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 . For example , X-ray pulsars are probably old rotationally-powered pulsars that have already lost most of their power , and have only become visible again after their binary companions had expanded and began transferring matter on to the neutron star .