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The canonical view that neutron stars are formed in the explosion of massive stars has at best modest observational support. Elemental abundances in the optical filaments of the Crab Nebula and in the LMC supernova remnant 0540-693 suggest massive progenitors for the young neutron stars in these two cases, but in most of the remnants which appear to house neutron stars, we have little or no evidence as to the nature of the parent star. In no case has the bulk of the supernova ejecta been assayed through direct observations of its X-ray emission. We present here the first X-ray spectra of composite supernova remnants which show clearly the composition of the parent star. Our principal example is the SNR G29.7-0.3, a classic composite remnant with a radio and X-ray synchrotron nebula at its center and a surrounding shell of emission at both wavelengths. We demonstrate a spectral decomposition of the two components using observations obtained with ASCA. We measure the luminosities of the Crab-like and shell components, estimate the age of the SNR and constrain the mass of the remnant's progenitor. We then go on to compare this result with the ASCA spectra of other such composite remnants in order to test the notion that the core-collapse supernova of a massive star is the only route to neutron star formation.
