Cooling Neutron Stars in Young Supernova Remnants

In quick succession, four young cooling neutron star candidates have been discovered in supernova remnants. These objects show no direct signs of magnetospheric activity. Here, we present a ~ 20 ks Advanced Satellite for Cosmology and Astrophysics (ASCA) observation of the intriguing X-ray source 1E 1207.4-5209. The source is situated near the center of G 296.5+10.0, one of the original barrel-shaped supernova remnants (SNR). ASCA and ROSAT PSPC data are very well described by a blackbody model of temperature kT = 0.28 keV with a foreground absorbing column of N_H = 4 x 10(20) cm(-2) in the range 0.1 - 10.0 keV. At its likely distance of 2 kpc, the luminosity L_X ~ 10(33) d_2(2) erg s(-1) implies a radiating surface area A ~ 30d₂(2) km(2) , which is significantly less than the total area of a canonical neutron star. Despite the large number of detected photons we see no evidence for rotationally induced X-ray pulsations. Surprisingly, no pulsar-like behavior is found in 1E 1207.4-5209, except that a faint radio nebulosity surrounding it may well be a pulsar-powered plerion. We deduce that the neutron star may have born spinning very slowly (a birth period P ~ 0.5 s) and is a weak pulsar, strengthening our belief that the observed radiation is indeed due to surface cooling. We suggest that these cooling objects may be born with large gtrsim 10(13) G fields and slow rotation. Such objects are candidates for ``injection'', a thorny issue in pulsar astrophysics.