On the nature of the Monoceros supernova remnant.

A dynamical expansion time of 300,000 years and an envelope of 100 km/sec are derived for the filamentary Monoceros Loop supernova remnant. Through arguments connecting three O stars with the supernova remnant, a lower limit of 25-30 solar masses is derived for the progenitor star. A thermal soft X-ray point source is predicted based on the hypothesis of a cooling neutron star. By analogy, we interpret the Cygnus Loop X-ray source as a neutron star.