can only be seen again after their binary companions expanded and matter from them started falling onto the neutron star.