A COOLING NEUTRON-STAR IN SUPERNOVA REMNANT G296.5+10.0

We present a similar to 20 ks Advanced Satellite for Cosmology and Ast rophysics (ASCA) observation of the intriguing X-ray source 1E 1207.4- 5209. The source is situated near the center of G296.5+10.0, one of th e original barrel-shaped supernova remnants (SNRs). ASCA and ROSAT PSP C 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. Previous stringent upper limits for opti cal and radio emission from the source support a cooling neutron star hypothesis. At its likely distance of 2 kpc, the luminosity L(X) simil ar to 10(33)d(2)(2) ergs s(-1) implies a radiating surface area A simi lar to 30d(2)(2) km(2), which is significantly less than the total are a of a canonical neutron star. Despite the large number of detected ph otons, we see no evidence for rotationally induced X-ray pulsations. S urprisingly, no pulsar-like behavior is found in 1E 1207.4-5209, excep t that a faint radio nebulosity surrounding it may well be a pulsar-po wered plerion. We deduce that the neutron star may have been born spin ning very slowly (a birth period P similar to 0.5 s) and is a weak pul sar, which strengthens our belief that the observed radiation is indee d due to surface cooling.